How to Prevent Execution Plan Troubles when Querying Skewed Data, with jOOQ

One of the biggest advantages of using jOOQ is that you can change all of your complex application’s generated SQL with just a few lines of code. In this article, we’ll look into how to solve some common bind peeking issues just like that, without touching your application code, without the need to explain this advanced SQL performance issue to every team member, and best of all: for good.

What are Bind Values Good For?

First of all, bind variables are a good thing. They:

The latter bullet doesn’t affect all dialects, just like this article doesn’t. Commercial DBMS like Oracle, SQL Server, etc. ship with a powerful execution plan cache. They are designed to run thousands of distinct queries with very complex plans. Planning these queries takes time (I’ve seen Oracle SQL being planned for 5 seconds!) and you don’t want the DBMS to re-execute this planning work every time the query is run, which may be thousands of times per second.

Usually, this execution plan cache takes the SQL string (or a hash of it), and associates meta data like alternative possible execution plans to it. When the SQL string changes, the cache lookup fails and the “new” query has to be planned again.

I say “new”, because it might be the “same” query to the user, even if the string is different. For example:

SELECT * FROM book WHERE id = 1;
SELECT * FROM book WHERE id = 2;

Now we have two times the “same” query, but each of them are “new”. Oracle will plan them both afresh. So, we use bind variables, instead:

SELECT * FROM book WHERE id = ?;
SELECT * FROM book WHERE id = ?;

What is the Bind Peeking Issue?

In some cases, the cached plan is not optimal. This is the case when the actual bind value is significant to the planning, e.g. the value 1 would produce a vastly different plan than the value 2, or much more likely the value DELETED produces a different plan from PROCESSED or NEW.

This problem has been discussed in our previous blog post Why You Should Design Your Database to Optimise for Statistics.

“Bind Peeking” is a technique by the Oracle database (it is done by others, too, but maybe not called “Bind Peeking”) to have a “peek” at the bind variable to get a more accurate plan than the average plan if we don’t know the bind value. This can go both ways, good or bad, so there were numerous fixes / patches / workarounds in previous Oracle versions. Some interesting articles on the topic:

Databases slowly get to truly adaptive query execution models where execution plans can be fixed in-flight when the estimates were clearly wrong. Db2 is quite strong at this, and Oracle is getting better.

But even then, some times planners get it wrong, simply because they can’t reasonably estimate the cardinalities produced by a simple predicate like

WHERE x = ?

… just because the overall query is very complex and some SQL transformation did not apply.

Preventing the Problem by Avoiding Bind Values

Again. Please use bind values by default. They’re a good thing by default. Not all data is as skewed as the one I presented in my other blog post. But some data is almost always skewed: Enumeration types.

When you have an enum like:

enum ProcessingState {
  NEW,
  PROCESSING,
  EXECUTED,
  DELETED
}

Or in PostgreSQL:

CREATE TYPE processing_state AS ENUM (
  'new',
  'processing',
  'executed',
  'deleted'
);

Or even just encoded as a CHECK constraint:

CREATE TABLE transaction (
  -- ...
  processing_state VARCHAR(10) CHECK (processing_state IN (
    'new',
    'processing',
    'executed',
    'deleted'
  ))
  -- ...
);

In that case, you will very likely have highly skewed data. For example, a quick query might yield:

SELECT processing_state, count(*)
FROM transaction
GROUP BY processing_state

Resulting in:

+------------------+----------+
| processing_state |    count |
+------------------+----------+
| new              |    10234 |
| processing       |       15 |
| executed         | 17581684 |
| deleted          |    83193 |
+------------------+----------+

Now, do you think you’ll profit from indexing PROCESSING_STATE equally, when looking for NEW or PROCESSING versus when looking for EXECUTED values? Do you want the same plans? Do you want an average plan, which might not use the index, when in fact you should use it (looking for PROCESSING)?

Not only that, but it’s also quite unlikely that your queries are so generic as that the individual PROCESSING_STATE values can be used interchangeably. For example, a query looking for DELETED states might be run by a housekeeping batch job that wants to remove the logically deleted transactions for good. It will never query anything other than DELETED states. So, might as well inline, right?

Now, if you write a query like this:

SELECT *
FROM transaction
WHERE processing_state = 'processing';

With jOOQ, you can create an “inline” on a per-query basis using DSL.inline("processing") (as opposed to DSL.val("processing"), which is used by default, implicitly:

// These generate a ? bind value
ctx.selectFrom(TRANSACTION)
   .where(TRANSACTION.PROCESSING_STATE.eq("processing"))
   .fetch();

ctx.selectFrom(TRANSACTION)
   .where(TRANSACTION.PROCESSING_STATE.eq(val("processing")))
   .fetch();

// This creates an inline 'processing' literal
ctx.selectFrom(TRANSACTION)
   .where(TRANSACTION.PROCESSING_STATE.eq(inline("processing")))
   .fetch();

As always, assuming this static import:

import static org.jooq.impl.DSL.*;

But now, you have to think of doing that every single time you query PROCESSING_STATE, and all the other similar columns.

Prevent it for Good

Much better, prevent it for good. You can create a very simple Binding like this:

class AlwaysInlineStringBinding implements Binding<String, String> {

    final Binding<?, String> delegate = VARCHAR.getBinding();

    @Override
    public Converter<String, String> converter() {
        return Converters.identity(String.class);
    }

    @Override
    public void sql(BindingSQLContext<String> ctx) 
    throws SQLException {
        ctx.render().visit(inline(ctx.value()));
    }

    @Override
    public void register(BindingRegisterContext<String> ctx) 
    throws SQLException {
        delegate.register(ctx);
    }

    // No need to set anything
    @Override
    public void set(BindingSetStatementContext<String> ctx) 
    throws SQLException {}

    @Override
    public void set(BindingSetSQLOutputContext<String> ctx) 
    throws SQLException {
        delegate.set(ctx);
    }

    @Override
    public void get(BindingGetResultSetContext<String> ctx) 
    throws SQLException {
        delegate.get(ctx);
    }

    @Override
    public void get(BindingGetStatementContext<String> ctx) 
    throws SQLException {
        delegate.get(ctx);
    }

    @Override
    public void get(BindingGetSQLInputContext<String> ctx) 
    throws SQLException {
        delegate.get(ctx);
    }
}

Or, starting from jOOQ 3.15, even simpler, and generic:

class AlwaysInlineStringBinding 
extends DefaultBinding<String, String> {
    public AlwaysInlineStringBinding() {
        super(DefaultBinding.binding(VARCHAR));
    }

    @Override
    public void sql(BindingSQLContext<String> ctx) 
    throws SQLException {
        ctx.render().visit(inline(ctx.value()));
    }

    // No need to set anything
    @Override
    public void set(BindingSetStatementContext<T> ctx) 
    throws SQLException {}
}

Or even generic:

class AlwaysInlineBinding<T> extends DefaultBinding<T, T> {
    public AlwaysInlineBinding(DataType<T> type) {
        super(DefaultBinding.binding(type));
    }

    @Override
    public void sql(BindingSQLContext<T> ctx) 
    throws SQLException {
        ctx.render().visit(inline(ctx.value()));
    }

    // No need to set anything
    @Override
    public void set(BindingSetStatementContext<T> ctx) 
    throws SQLException {}
}

All this does is generate inline values instead of the ? bind parameter marker, and skip setting any value to the JDBC PreparedStatement (or reactive R2DBC Statement, starting from jOOQ 3.15. This will work just the same!)

Try it very easily, yourself (using the jOOQ 3.15 version):

@Test
public void testAlwaysInlineBinding() {
    DSLContext ctx = DSL.using(DEFAULT);
    DataType<Integer> t = INTEGER.asConvertedDataType(
        new AlwaysInlineBinding<>(INTEGER));

    Field<Integer> i = field("i", INTEGER);
    Field<Integer> j = field("j", t);
    Param<Integer> a = val(1);
    Param<Integer> b = val(1, INTEGER.asConvertedDataType(
        new AlwaysInlineBinding<>(INTEGER)));

    // Bind value by default
    assertEquals("?", ctx.render(a));
    assertEquals("1", ctx.renderInlined(a));
    assertEquals("1", ctx.render(b));
    assertEquals("1", ctx.renderInlined(b));

    // Bind value by default in predicates
    assertEquals("i = ?", ctx.render(i.eq(a)));
    assertEquals("i = 1", ctx.renderInlined(i.eq(a)));
    assertEquals("i = 1", ctx.render(i.eq(b)));
    assertEquals("i = 1", ctx.renderInlined(i.eq(b)));
    assertEquals("i = ?", ctx.render(i.eq(1)));
    assertEquals("i = 1", ctx.renderInlined(i.eq(1)));

    // No more bind values in predicates!
    assertEquals("j = 1", ctx.render(j.eq(a)));
    assertEquals("j = 1", ctx.renderInlined(j.eq(a)));
    assertEquals("j = 1", ctx.render(j.eq(b)));
    assertEquals("j = 1", ctx.renderInlined(j.eq(b)));
    assertEquals("j = 1", ctx.render(j.eq(1)));
    assertEquals("j = 1", ctx.renderInlined(j.eq(1)));
}

Of course, instead of doing the above programmatically, you’ll attach this Binding to all relevant columns using the code generator’s forced type configuration

Conclusion

Please use bind values by default. Both in jOOQ and elsewhere. It’s a very good default.

