Month: September 2018

How to Use SQL UPDATE .. RETURNING to Run DML More Efficiently

Posted on by lukaseder
At a customer site, I recently refactored a “slow-by-slow” PL/SQL loop and turned that into an efficient set based UPDATE statement saving many lines of code and running much faster. In this blog post, I will show how that can be done. The blog post will focus on Oracle and UPDATE, but rest assured, this technique can be implemented in other databases too, and also with other DML statements, such as INSERT, DELETE, and depending on the vendor, even MERGE.

The Schema

The original logic that needed refactoring worked on the following data set (simplified for this blog post): 

-- Table definition
CREATE TABLE t (
  id NUMBER(10) GENERATED ALWAYS AS IDENTITY NOT NULL PRIMARY KEY,
  category NUMBER(10) NOT NULL,
  counter NUMBER(10),
  text VARCHAR2(10) NOT NULL
);

-- Sample data
INSERT INTO t (category, text)
SELECT dbms_random.value(1, 10), dbms_random.string('a', 10)
FROM dual
CONNECT BY level <= 100;

-- Output of data
SELECT *
FROM t
ORDER BY counter DESC NULLS LAST, category, id;
The sample data generated above might look like this: 
ID   CATEGORY   COUNTER   TEXT
16   1                    UIXSzJxDez
25   1                    hkvvrTRbTC
29   1                    IBOJYveDgf
44   1                    VhcwOugrWB
46   1                    gBJFJrPQYy
47   1                    bVzfHznOUj
10   2                    KpHHgsRXwR
11   2                    vpkhTrkaaU
14   2                    fDlNtRdvBE
So, there were certain records belonging to some category, and there’s a counter indicating how often each record has been encountered in some system.

The “slow-by-slow” PL/SQL Logic

(“slow-by-slow” rhymes with “row-by-row”. You get the idea) Every now and then, there was a message from another system that should:
  • Fetch all the rows of a category
  • Increase the counter on each element of that category
  • Concatenate all the texts of that category and return those
Sounds like something that can be done very easily using a loop. In PL/SQL (but imagine you could be doing this in Java just the same): 

SET SERVEROUTPUT ON
DECLARE
  v_text VARCHAR2(2000);
  v_updated PLS_INTEGER := 0;
BEGIN
  FOR r IN (
    SELECT * FROM t WHERE category = 1
  ) LOOP
    v_updated := v_updated + 1;
    
    IF v_text IS NULL THEN
      v_text := r.text;
    ELSE
      v_text := v_text || ', ' || r.text;
    END IF;
    
    IF r.counter IS NULL THEN
      UPDATE t SET counter = 1 WHERE id = r.id;
    ELSE
      UPDATE t SET counter = counter + 1 WHERE id = r.id;
    END IF;
  END LOOP;
  
  COMMIT;
  dbms_output.put_line('Rows updated: ' || v_updated);
  dbms_output.put_line('Returned:     ' || v_text);
END;
/
The result of this block would be: 
Rows updated: 6
Returned:     UIXSzJxDez, hkvvrTRbTC, IBOJYveDgf, VhcwOugrWB, gBJFJrPQYy, bVzfHznOUj
And the data is now: 
ID   CATEGORY   COUNTER   TEXT
16   1          1         UIXSzJxDez
25   1          1         hkvvrTRbTC
29   1          1         IBOJYveDgf
44   1          1         VhcwOugrWB
46   1          1         gBJFJrPQYy
47   1          1         bVzfHznOUj
10   2                    KpHHgsRXwR
11   2                    vpkhTrkaaU
14   2                    fDlNtRdvBE
Wonderful. What’s wrong with this? The logic is straightforward and runs quite quickly. Until you run this many many many times per second – then it suddenly starts to hurt.

Thinking Set Based

Whenever you work with RDBMS, try to think in terms of data sets and try running a bulk operation on such a data set. (Exceptions exist, see caveats below). The modification of the data can be written in a single SQL statement, instead of updating the same table many times. Here’s the SQL statement in Oracle, that does precisely the same thing: 

SET SERVEROUTPUT ON
DECLARE
  v_text VARCHAR2(2000);
  v_updated PLS_INTEGER := 0;
BEGIN
  UPDATE t
  SET counter = nvl(counter, 0) + 1
  WHERE category = 1
  RETURNING
    listagg (text, ', ') WITHIN GROUP (ORDER BY text),
    count(*)
  INTO
    v_text,
    v_updated;
    