But sometimes, your data is skewed, and you as a developer, you probably know that. In those cases, sometimes, “inline values” as we call them (or constants, literals, etc.) may be the better choice to help the optimiser get better at estimating. Even if the optimiser’s estimates are good the first time, the plan may switch in production for some weird reason, including some plans being purged because the cache is full, or the DBA clicked a button, or whatever.

And that’s when your query might be unnecessarily slow all of a sudden. No more need. When you have enum types, or similar, just use the above simple trick, apply to all of your schema where it makes sense, and forget this problem for good.

Side note:

Of course, the other way round is just as easy. When you have inline literals that you want to switch over to bind values, you can do it just the same way, e.g. when you use jOOQ’s parsing connection to translate between dialects, or to patch your wrong ORM-generated SQL!

Use ResultQuery.collect() to Implement Powerful Mappings

In our opinion, any Iterable<T> should offer a <R> collect(Collector<T, ?, R>) method to allow for transforming the the content to something else using standard JDK collectors, jOOλ collectors from org.jooq.lambda.Agg or your own.

When using jOOQ, you don’t have to wait for the JDK to finally add these useful utilities to the Iterable API. jOOQ’s ResultQuery<R> already implements Iterable<R>, and offers additional convenience like collect() on top of it.

For example, using a Java 16 record type:

record Book (int id, String title) {}

List<Book> books =
ctx.select(BOOK.ID, BOOK.TITLE)
   .from(BOOK)
   .collect(Collectors.mapping(
        r -> r.into(Book.class),
        Collectors.toList()
   ));

There are other ways to map things, but why not use a Collector. The best thing about Collector types is, they compose, type safely, and arbitrarily, almost like Stream pipelines.

I found a very interesting use-case recently on Stack Overflow. The problem there was that fetchGroups() is quite simple and not left-join aware, meaning that when an AUTHOR (parent) has no BOOK (child), instead of an empty list, there will be a list with a single NULL item:

Map<AuthorRecord, List<BookRecord>> result =
ctx.select()
   .from(AUTHOR)
   .leftJoin(BOOK).onKey()
   .fetchGroups(AUTHOR, BOOK);

The above works well for inner joins, but it doesn’t really make sense for left joins. We should fix this in jOOQ, of course (https://github.com/jOOQ/jOOQ/issues/11888), but using Collectors, you can already work around this problem today.

Simply write

Map<AuthorRecord, List<BookRecord>> result =
ctx.select()
   .from(AUTHOR)
   .leftJoin(BOOK).onKey()
   .collect(groupingBy(
        r -> r.into(AUTHOR), 
        filtering(
            r -> r.get(BOOK.ID) != null, 
            mapping(
                r -> r.into(BOOK), 
                toList()
            )
        )
    ));

// All assuming the usual static imports:
import static org.jooq.impl.DSL.*;
import static com.example.generated.Tables.*;
import static java.util.stream.Collectors.*;

Step by step:

  1. Group results by AUTHOR, mapping keys to AuthorRecord just like jOOQ’s fetchGroups()
  2. For each AUTHOR filter out those BOOK records whose BOOK.ID is null. Given that BOOK.ID is the primary key, the only reason why it could be null is because of the left join
  3. Map values to BookRecord, just like jOOQ’s fetchGroups()
  4. Collect the child records into a list.

And you’re done. Just like when you use the ResultQuery as an Iterable in a foreach loop, the collect() call executes your query automatically, managing all resources, bypassing the intermediate Result data structure, which isn’t needed here.

How to Get an RDBMS Server Version with SQL

Do you need to know what RDBMS Server version you’re on, and you only have SQL at your disposal? No problem. Most RDBMS provide you with that information in some form of meta data table.

Here’s how:

-- CockroachDB
select version();

-- Db2
select service_level from table (sysproc.env_get_inst_info()) t

-- Derby
select getdatabaseproductversion() from (values (1)) t (a);

-- Exasol
select param_value 
from exa_metadata 
where param_name = 'databaseProductVersion';

-- Firebird
select rdb$get_context('SYSTEM', 'ENGINE_VERSION')
from rdb$database;

-- H2
select h2version(); 

-- HANA
select * from m_database;

-- HSQLDB
select character_value
from information_schema.sql_implementation_info
where implementation_info_name = 'DBMS VERSION'

-- Informix
select dbinfo('version', 'full') from systables where tabid = 1;

-- MariaDB
select version();

-- MemSQL (SingleStore)
select version();

-- MySQL
select version();

-- Oracle
select * from v$version;

-- PostgreSQL
select version();

-- Snowflake
select current_version();

-- SQL Server
select @@version;

-- SQLite
select sqlite_version();

-- Teradata
select infodata from dbc.dbcinfov where infokey = 'VERSION'

Missing this info for your own RDBMS? Feel free to comment.

Use IN List Padding to Your JDBC Application to Avoid Cursor Cache Contention Problems

A problem few developers are aware of is the possibility of running into “cursor cache contention” or “execution plan cache contention” problems when using IN lists in SQL. The problem that is described in lengths in previous articles, can be summarised as this.

All of these are distinct SQL queries and need to be parsed / planned / cached as possibly distinct execution plans in RDBMS that have strong plan caches (e.g. Db2, Oracle, SQL Server):

SELECT * FROM t WHERE id IN (?);
SELECT * FROM t WHERE id IN (?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?, ?);
SELECT * FROM t WHERE id IN (?, ?, ?, ?, ?);

While this is never a problem on developer machines, this can produce significant problems in production. I’ve seen this take down entire Oracle instances during peak loads. While RDBMS vendors should work on avoiding the severe problems this can cause, you can work around it using a trick that we invented at jOOQ (and Hibernate also has it now):

IN list padding

The trick is very simple. Just “pad” your IN lists to the nearest power of 2, and repeat the last value until the end:

SELECT * FROM t WHERE id IN (?);                      -- Left as it is
SELECT * FROM t WHERE id IN (?, ?);                   -- Left as it is
SELECT * FROM t WHERE id IN (?, ?, ?, ?);             -- Padded 3 to 4
SELECT * FROM t WHERE id IN (?, ?, ?, ?);             -- Left as it is
SELECT * FROM t WHERE id IN (?, ?, ?, ?, ?, ?, ?, ?); -- Padded 5 to 8

It’s really a hack and there are better solutions to avoiding this problem, including using arrays or temporary tables, but your production system may be down and you need a quick fix.

jOOQ has supported IN list padding for years now, since jOOQ 3.9 (late 2016), but with the relatively new parser and the ParsingConnection, you can now also apply this technique to any arbitrary SQL query in your non-jOOQ based system. Here’s a simple example:

// Any arbitrary JDBC Connection is wrapped by jOOQ here and replaced
// by a "ParsingConnection", which is also a JDBC Connection
DSLContext ctx = DSL.using(connection);
ctx.settings().setInListPadding(true);
Connection c = ctx.parsingConnection();

// Your remaining code is left untouched. It is unaware of jOOQ
for (int i = 0; i < 10; i++) {
    try (PreparedStatement s = c.prepareStatement(

        // This alone is reason enough to use jOOQ instead, 
        // but one step at a time :)
        "select 1 from dual where 1 in (" +
            IntStream.rangeClosed(0, i)
                     .mapToObj(x -> "?")
                     .collect(Collectors.joining(", ")) +
        ")")
    ) {
        for (int j = 0; j <= i; j++)
            s.setInt(j + 1, j + 1);

        try (ResultSet rs = s.executeQuery()) {
            while (rs.next())
                System.out.println(rs.getInt(1));
        }
    }
}

The above example just generates and runs 10 queries of this form:

select 1 from dual where 1 in (?)
select 1 from dual where 1 in (?, ?)
select 1 from dual where 1 in (?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?)
select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)

But that’s not what is being executed. In the DEBUG logs, we can see the following:

Translating from         : select 1 from dual where 1 in (?)
Translating to           : select 1 from dual where 1 in (?)
Translating from         : select 1 from dual where 1 in (?, ?)
Translating to           : select 1 from dual where 1 in (?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating from         : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Translating to           : select 1 from dual where 1 in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)

And just like that, our legacy application can run in production again, and you’ll have time to fix this more thoroughly.

Conclusion

While jOOQ is mostly an internal DSL for writing type safe, embedded SQL in Java, you can use it for a lot of other things too on any JDBC based application. The above example is using the ParsingConnection that can parse all your SQL statements and translate / transform them to anything else, including other dialects.

Never Again Forget to Call .execute() in jOOQ

jOOQ’s DSL, like any fluent API, has one big caveat. It’s very easy to forget to call .execute(). And when you do, chances are, you’re going to be staring at your code for minutes, because everything looks perfect:

ctx.insertInto(T)
   .columns(T.A, T.B)
   .values(1, 2);

Staring… staring… staring… Why is it not inserting that row?

“Aaaah, not again!!”

This is how it’s done:

ctx.insertInto(T)
   .columns(T.A, T.B)
   .values(1, 2)
   .execute();

In principle, this kind of mistake can happen with any fluent API. E.g. StringBuilder

sb.append("a").append("b"); // Not consuming the result

Or streams:

Stream.of(1, 2).peek(System.out::println); // Not so much peeking

But it usually doesn’t happen as much, because the difference to jOOQ is that

  • jOOQ’s DML statements (INSERT, UPDATE, DELETE, MERGE) and DDL statements (CREATE, ALTER, DROP, TRUNCATE), and a few other produce a side effect
  • That side effect is the only thing we care about. The result (the update count) is mostly irrelevant

And as such, we don’t care about the result of execute(), which is an int. No one forgets to call fetch() on a jOOQ ResultQuery:

ctx.select(T.A, T.B)
   .from(T); // Well duh

Because without calling fetch() or something similar, we’re not getting any results, and we want the results, just like with the StringBuilder or the Stream. But we don’t want the execute() result.

As such, even we, when writing jOOQ’s integration tests, occasionally forget to call this silly little method.

No more!

When it happened again this week…

… I finally created an issue to think about it: https://github.com/jOOQ/jOOQ/issues/11718. And I created an issue to wonder if JetBrains could do something about it: https://youtrack.jetbrains.com/issue/IDEA-265263

And they already can! Apart from the org.jetbrains.annotations.Contract annotation, which is there for precisely this reason, apparently, it’s also possible to mimick the JSR-305 @CheckReturnValue annotation on every method “whose return value needs to be checked” (i.e. a method that has no side effect, or whose side effect is to mutate only “this“).

I added this annotation, I added it to all the relevant jOOQ API, which was a bit of yak shaving (https://github.com/jOOQ/jOOQ/commit/f2b529a2305f8c5f8d037776687887a5acd50b11) and voilà

image

As you can see, IntelliJ now warns the user whenever they forget to consume the result of any of jOOQ’s DSL methods (by calling execute(), passing it to some method that consumes it, etc.)

Thanks again to Tagir Valeev from JetBrains for walking me through this and even improving the @Contract annotation, which jOOQ might use in a future version.

Calculating Pagination Metadata Without Extra Roundtrips in SQL

When paginating results in SQL, we use standard SQL OFFSET .. FETCH or a vendor specific version of it, such as LIMIT .. OFFSET. For example:

SELECT first_name, last_name
FROM actor
ORDER BY actor_id
OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY

As always, we’re using the Sakila database for this example.

This is rather straightforward. It will give us page 2 out of N pages, with a page size of 10. But how do we calculate these values? How do we know we’re on page 2? How do we know the number of pages N? Can we calculate this without an extra round-trip e.g. to calculate the total number of actors:

-- Yuck, a second round-trip!
SELECT COUNT(*)
FROM actor

We can do it with a single SQL query and window functions, but before I explain how to do this, please consider reading this article on why OFFSET pagination is a bad thing for your performance

If you’re still convinced OFFSET pagination is what you need, as opposed to keyset pagination, let’s look at how to calculate the above meta data with SQL.

What Metadata Do We Need?

The metadata we typically need to paginate using OFFSET are these:

  • TOTAL_ROWS: The total number of records if we hadn’t paginated
  • CURRENT_PAGE: The current page we’re on
  • MAX_PAGE_SIZE: The maximum page size
  • ACTUAL_PAGE_SIZE: The actual page size (when on the last page)
  • ROW: The actual offsets of the returned rows
  • LAST_PAGE: Whether we are on the last page

The maximum page size is something we set to the query, so it doesn’t have to be calculated. Everything else needs to be calculated. And here’s how to do that in a single query

SELECT 
  t.first_name, 
  t.last_name,

  -- Calculate some meta data
  COUNT(*) OVER () AS actual_page_size,
  MAX(row) OVER () = total_rows AS last_page,

  -- Metadata from the subquery
  total_rows,
  row,
  ((row - 1) / :max_page_size) + 1 AS current_page
FROM (
  SELECT
    u.*,

    -- Calculate some meta data, repeating the ORDER BY from
    -- the original query
    COUNT(*) OVER () AS total_rows,
    ROW_NUMBER () OVER (ORDER BY u.actor_id) AS row

  -- Original query with all the predicates, joins, as a derived table
  FROM (
    SELECT *
    FROM actor
  ) AS u

  -- Ordering and pagination done here, where :offset is
  -- The maximum row value of the previous page + 1
  ORDER BY u.actor_id
  OFFSET :offset ROWS
  FETCH NEXT :max_page_size ROWS ONLY
) AS t
ORDER BY t.actor_id

That’s it. Impressive? Don’t be scared, I’ll walk you through these things step by step. And if you ever get confused by SQL syntax, consider this article explaining the logical order of SQL operations, which is, for our example:

  • FROM (recurse ordering for derived tables)
  • WHERE (which the example omitted)
  • WINDOW calculations
  • SELECT (the projection)
  • ORDER BY
  • OFFSET .. FETCH

Annotating our query, ordering operations logically as 1.1, 1.2, 2.1, 2.2, 2.3, 2.4, 2.5, 3.1, 3.2, 3.3, 3.4:

-- 3.3
SELECT 
  t.first_name, 
  t.last_name,

  -- 3.2
  COUNT(*) OVER () AS actual_page_size,
  MAX(row) OVER () = total_rows AS last_page,

  -- 3.3
  total_rows,
  row,
  ((row - 1) / :max_page_size) + 1 AS current_page
-- 3.1
FROM (
  -- 2.3
  SELECT
    u.*,

    -- 2.2
    COUNT(*) OVER () AS total_rows,
    ROW_NUMBER () OVER (ORDER BY u.actor_id) AS row

  -- 2.1
  FROM (

    -- 1.2
    SELECT *
    -- 1.1
    FROM actor
  ) AS u

  -- 2.4
  ORDER BY u.actor_id

  -- 2.5
  OFFSET :offset ROWS
  FETCH NEXT :max_page_size ROWS ONLY
) AS t

-- 3.4
ORDER BY t.actor_id

Step by step explanation

First off, the original query SELECT * FROM actor is wrapped as a derived table called u. You can do almost anything you want with this original query, applying only a few transformations:

  • 1.1, 1.2, 2.1: You need to project (SELECT clause) the columns that your original query projected, plus the columns that you need for ORDER BY. Because I’m projecting the right things in the outermost query, and because there’s no DISTINCT clause in the original query, I conveniently projected *. Alternatively, I could have projected FIRST_NAME, LAST_NAME (because that is projected in the original query), and ACTOR_ID (because that’s what we ORDER BY).
  • 2.2: On that derived table u, we’re now able to calculate some metadata, including TOTAL_ROWS as COUNT(*) OVER () and ROW as ROW_NUMBER () OVER (ORDER BY t.actor_id). The COUNT(*) OVER () window function has an empty window specification OVER (), meaning it calculates all the rows that result from the FROM, WHERE, GROUP BY, HAVING clauses, i.e. from u in our particular example. Without a second round-trip! The ROW_NUMBER () OVER (ORDER BY u.actor_id) orders all the rows in u by u.actor_id and assigns unique row numbers to them, according to that ordering.
  • 2.3: The window functions are calculated implicitly because they’re located in the projection of this derived table. We’re also again going to conveniently project everything from u.*, because the outer-most query is the one that projects columns explicitly.
  • 2.4: The original ordering has been moved here because there is no guarantee that the ordering would have been maintained if we had ordered the contents of u. But we need the ordering to calculate OFFSET .. FETCH right after
  • 2.5: This is where we paginate. The OFFSET corresponds to the maximum ROW value that we’ve encountered before. We start at 0, and with a page size of 15, we use 15 on the next page. Remember that while indexes are 1 based in SQL, OFFSET is 0 based.
  • 3.1: All of the above is wrapped again in a derived table, in order to make further calculations on it, namely:
  • 3.2: We can again calculate COUNT(*) OVER (), calculating the total number of rows that result from the FROM, WHERE, GROUP BY, HAVING clauses, i.e. from t in our particular example. This time, the number of rows can be no more than MAX_PAGE_SIZE, because that’s what the FETCH (or LIMIT) clause inside of t says. But it can be less, too, so this is what we use to calculate the ACTUAL_PAGE_SIZE. Finally, we compare MAX(row) OVER () = total_rows to see if we’re on the last page, meaning the highest value for row in the current page resulting from t is compared to the total row count. Another way to calculate the LAST_PAGE value would be if ACTUAL_PAGE_SIZE < MAX_PAGE_SIZE, i.e. COUNT(*) OVER () < :MAX_PAGE_SIZE.
  • 3.3: In addition to the usual projection of the original columns FIRST_NAME, LAST_NAME (we’re no longer projecting * now!), we’re doing some final calculations including dividing ROW / TOTAL_ROWS to get the page number. You can calculate more things, such as TOTAL_ROWS / MAX_PAGE_SIZE to get the TOTAL_PAGES value.
  • 3.4: Finally, we have to ORDER BY t.actor_id again, don’t let anyone tell you otherwise. In SQL, if you do not ORDER BY, then the ordering is undefined. Sure, it would be silly for an optimiser to re-order things without any good reason. We’ve already ordered the contents of our subqueries in 2.4, but there is no guarantee, that this ordering is stable. Just add DISTINCT, UNION, or a JOIN resulting in a hash join or some random other operator to your query, and the ordering breaks. So, always ORDER BY if ordering is important to you.