  COMMIT;
  dbms_output.put_line('Rows updated: ' || v_updated);
  dbms_output.put_line('Returned:     ' || v_text);
END;
/
Again, the same output: 
Rows updated: 6
Returned:     UIXSzJxDez, hkvvrTRbTC, IBOJYveDgf, VhcwOugrWB, gBJFJrPQYy, bVzfHznOUj
And the data set is now: 
ID   CATEGORY   COUNTER   TEXT
16   1          2         UIXSzJxDez
25   1          2         hkvvrTRbTC
29   1          2         IBOJYveDgf
44   1          2         VhcwOugrWB
46   1          2         gBJFJrPQYy
47   1          2         bVzfHznOUj
10   2                    KpHHgsRXwR
11   2                    vpkhTrkaaU
14   2                    fDlNtRdvBE
Below, you can see each piece of logic of the original PL/SQL block, and the corresponding logic in the revised SQL statement There are 4 areas of interest:
  1. Red: The category predicate
    In the PL/SQL version, this predicate is a simple access predicate for the SELECT statement, over whose implicit cursor we’re iterating. In the set based SQL version, that predicate has been moved into the single bulk UPDATE statement. Thus: we’re modifying the exact same set of rows.
  2. Blue: The number of updated rows
    Before, we had a count variable that counted the number of iterations over the implicit cursor. Now, we can simply count the number of rows being updated in the bulk update statement, conveniently in the RETURNING clause. An alternative (in Oracle) would have been to use SQL%ROWCOUNT, which is available for free after a single bulk UPDATE statement.
  3. Orange: The string concatenation
    The requirement was to concatenate all the texts which are being updated. In the “slow-by-slow” PL/SQL approach, we’re again keeping around a local variable and concatenate new values to it, doing some NULL handling, initially. In the set based SQL version, we can simply use LISTAGG() in the RETURNING clause. Notice, there seems to be a bug with this usage of LISTAGG. The ORDER BY clause has no effect.
  4. Green: The actual update
    In the “slow-by-slow” version, we run 1 UPDATE statement per row, which can turn out to be devastating, if we’re updating a lot of rows. Besides, in this particular case, the developer(s) have been unaware of the possibility of NULL handling using NVL() (or COALESCE() or similar). There is really only one UPDATE statement necessary here.
That already looks a lot neater.

How does it perform?

In a quick test script, which I’ve linked here, I could observe the following times for the above test data set, when running each approach 5 x 10000 times: 
Run 1, Statement 1 : 2.63841 (avg : 2.43714)
Run 1, Statement 2 : 1.11019 (avg : 1.04562)
Run 2, Statement 1 : 2.35626 (avg : 2.43714)
Run 2, Statement 2 : 1.05716 (avg : 1.04562)
Run 3, Statement 1 : 2.38004 (avg : 2.43714)
Run 3, Statement 2 : 1.05153 (avg : 1.04562)
Run 4, Statement 1 : 2.47451 (avg : 2.43714)
Run 4, Statement 2 : 1.00921 (avg : 1.04562)
Run 5, Statement 1 : 2.33649 (avg : 2.43714)
Run 5, Statement 2 : 1.00000 (avg : 1.04562)
As always, I’m not publishing actual benchmark times, but relative times compared to the fastest run. The set based approach is consistently 2.5x faster on my machine (Oracle 18c on Docker on Windows 10 / SSD). This is updating 6 rows per execution. When we remove the WHERE category = 1 predicate, updating the entirety of the 100 rows each time, we get even more drastic results. I’m now running this 5 x 2000 times to get: 
Run 1, Statement 1 : 10.21833 (avg : 11.98154)
Run 1, Statement 2 : 1.219130 (avg : 1.739260)
Run 2, Statement 1 : 10.17014 (avg : 11.98154)
Run 2, Statement 2 : 3.027930 (avg : 1.739260)
Run 3, Statement 1 : 9.444620 (avg : 11.98154)
Run 3, Statement 2 : 1.000000 (avg : 1.739260)
Run 4, Statement 1 : 20.54692 (avg : 11.98154)
Run 4, Statement 2 : 1.193560 (avg : 1.739260)
Run 5, Statement 1 : 9.527690 (avg : 11.98154)
Run 5, Statement 2 : 2.255680 (avg : 1.739260)
At this point, no one needs to be convinced anymore that a set based approach is much better for updating your data than a row-by-row approach in a language like PL/SQL or Java, etc.

Caveats

Bulk updates are much better than row-by-row (remember: “slow-by-slow”) updates, regardless if you’re using PL/SQL or Java or whatever client language. This is because the optimiser can plan the update much more efficiently when it knows which rows will be updated in bulk, rather than seeing each individual row update afresh, not being able to plan ahead for the remaining number of updates. However, in situations where a lot of other processes are reading the same data while you’re bulk updating them, you need to be more careful. In such cases, a bulk update can cause trouble keeping locks and log files busy while you’re updating and while the other processes need to access the data prior to your update. One size never fits all, but at least, in every situation where you loop over a result set to update some data (or fetch additional data), ask yourself: Could I have written that logic in a single SQL statement? The answer is very often: Yes.