And we’re done!

How to do it in jOOQ?

This is the kind of use-case where jOOQ really really shines, because all of this is about dynamic SQL. Your actual business logic is contained in the deeply nested u table. Everything else is “presentation logic”, which is implemented in SQL for very obvious reasons: To improve performance.

And because you want to implement all of this only once in some library of yours, instead of having to play this game on every query, you make this kind of query dynamic. The utility will look like this:

// Assuming as always the usual static imports, including:
// import static org.jooq.impl.DSL.*;
// import com.generated.code.Tables.*;

static Select<?> paginate(
    DSLContext ctx,
    Select<?> original,
    Field<?>[] sort,
    int limit,
    int offset
) {
    Table<?> u = original.asTable("u");
    Field<Integer> totalRows = count().over().as("total_rows");
    Field<Integer> row = rowNumber().over().orderBy(u.fields(sort))
        .as("row");

    Table<?> t = ctx
        .select(u.asterisk())
        .select(totalRows, row)
        .from(u)
        .orderBy(u.fields(sort))
        .limit(limit)
        .offset(offset)
        .asTable("t");

    Select<?> result = ctx
        .select(t.fields(original.getSelect().toArray(Field[]::new)))
        .select(
            count().over().as("actual_page_size"),
            field(max(t.field(row)).over().eq(t.field(totalRows)))
                .as("last_page"),
            t.field(totalRows),
            t.field(row),
            t.field(row).minus(inline(1)).div(limit).plus(inline(1))
                .as("current_page"))
        .from(t)
        .orderBy(t.fields(sort));

    // System.out.println(result);
    return result;
}

Notice the println for debugging? It will print again something like our original query (but you’ll also see that in your debug log output, by default, with jOOQ):

select
  t.ACTOR_ID,
  t.FIRST_NAME,
  t.LAST_NAME,
  count(*) over () as actual_page_size,
  (max(t.row) over () = t.total_rows) as last_page,
  t.total_rows,
  t.row,
  ((t.row / 15) + 1) as current_page
from (
  select
    u.*,
    count(*) over () as total_rows,
    row_number() over (order by u.ACTOR_ID) as row
  from (
    select
      ACTOR.ACTOR_ID,
      ACTOR.FIRST_NAME,
      ACTOR.LAST_NAME
    from ACTOR
  ) as u
  order by u.ACTOR_ID
  offset 30 rows
  fetch next 15 rows only
) as t
order by t.ACTOR_ID

And here’s how you call the utility:

System.out.println(
    paginate(
        ctx,
        ctx.select(ACTOR.ACTOR_ID, ACTOR.FIRST_NAME, ACTOR.LAST_NAME)
           .from(ACTOR),
        new Field[] { ACTOR.ACTOR_ID },
        15,
        30
    ).fetch()
);

Notice that you can plug in arbitrary SQL fragments into that utility and paginate them. Irrespective of the complexity (including joins, other window functions, grouping, recursion, and what not), jOOQ will have you covered and will now paginate things for you.

The result of the above is:

+--------+----------+---------+----------------+---------+----------+----+------------+
|ACTOR_ID|FIRST_NAME|LAST_NAME|actual_page_size|last_page|total_rows| row|current_page|
+--------+----------+---------+----------------+---------+----------+----+------------+
|      31|SISSY     |SOBIESKI |              15|false    |       200|  31|           3|
|      32|TIM       |HACKMAN  |              15|false    |       200|  32|           3|
|      33|MILLA     |PECK     |              15|false    |       200|  33|           3|
|      34|AUDREY    |OLIVIER  |              15|false    |       200|  34|           3|
|      35|JUDY      |DEAN     |              15|false    |       200|  35|           3|
|      36|BURT      |DUKAKIS  |              15|false    |       200|  36|           3|
|      37|VAL       |BOLGER   |              15|false    |       200|  37|           3|
|      38|TOM       |MCKELLEN |              15|false    |       200|  38|           3|
|      39|GOLDIE    |BRODY    |              15|false    |       200|  39|           3|
|      40|JOHNNY    |CAGE     |              15|false    |       200|  40|           3|
|      41|JODIE     |DEGENERES|              15|false    |       200|  41|           3|
|      42|TOM       |MIRANDA  |              15|false    |       200|  42|           3|
|      43|KIRK      |JOVOVICH |              15|false    |       200|  43|           3|
|      44|NICK      |STALLONE |              15|false    |       200|  44|           3|
|      45|REESE     |KILMER   |              15|false    |       200|  45|           3|
+--------+----------+---------+----------------+---------+----------+----+------------+

Or, on the last page, with offset 195

+--------+----------+---------+----------------+---------+----------+----+------------+
|ACTOR_ID|FIRST_NAME|LAST_NAME|actual_page_size|last_page|total_rows| row|current_page|
+--------+----------+---------+----------------+---------+----------+----+------------+
|     196|BELA      |WALKEN   |               5|true     |       200| 196|          14|
|     197|REESE     |WEST     |               5|true     |       200| 197|          14|
|     198|MARY      |KEITEL   |               5|true     |       200| 198|          14|
|     199|JULIA     |FAWCETT  |               5|true     |       200| 199|          14|
|     200|THORA     |TEMPLE   |               5|true     |       200| 200|          14|
+--------+----------+---------+----------------+---------+----------+----+------------+

Conclusion

jOOQ is all about dynamic SQL. There’s hardly any SQL feature left that jOOQ doesn’t support. This includes window functions, for example, but also making sure that your dynamic SQL works on a large number of SQL dialects, irrespective of the little syntactic details.

You can build your own libraries to construct re-usable SQL elements from other SQL building blocks as this article has shown, to dynamically create single-query OFFSET pagination meta data calculation, without performing additional database round trips.

Simulating Latency with SQL / JDBC

I’ve run across a fun little trick to simulate latency in your development environments when testing some SQL queries. Possible use-cases including to validate that backend latency won’t bring down your frontend, or that your UX is still bearable, etc.

The solution is PostgreSQL and Hibernate specific, though to doesn’t have to be. Besides, it uses a stored function to work around the limitations of a VOID function in PostgreSQL, but that can be worked around differently as well, without storing anything auxiliary to the catalog.

To remove the Hibernate dependency, you can just use the pg_sleep function directly using a NULL predicate, but don’t try it like this!

select 1
from t_book
-- Don't do this!
where pg_sleep(1) is not null;

This will sleep 1 second per row (!). As can be seen in the explain plan. Let’s limit to 3 rows to see:

explain analyze
select 1
from t_book
where pg_sleep(1) is not null
limit 3;

And the result is:

Limit  (cost=0.00..1.54 rows=3 width=4) (actual time=1002.142..3005.374 rows=3 loops=1)
   ->  Seq Scan on t_book  (cost=0.00..2.05 rows=4 width=4) (actual time=1002.140..3005.366 rows=3 loops=1)
         Filter: (pg_sleep('1'::double precision) IS NOT NULL)
 Planning Time: 2.036 ms
 Execution Time: 3005.401 ms

As you can see, the whole query took about 3 seconds for 3 rows. In fact, this is also what happens in Gunnar’s example from the tweet, except that he was filtering by ID, which “helps” hide this effect.

We can use what Oracle calls scalar subquery caching, the fact that a scalar subquery can be reasonably expected to be side-effect free (despite the obvious side-effect of pg_sleep), meaning that some RDBMS cache its result per query execution.

explain analyze
select 1
from t_book
where (select pg_sleep(1)) is not null
limit 3;

The result is now:

Limit  (cost=0.01..1.54 rows=3 width=4) (actual time=1001.177..1001.178 rows=3 loops=1)
   InitPlan 1 (returns $0)
     ->  Result  (cost=0.00..0.01 rows=1 width=4) (actual time=1001.148..1001.148 rows=1 loops=1)
   ->  Result  (cost=0.00..2.04 rows=4 width=4) (actual time=1001.175..1001.176 rows=3 loops=1)
         One-Time Filter: ($0 IS NOT NULL)
         ->  Seq Scan on t_book  (cost=0.00..2.04 rows=4 width=0) (actual time=0.020..0.021 rows=3 loops=1)
 Planning Time: 0.094 ms
 Execution Time: 1001.223 ms

We’re now getting the desired one-time filter. However, I don’t really like this hack, because it depends on an optimisation, which is optional, not a formal guarantee. This may be good enough for a quick simulation of latency, but don’t depend on this kind of optimisation in production lightheartedly.

Another approach that seems to guarantee this behaviour would be to use a MATERIALIZED CTE:

explain
with s (x) as materialized (select pg_sleep(1))
select *
from t_book
where (select x from s) is not null;

I’m now again using a scalar subquery, because I somehow need to access the CTE, and I don’t want to place it in the FROM clause, where it would impact my projection.

The plan being:

Result  (cost=0.03..2.07 rows=4 width=943) (actual time=1001.289..1001.292 rows=4 loops=1)
   One-Time Filter: ($1 IS NOT NULL)
   CTE s
     ->  Result  (...) (actual time=1001.262..1001.263 rows=1 loops=1)
   InitPlan 2 (returns $1)
     ->  CTE Scan on s  (cost=0.00..0.02 rows=1 width=4) (actual time=1001.267..1001.268 rows=1 loops=1)
   ->  Seq Scan on t_book  (cost=0.03..2.07 rows=4 width=943) (actual time=0.015..0.016 rows=4 loops=1)
 Planning Time: 0.049 ms
 Execution Time: 1001.308 ms

Again, containing a one-time filter, which is what we want here.

Using a JDBC based approach

If your application is JDBC based, you don’t have to simulate the latency by tweaking the query. You can simply proxy JDBC in one way or another. Let’s look at this little program:

try (Connection c1 = db.getConnection()) {

    // A Connection proxy that intercepts preparedStatement() calls
    Connection c2 = new DefaultConnection(c1) {
        @Override
        public PreparedStatement prepareStatement(String sql) 
        throws SQLException {
            sleep(1000L);
            return super.prepareStatement(sql);
        }
    };

    long time = System.nanoTime();
    String sql = "SELECT id FROM book";

    // This call now has a 1 second "latency"
    try (PreparedStatement s = c2.prepareStatement(sql);
        ResultSet rs = s.executeQuery()) {
        while (rs.next())
            System.out.println(rs.getInt(1));
    }

    System.out.println("Time taken: " + 
       (System.nanoTime() - time) / 1_000_000L + "ms");
}

Where:

public static void sleep(long time) {
    try {
        Thread.sleep(time);
    }
    catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
}

For simplicity reasons, this uses jOOQ’s DefaultConnection which acts as a proxy, conveniently delegating all the methods to some delegate connection, allowing for overriding only specific methods. The output of the program is:

1
2
3
4
Time taken: 1021ms

This simulates the latency on the prepareStatement() event. Obviously, you’d be extracting the proxying into some utility in order not to clutter your code. You could even proxy all your queries in development and enable the sleep call only based on a system property.

Alternatively, we could also simulate it on the executeQuery() event:

try (Connection c = db.getConnection()) {
    long time = System.nanoTime();

    // A PreparedStatement proxy intercepting executeQuery() calls
    try (PreparedStatement s = new DefaultPreparedStatement(
        c.prepareStatement("SELECT id FROM t_book")
    ) {
        @Override
        public ResultSet executeQuery() throws SQLException {
            sleep(1000L);
            return super.executeQuery();
        };
    };

        // This call now has a 1 second "latency"
        ResultSet rs = s.executeQuery()) {
        while (rs.next())
            System.out.println(rs.getInt(1));
    }

    System.out.println("Time taken: " +
        (System.nanoTime() - time) / 1_000_000L + "ms");
}

This is now using the jOOQ convenience class DefaultPreparedStatement. If you need these, just add the jOOQ Open Source Edition dependency (there’s nothing RDBMS specific in these classes), with any JDBC based application, including Hibernate:

<dependency>
  <groupId>org.jooq</groupId>
  <artifactId>jooq</artifactId>
</dependency>

Alternatively, just copy the sources of the classes DefaultConnection or DefaultPreparedStatement if you don’t need the entire dependency, or you just proxy the JDBC API yourself.

A jOOQ based solution

If you’re already using jOOQ (and you should be!), you can do this even more easily, by implementing an ExecuteListener. Our program would now look like this:

try (Connection c = db.getConnection()) {
    DSLContext ctx = DSL.using(new DefaultConfiguration()
        .set(c)
        .set(new CallbackExecuteListener()
            .onExecuteStart(x -> sleep(1000L))
        )
    );

    long time = System.nanoTime();
    System.out.println(ctx.fetch("SELECT id FROM t_book"));
    System.out.println("Time taken: " +
        (System.nanoTime() - time) / 1_000_000L + "ms");
}

Still the same result:

+----+
|id  |
+----+
|1   |
|2   |
|3   |
|4   |
+----+
Time taken: 1025ms

The difference is that with a single intercepting callback, we can now add this sleep to all types of statements, including prepared statements, static statements, statements returning result sets, or update counts, or both.

Translating Stored Procedures Between Dialects

In the past years, we’ve invested a lot of effort into improving our procedural language capabilities in jOOQ. What started with a simple internal API to support the emulations of DDL clauses like these:

-- Some dialect that supports this
create table if not exists t (i varchar(10));

-- Db2
begin
  declare continue handler for sqlstate '42710' begin end;
  execute immediate 'create table T (I varchar(10))';
end

-- Oracle
begin
  execute immediate 'create table T (I varchar2(10))';
exception
  when others then
    if sqlerrm like 'ORA-00955%' then null;
    else raise;
    end if;
end;

-- SQL Server
begin try
  create table T (I varchar(10))
end try
begin catch
  if error_number() != 2714 throw;
end catch

… evolved into a full fledged API for all sorts of procedural logic executed in your database server.

Anonymous blocks

The above examples show what most RDBMS call “anonymous blocks”, similar to Java’s anonymous classes, i.e. elements of procedural logic that do not have a name.

Depending on the database, these blocks are interpreted on the fly, or compiled and cached like ordinary SQL statements. They can be very useful for a variety of reasons:

  • To create atomic ad-hoc units of code, executed in a single server round trip, similar to the above DDL scripts with integrated exception handling
  • To create dynamic procedural code. This may be esoteric for many, but exactly the right thing to do for others. All of jOOQ is about dynamic SQL, so why not also dynamic PL/SQL, T-SQL, pgplsql, etc?
  • To work around limitations imposed by Conway’s Law, when there is no way you can get the necessary GRANT or other bureaucratic token to deploy your procedure in production at your discretion. I mean, this is still a thing in a lot of companies.
  • A lesser case of Conway’s Law may be when you’re a product vendor, and you don’t know if you can create procedures on your clients’ production system. Just don’t. Run your procedural logic as an anonymous block if you can’t, or as a procedure if you can. Same jOOQ code.
  • If your procedural code changes very frequently (even dynamically), storing it might cause tricky issues. If you’ve ever worked with Oracle and ran into the dreaded latch free event, you know what I mean.

I’m by no means advocating you should use anonymous blocks over stored procedures in general. If you can, store your code in the database for better performance and re-use. But sometimes you can’t, and sometimes you shouldn’t.

So, jOOQ supports – as always – a mix of various procedural logic elements, including:

  • Blocks with variable declarations
  • IF statement
  • Loops including LOOP, WHILE, REPEAT, FOR loops
  • EXIT (or LEAVE) and CONTINUE (or ITERATE) for loop control flow
  • RETURN to return from procedures or functions
  • GOTO (ghasp!)
  • SIGNAL or RAISE
  • Labels
  • CALL statement to call other stored procedures
  • EXECUTE statement (for running dynamic SQL from within procedural logic. Which level of inception is that?)

And we’re adding more support all the time. The Java code might look something like this:

Variable<Integer> i = var(name("i"), INTEGER);
ctx.begin(
  for_(i).in(1, 10).loop(
    insertInto(T).columns(T.COL).values(i)
  )
).execute();

Assuming you cannot run a bulk insert statement for some reason, this might be the way to go. It translates to various dialects as follows.

Db2 and MySQL (which doesn’t support anonymous blocks, but statement batches)

begin
  declare I bigint;
  set I = 1;
  while I <= 10 do
    insert into T (COL) values (I);
    set I = (I + 1);
  end while;
end;

PostgreSQL

do $$
begin
  for I in 1 .. 10 loop
    insert into T (COL) values (I);
  end loop;
end;
$$

Oracle

begin
  for I in 1 .. 10 loop
    insert into T (COL) values (I);
  end loop;
end;

SQL Server

begin
  declare @I bigint = 1;
  while @I <= 10 begin
    insert into T (COL) values (I);
    set @I = (@I + 1);
  end;
end;

As always with jOOQ, you don’t have to start out with writing jOOQ API based code. While this is the recommended approach when your procedural (or SQL) logic is dynamic, jOOQ can also parse and translate static SQL in string form. The babelfish of SQL. Play around with it here to learn more: https://www.jooq.org/translate/

Storing the code as a procedure

If you don’t have any of the above use-cases, you will want to store this code as a procedure (or function):

  • For greater re-use
  • For better performance

In that case, starting from jOOQ 3.15, you can use our CREATE PROCEDURE, CREATE FUNCTION, or even CREATE TRIGGER support.

Note: CREATE PACKAGE is high on our wish list, but might not make it into 3.15 anymore. If packages are used for namespacing only, they might be emulated using schemas in other dialects. Other package level features, such as package state may be more difficult to translate.

The previous anonymous block can be easily wrapped in a DSLContext.createProcedure() call

Variable<Integer> i = var("i", INTEGER);
Parameter<Integer> i1 = in("i1", INTEGER);
Parameter<Integer> i2 = in("i2", INTEGER);

ctx.createProcedure("insert_into_t")
   .parameters(i1, i2)
   // You may or may not wrap your block in BEGIN .. END.
   // jOOQ will figure it out per dialect...
   .as(for_(i).in(i1, i2).loop(
     insertInto(T).columns(T.COL).values(i)
   ))
   .execute();

Which would produce the following procedures:

Db2 and MySQL

create procedure INSERT_INTO_T(
  I1 integer,
  I2 integer
)
begin
  declare I bigint;
  set I = I1;
  while I <= I2 do
    insert into T (COL) values (I);
    set I = (I + 1);
  end while;
end;

MariaDB

create procedure INSERT_INTO_T(
  I1 int,
  I2 int
)
begin
  for I in I1 .. I2 do
    insert into T (COL) values (I);
  end for;
end;

Oracle

create procedure INSERT_INTO_T(
  I1 number,
  I2 number
)
as
begin
  for I in I1 .. I2 loop
    insert into T (COL) values (I);
  end loop;
end;

PostgreSQL

create procedure INSERT_INTO_T(
  I1 int,
  I2 int
)
language plpgsql
as
$$
begin
  for I in I1 .. I2 loop
    insert into T (COL) values (I);
  end loop;
end;
$$

SQL Server

create procedure INSERT_INTO_T
  @I1 int,
  @I2 int
as
begin
  declare @I bigint = @I1;
  while @I <= @I2 begin
    insert into T (COL) values (@I);
    set @I = (@I + 1);
  end;
end;

Play around with it here to learn more: https://www.jooq.org/translate/. We’re also looking forward to your bug reports and/or feature requests here: https://github.com/jOOQ/jOOQ/issues/new/choose.

Tricky transformations

Procedural languages are standardised via the ISO/IEC 9075-4 standard, and some RBDMS surprisingly agree to a large extent on the standard, including:

  • Db2
  • HSQLDB
  • MariaDB
  • MySQL

Others do less so, but all procedural languages agree on the fact that they are very simple languages, without any such “fancy” things like subtype or parametric polymorphism (OK, PL/SQL has some subtype polymorphism, but not a very sophisticated one. We won’t support it for now), lambda expressions, dynamic dispatch, algebraic data types, etc. etc.

What they do have in common is a tight integration with the SQL language, which is where they shine.

But there are subtle differences, nonetheless. For example, they differ in where you can declare variables. Some have block scope, others don’t. And some adhere to the standard, where LEAVE requires a label, others don’t.

Imagine you write this “fantasy” jOOQ code

Name t = unquotedName("t");
Name a = unquotedName("a");
Variable<Integer> i = var(unquotedName("i"), INTEGER);

ctx.begin(
     insertInto(t).columns(a).values(1),
     declare(i).set(2),
     loop(
       insertInto(t).columns(a).values(i),
       i.set(i.plus(1)),
       if_(i.gt(10)).then(loop(exit()), exit())
     )
   )
   .execute();

This is just a more complicated version of the original loop, which inserts values 1-10 into a table. There’s no reason other than to show off the transformation capabilities for the nesting of loop(exit()), as well as the infinite LOOP with EXIT usage, rather than the indexed FOR loop.

There are a few things that don’t always work exactly like this in some dialects!

Let’s look at what Db2 does with this.

begin
  -- Variable declarations need to be "pulled up" to the beginning
  -- of the block, i.e. before the INSERT statement
  declare i integer;
  insert into t (a) values (1);

  -- While irrelevant to this example, the init value for the
  -- variable declaration must remain at the original location
  set i = 2;

  -- We need a label to be able to leave this loop
  alias_1:
  loop
    insert into t (a) values (i);
    set i = (i + 1);
    if i > 10 then

      -- Same here, a label is required
      alias_2:
      loop
        leave alias_2;
      end loop;
      leave alias_1;
    end if;
  end loop;
end

If we don’t use EXIT on a loop, then there won’t be a label. Or, you can obviously label your loops explicitly, which is always recommended. But sometimes, you don’t have that in your original source code.

What does Oracle do with this?

Oracle has a slightly different syntax here:

declare
  i number(10);
begin
  insert into t (a) values (1);
  i := 2;
  loop
    insert into t (a) values (i);
    i := (i + 1);
    if i > 10 then
      loop
        exit;
      end loop;
      exit;
    end if;
  end loop;
end;

The main difference being that the declaration is also pulled up, but a separate DECLARE block is required to declare variables outside of BEGIN .. END. Label-less EXIT is supported natively, so nothing needs to be transformed here.

If you’re interested in how this transformations work, read this post.

Conclusion

Whether you’re migrating off one dialect onto another, or whether you’re supporting several dialects at once, or you’re writing dynamic SQL and dynamic procedural logic, or you just like writing things in Java rather than native SQL, or you suffer from Conway’s Law and cannot store your procedural code easily, jOOQ can help you with those endeavours.

For a while now, jOOQ has supported procedural statements as anonymous blocks for the most popular dialects. Starting from jOOQ 3.15, we’ll also support storing this logic in the database in a dialect agnostic way, as well as parsing / translating procedural code on our website, or as a library / CLI or JDBC proxy to replace your SQL / procedural code ad-hoc in a legacy JDBC application.

Stay tuned for more in this exciting area of jOOQ development!

Implementing a generic REDUCE aggregate function with SQL

So, @rotnroll666 nerd sniped me again. Apparently, the Neo4j Cypher query language supports arbitrary reductions, just like any functional collection API, oh say, the JDK Stream API:

Stream.of(2, 4, 3, 1, 6, 5)
      .reduce((i, j) -> i * j)
      .ifPresent(System.out::println); // Prints 720

SQL doesn’t have this, yet it would be very useful to be able to occasionally do that. An arbitrary reduction can be implemented “easily” in SQL. Let’s look at the above multiplication reduction. In PostgreSQL, you’d write it like this:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  (
    with recursive
      u(i, o) as (
         select i, o
         from unnest(array_agg(t.i)) with ordinality as u(i, o)
      ),
      r(i, o) as (
        select u.i, u.o from u where o = 1
        union all 
        select r.i * u.i, u.o from u join r on u.o = r.o + 1
        --     ^^^^^^^^^ reduction
      )
    select i from r
    order by o desc
    limit 1
  )
from t;

Woah. That’s a bit of a syntactic beast. Let’s decompose it.

The aggregate function

First off, if we were summing the values, we’d use the built-in SUM function, like this:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select sum(i)
from t;

That would produce 21. If you’re willing to lose precision, you could emulate PRODUCT() using logarithms. But we wrote REDUCE(), a hypothetical one, like this:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select reduce(
  t1.i * t2.i referencing accumulated as t1, accumulating as t2
)
from t;

This is SQL, so the lambda expression would obviously use a ton of keywords, completely novel and unique to this particular function, and you’d need jOOQ to make it composable 😁. Essentially, we’d have some sort of reduction expression based on two pseudo tables:

  • The accumulated table containing the result
  • The accumulating table (or rather row)

A reduction is a generic aggregate function that operates on groups. So, we will have to re-use some SQL aggregate function mechanism to achieve the desired behaviour.

Using ARRAY_AGG() to get the aggregation effect

First off, let’s do some aggregation. PostgreSQL’s ARRAY_AGG() is perfect for this job, because it

  • Aggregates
  • Yet kinda leaves the data untouched, unlike e.g. SUM()

In a way, it’s a collection like Stream.collect(), not a reduction.

If we use ARRAY_AGG() in a correlated subquery, we’ll still get the aggregation effect, but we can unnest the array again to a table, in order to operate on it. You can see this in the following example:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  (
    select string_agg(i::text, ', ')
    from unnest(array_agg(t.i)) as u(i)
  )
from t;

This yields:

2, 4, 3, 1, 6, 5

Not a very useful thing to do, aggregate, unnest, and aggregate again, but it shows the power of nesting an aggregate function in a correlated subquery’s FROM clause. If your RDBMS doesn’t have arrays, maybe you can do the same thing using JSON_ARRAYAGG and JSON_TABLE, or XMLAGG and XMLTABLE.

Disclaimer: PostgreSQL often Does The Right Thing™. I think you’d be more hard pressed to juggle with SQL syntax as elegantly in most other RDBMS, so this approach isn’t portable. But as Lætitia Avrot so elegantly put it:

Next step, generate row numbers

There are mainly 2 ways how we can generate row numbers in our example:

Adapting our previous example for some visualisation:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  (
    select string_agg(row(i, o)::text, ', ')
    from unnest(array_agg(t.i)) with ordinality as u(i, o)
  )
from t;

(Awesome, that row constructor!)

This produces:

(2,1), (4,2), (3,3), (1,4), (6,5), (5,6)

Doesn’t look fancy, but imagine we group by even numbers:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  i % 2,
  (
    select string_agg(row(i, o)::text, ', ')
    from unnest(array_agg(t.i)) with ordinality as u(i, o)
  )
from t
group by i % 2;

The result is now:

i % 2string_agg
0(2,1), (4,2), (6,3)
1(3,1), (1,2), (5,3)

It’s a bit weird, right? We GROUP BY in the outer query, and the entire correlated subquery is the aggregate function based on the fact that its FROM clause contains ARRAY_AGG(). This isn’t so much different from this query:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 1 + sum(i) + 2
from t;

We’re used to building scalar expressions from aggregate functions all the time. This is nothing fancy. We can easily also just wrap the function in another subquery:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select (select 1 + sum(i) + 2)
from t;

From here, it’s not far fetched to extend the aggregate-function-in-scalar-subquery approach to the FROM clause, and then unnesting the aggregation again. This may not “click” immediately. The GROUP BY clause in SQL is a bit weird, syntactically.

Remark: Regrettably, PostgreSQL doesn’t allow using aggregate functions in the FROM clause on the same query level like in a correlated subquery. I was going to show a fancy LATERAL version, but this doesn’t work (yet).

Now, recurse

The final bit is the recursion with the r table:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  (
    with recursive
      u(i, o) as (
         select i, o
         from unnest(array_agg(t.i)) with ordinality as u(i, o)
      ),
      r(i, o) as (
        select u.i, u.o from u where o = 1
        union all 
        select r.i * u.i, u.o from u join r on u.o = r.o + 1
        --     ^^^^^^^^^ reduction
      )
    select i from r
    order by o desc
    limit 1
  )
from t;

We simply recurse on the ordinality. The first subquery of UNION ALL produces the first row of our data, namely (1, 1). The next iterations just always multiply the result of r.i by the value of u.i from the next row by ordinality. This is probably best shown visually:

r.ir.ou.i
2 = u.i (first iteration)12
8 = prev r.i * u.i24
24 = prev r.i * u.i33
24 = prev r.i * u.i41
144 = prev r.i * u.i56
720 = prev r.i * u.i65

Finally, we don’t care about SQL’s set-based way of working. I.e. we don’t care about the whole set of multiplications that are shown in the table above. We only care about the last row, ordered by the ordinality, which contains our result in r.i

Done!

Using group by

Just as shown before, we can easily add a GROUP BY clause to the outer query. E.g. let’s multiply odd and even numbers separately:

with t(i) as (values (2), (4), (3), (1), (6), (5))
select 
  i % 2,
  (
    with recursive
      u(i, o) as (
         select i, o
         from unnest(array_agg(t.i)) with ordinality as u(i, o)
      ),
      r(i, o) as (
        select u.i, u.o from u where o = 1
        union all 
        select r.i * u.i, u.o from u join r on u.o = r.o + 1
      )
    select i from r
    order by o desc
    limit 1
  ),
  string_agg(i::text, ' * ')
from t
group by i % 2

I’ve added another aggregate function STRING_AGG() for good measure to get:

i % 2istring_agg
0482 * 4 * 6
1153 * 1 * 5

Wonderful, isn’t it? Now, I wasn’t able to just add an OVER() clause right there. That produced

SQL Error [42P20]: ERROR: window functions are not allowed in functions in FROM

Maybe that will work as well, in the near future? Or, I might come up with another hack to make it work, in case of which I’ll update this post.

jOOQ support

Obviously, this will be supported in jOOQ soon: https://github.com/jOOQ/jOOQ/issues/11385. The syntax will be again much more bearable:

ctx.select(T.I.mod(inline(2)), reduce(T.I, (i1, i2) -> i1.times(i2)))
   .from(T.I)
   .groupBy(T.I.mod(inline(2)))
   .fetch();

Other emulations using actual CREATE AGGREGATE FUNCTION will be investigated as well, in the near future.

jOOQ Internals: Pushing up SQL fragments

Over the past 13 years, jOOQ has accrued quite some internal features, which you, the user, are not exposed to. One very interesting feature is the capability for any arbitrary jOOQ expression tree element to push a SQL fragment up to a higher level.

How does it work?

The jOOQ expression tree model

When you write a jOOQ query, you’re really creating an expression tree of your SQL statement that just happens to look like actual SQL syntax. For example:

ctx.select(BOOK.ID, BOOK.TITLE)
   .from(BOOK)
   .where(BOOK.AUTHOR_ID.eq(1))
   .fetch();

In jOOQ, this is nothing else than an expression tree of this form

                     Select (Statement)
                             |
          +------------------+-----------------+
          |                  |                 |
        Select             From              Where
          |                  |                 |
    +-----+------+           |                 |
    |            |           |                 |
TableField  TableField     Table        CompareCondition
  |     |     |     |        |           |     |       |
Table Field Table Field    BOOK    TableField  EQ     Val
  |     |     |     |                |     |           |
BOOK    ID   BOOK TITLE            Table Field         1
                                     |     |
                                    BOOK AUTHOR_ID

When generating the SQL, jOOQ mostly just traverses the tree brepth first (just kidding. It’s mostly depth first, but some child elements on the same level are often considered, prior to going down one level), collecting each element into a StringBuilder to the expected form:

SELECT book.id, book.title
FROM book
WHERE book.author_id = 1

Each of these so-called QueryPart expression tree elements can decide for itself how it’s going to render its SQL. For example, the CompareCondition will roughly generate this sequence:

<lhs> <operator> <rhs>

… further delegating SQL generation to its children, whatever they may be. A TableField will decide whether to fully / partially / or not qualify its Field reference at all, etc., e.g. based on the schema mapping (multi tenancy) feature.

If you’re using functions, such as Substring, that function can decide on its own how it should generate its SQL. As can be seen in the manual, these are all the same:

-- ACCESS
mid('hello world', 7)

-- ASE, SQLDATAWAREHOUSE, SQLSERVER
substring('hello world', 7, 2147483647)

-- AURORA_MYSQL, AURORA_POSTGRES, COCKROACHDB, CUBRID, H2, 
-- HANA, HSQLDB, IGNITE, MARIADB, MEMSQL, MYSQL, POSTGRES, REDSHIFT, 
-- SNOWFLAKE, SYBASE, VERTICA
substring('hello world', 7)

-- DB2, DERBY, INFORMIX, ORACLE, SQLITE
substr('hello world', 7)

-- FIREBIRD, TERADATA
substring('hello world' FROM 7)

This way, a jOOQ expression tree can emulate any syntax in any dialect.

But what if the emulation is not local?

Non local emulations

Sometimes, a QueryPart needs to assume the presence of a non-local syntactic element to work. A recent case was https://github.com/jOOQ/jOOQ/issues/11366.

When writing this procedural logic in jOOQ:

for_(i).in(1, 10).loop(
    insertInto(t).columns(a).values(i)
)

Of course, you wouldn’t do this. You’d write a bulk insert statement instead and solve this with SQL only! But you have your reasons, right?

Then, the indexed FOR loop may have to be emulated in some dialects, using an equivalent WHILE statement. So, instead of this straight forward procedural SQL generation, which we might get in Oracle for example:

FOR i IN 1 .. 10 LOOP
  INSERT INTO t (a) VALUES (i);
END LOOP;

… we generate this in MySQL, more or less:

DECLARE i INT DEFAULT 1;
WHILE i <= 10 DO
  INSERT INTO t (a) VALUES (i);
  SET i = i + 1;
END WHILE;

This can still be done completely locally, as shown before. There’s a FOR expression tree element that can locally generate the DECLARE and WHILE query parts, instead. But what if local variables aren’t possible? What if there’s no block scope, as in Firebird?

In Firebird, all your local variables must be declared in a top level declaration section. If we run the above in an anonymous block, the correct generated procedural SQL would be this:

EXECUTE BLOCK AS
  DECLARE i INT;
BEGIN
  :i = 1; -- The loop variable must still be initialised locally
  WHILE (:i <= 10) DO BEGIN
    INSERT INTO t (a) VALUES (i);
    :i = :i + 1;
  END
END

This is still the case when we further nest the loop in procedural control flow elements, such as:

for_(i).in(1, 10).loop(
    for_(j).in(1, 10).loop(
        insertInto(t).columns(a, b).values(i, j)
    )
)

Of course, you still wouldn’t do this. You’d write a bulk insert statement from a cartesian product instead and solve this with SQL only! But alas, let’s keep the example simple

We now have nested variable declarations, which still work well in MySQL

DECLARE i INT DEFAULT 1;
WHILE i <= 10 DO 
  BEGIN
    DECLARE j INT DEFAULT 1;
    WHILE j <= 10 DO
      INSERT INTO t (a, b) VALUES (i, j);
      SET j = j + 1;
    END WHILE;
  END
  SET i = i + 1;
END WHILE;

But in Firebird, the declarations will both have to be pushed up to the top:

EXECUTE BLOCK AS
  DECLARE i INT;
  DECLARE j INT;
BEGIN
  :i = 1; -- The loop variable must still be initialised locally
  WHILE (:i <= 10) DO BEGIN
    :j = 1; -- The loop variable must still be initialised locally
    WHILE (:j <= 10) DO BEGIN
      INSERT INTO t (a, b) VALUES (i, j);
      :j = :j + 1;
    END
    :i = :i + 1;
  END
END

This doesn’t handle all the edge cases yet (e.g. some dialects allow for “hiding” local variables, such as PL/SQL) but it goes a long way already for simple procedures, functions, and triggers, all of which will be supported starting in jOOQ 3.15.

How does it work?

Alternative 1: Expression tree transformation

There are numerous ways to make such an expression tree transformation work. In the long run, we will re-design our internal expression tree model, and make it available also as a public API to those who wish to use the jOOQ parser and expression tree transformation as a standalone product. To some extent, this is already possible using the VisitListener SPI as shown in this post about row level security, but the current implementation is complex.

Also, it is relatively rare (so far) for an expression tree to require non-local transformation. This means that eagerly trying to look for possible candidates every time is probably overkill.

Alternative 2: Lazy expression tree transformation

We could transform the expression tree “lazily”, i.e. still assuming it is unnecessary, and when it is, throw an exception and start again. We actually do this, the “pattern” in jOOQ is called a ControlFlowSignal, and it is mainly used to work around the maximum number of bind parameters per statement limit of different dialects. I.e. we just count bind values, and if there are more than 2000 in SQL Server (2100 are supported by SQL Server, but only 2000 by jtds), then we just re-generate the SQL from scratch using inline values in a static statement.

As always with jOOQ, you can re-configure these limits to your own value.

Another case is when you forgot to turn on the ROWNUM to LIMIT transformation when migrating off Oracle’s old ROWNUM filtering. It would be silly to eagerly search for ROWNUM instances every time. Instead, we just re-generate the entire SQL query when we encounter one, and when you’re not using Oracle.

The assumption here is that these things happens very rarely, and if they do, you haven’t thought of it, and you don’t want the query to fail in production. The fact that a probably already slow query takes just a bit more time for jOOQ to generate is a price worth paying for the query to still Just Work™

Alternative 3: Patching generated SQL strings

Now, this is what we are actually doing.

Better assume almost all SQL transformations are local (as in the Substring example), and patch the SQL in case they’re not. In the end, we’re just generating SQL strings! And as such, why not introduce an infrastructure where we can put special markers to special text areas, and then replace that area with alternative SQL content.

Without the fix #11366, the generated code may have looked like this:

EXECUTE BLOCK AS
  -- Special marker here
BEGIN 
  -- Entering scope
  DECLARE i INT DEFAULT 1;
  WHILE (:i <= 10) DO BEGIN
    DECLARE j INT DEFAULT 1;
    WHILE (:j <= 10) DO BEGIN
      INSERT INTO t (a, b) VALUES (i, j);
      :j = :j + 1;
    END
    :i = :i + 1;
  END
  -- Exiting scope
END

This doesn’t work in Firebird, so we apply the fix. Note there’s a cheap, special marker that is placed by the SQL generation of anonymous blocks, but also for procedures, functions, and triggers, e.g.

CREATE FUNCTION func1()
RETURNS INTEGER
AS 
  -- Special marker here
BEGIN
  -- Entering scope
  RETURN 1;
  -- Exiting scope
END

Now, whenever the org.jooq.impl.DeclarationImpl query part generates its SQL locally, then, instead of generating something like:

DECLARE i INT DEFAULT 1;
DECLARE j INT;

We generate (locally)

:i = 1;
-- j isn't initialised here, so nothing needs to be done locally

At the same time, we push the org.jooq.impl.DeclarationImpl into some context variable that is visible to the entire scope (see “entering scope” and “exiting scope” comments).

As soon as we exit the scope, we must render all collected declarations, this time without the defaults, e.g.

DECLARE i INT;
DECLARE j INT;

… and then insert that rendered SQL right where the marker was located. All subsequent markers, if any, are then shifted by the difference of text lengths, of course.

Applications in jOOQ

This is currently used a few times within jOOQ:

  • To emulate calling Oracle PL/SQL functions with BOOLEAN parameters / return values. We patch the generated SQL producing a synthetic WITH clause with some BOOLEAN to NUMBER translation logic. Since Oracle 12c, Oracle supports PL/SQL embedded in WITH, which is quite a nifty feature!
  • The entire implicit JOIN feature is implemented this way! Markers delimit each table in the FROM clause (e.g. FROM BOOK), and if any path originating from any such marked table is encountered in the query (e.g. SELECT BOOK.AUTHOR.FIRST_NAME), then 1) instead of generating the path, a synthetic alias for the path is used to qualify the column, 2) instead of generating the FROM BOOK table, a synthetic LEFT JOIN or INNER JOIN is generated, joining all the necessary to-one relationships. An example will be shown below.
  • The above Firebird (and possibly also T-SQL, let’s see) procedural local variable scoping fixes are implemented this way.
  • A few emulations that require prepending SQL to the complete statement, such as CREATE OR REPLACE PROCEDURE x emulations that prepend DROP PROCEDURE x to CREATE PROCEDURE x work in a similar fashion. These types of emissions are “special” in that they add another statement to the statement batch. This means we’ll also have to be careful to skip any possible result sets or update counts this produces when calling the batch from JDBC.

Future applications may include:

  • More top level CTE, which are quite handy for a variety of emulations

An example for implicit joins:

SELECT
  book.author.first_name,
  book.author.last_name
FROM book -- Special marker around book

The above SQL doesn’t work in any dialect, it’s just jOOQ specific. We generate an alias for each unique path, based on the path’s hash code, so the query might look like this, instead:

SELECT
  -- The path isn't generated, but an alias for it is
  alias_xyz.first_name,
  alias_xyz.last_name
FROM (
  -- The marked "book" table is replaced by a join tree
  book 
    LEFT JOIN author AS alias_xyz 
    ON book.author_id = author.id
)

We simply replace book by (book LEFT JOIN ...) in the resulting SQL string. Thanks to our infrastructure of being able to define scopes and register context and variables for each scope, this works for arbitrary levels of nesting. We can always identify the appropriate book identifier for each path expression, even for things like this:

SELECT
  book.author.first_name,
  book.author.last_name,

  -- Different book tables here, because the nested scope 
  -- hides the "book" identifier from the outer scope
  (SELECT count(*) FROM book),
  (SELECT count(DISTINCT book.author.first_name) FROM book),

  -- Outer book, again
  (SELECT book.author.first_name)
FROM 
  book

The above being emulated like this, by patching two marked occurrences of book by the same join graph:

SELECT
  alias_xyz.first_name,
  alias_xyz.last_name,

  -- No patching done to this book
  (SELECT count(*) FROM book),

  -- The alias_xyz alias is used again, the path is the same
  (SELECT count(DISTINCT alias_xyz.first_name) 

  -- And the book table is patched again with the same left join
   FROM (
     book 
       LEFT JOIN author AS alias_xyz 
       ON book.author_id = author.id
  )),

  -- Outer book, again
  (SELECT alias_xyz.first_name)
FROM (
  book 
    LEFT JOIN author AS alias_xyz 
    ON book.author_id = author.id
)

It sounds as sophisticated as it sounds complex, but it works really really well.

Perhaps, in the future, expression tree transformation will be preferred over patching the result string. So far, with the current applications, this was the path of least resistance, and highest performance.