Other databases

A few other databases support similar language features. These include: The DB2 syntax is quite noteworthy, because:
  • It is very elegant
  • It corresponds to the SQL standard
The UPDATE statement would have been nested in a SELECT statement: 

SELECT 
  listagg (text, ', ') WITHIN GROUP (ORDER BY id),
  count(*)
FROM FINAL TABLE (
  UPDATE t
  SET counter = nvl(counter, 0) + 1
  WHERE category = 1
)

How to Write a Multiplication Aggregate Function in SQL

Posted on by lukaseder
Everyone knows the SQL SUM() aggregate function (and many people also know its window function variant). When querying the Sakila database, we can get the daily revenue (using PostgreSQL syntax): 

WITH p AS (
  SELECT
    CAST (payment_date AS DATE) AS date,
    amount
  FROM payment
)
SELECT
  date,
  SUM (amount) AS daily_revenue,
  SUM (SUM (amount)) OVER (ORDER BY date) AS cumulative_revenue
FROM p
GROUP BY date
ORDER BY date
The result will look something like this: 
date       |daily_revenue |cumulative_revenue 
-----------|--------------|-------------------
2005-05-24 |29.92         |29.92              
2005-05-25 |573.63        |603.55             
2005-05-26 |754.26        |1357.81            
2005-05-27 |685.33        |2043.14            
2005-05-28 |804.04        |2847.18            
2005-05-29 |648.46        |3495.64            
2005-05-30 |628.42        |4124.06            
2005-05-31 |700.37        |4824.43            
2005-06-14 |57.84         |4882.27            
...

Doing the same with multiplication

This is already quite useful. Very occasionally, however, we do not need to aggregate multiple values in a sum (through addition), but in a product (through multiplication). I’ve just stumbled upon such a case on Stack Overflow, recently. The question wanted to achieve the following result: 
date        factor          accumulated
---------------------------------------
1986-01-10  null            1000
1986-01-13  -0.026595745    973.4042548
1986-01-14  0.005464481     978.7234036
1986-01-15  -0.016304348    962.7659569
1986-01-16  0               962.7659569
1986-01-17  0               962.7659569
1986-01-20  0               962.7659569
1986-01-21  0.005524862     968.0851061
1986-01-22  -0.005494506    962.765957
1986-01-23  0               962.765957
1986-01-24  -0.005524862    957.4468078
1986-01-27  0.005555556     962.7659569
1986-01-28  0               962.7659569
1986-01-29  0               962.7659569
1986-01-30  0               962.7659569
1986-01-31  0.027624309     989.3617013
1986-02-03  0.016129032     1005.319148
1986-02-04  0.042328041     1047.872338
1986-02-05  0.04568528      1095.744679
If this were a Microsoft Excel spreadsheet, the ACCUMULATED column would simply start with 1000 and have the following formula in all other rows: 
accumulated(i) = accumulated(i - 1) * (1 + factor)
In other words (values truncated for simplicity): 
1000.0 = start
 973.4 = 1000.0 * (1 - 0.026)
 978.7 =  973.4 * (1 + 0.005)
 962.7 =  978.7 * (1 - 0.016)
 962.7 =  962.7 * (1 - 0.000)
 962.7 =  962.7 * (1 - 0.000)
 962.7 =  962.7 * (1 - 0.000)
 968.0 =  962.7 * (1 + 0.005)
 ...
This is exciting because we’re not only requiring multiplicative aggregation, but even cumulative multiplicative aggregation. So, another window function. But regrettably, SQL doesn’t offer a MUL() aggregate function, even if it were relatively simple to implement. We have two options:
  • Implementing a custom aggregate function (stay tuned for a future blog post)
  • Using a trick by summing logarithms, rather than multiplying operands directly
We’re implementing the latter for now. Check out this cool Wikipedia website about logarithmic identities, which we are going to blindly trust. In the middle of it, we have: 
bx * by = bx + y
Which leads to: 
logb(x * y) = logb(x) + logb(y)
How cool is that? And thus: 
x * y = blogb(x) + logb(y)
So, we can define any multiplication in terms of a bunch of exponentiation to some base (say e) and logarithms to some base (say e). Or, in SQL: 
x * y = EXP(LN(x) + LN(y))
Or, as an aggregate function: 
MUL(x) = EXP(SUM(LN(x)))
Heh! Our original problem can thus be solved very easily using this, as shown in my stack overflow answer: 

SELECT
  date,
  factor,
  1000 * (EXP(SUM(LN(1 + COALESCE(factor, 1)))
       OVER (ORDER BY date)) - 1) AS accumulated
FROM t
And we get the nice result as previously shown. You may have to replace LN() by LOG() depending on your database.

Caveat: Negative numbers

Try running this: 

SELECT LN(-1)
You’ll get: 
SQL Error [2201E]: ERROR: cannot take logarithm of a negative number
Logarithms are defined only for strictly positive numbers, unless your database is capable of handling complex numbers as well. In case of which a single zero value would still break the aggregation. But if your data set is defined to contain only strictly positive numbers, you’ll be fine – give or take some floating point rounding errors. Or, you’ll do some sign handling, which looks like this: 

WITH v(i) AS (VALUES (-2), (-3), (-4))
SELECT 
  CASE 
    WHEN SUM (CASE WHEN i < 0 THEN -1 END) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(LN(ABS(i)))) multiplication1
FROM v;

WITH v(i) AS (VALUES (-2), (-3), (-4), (-5))
SELECT 
  CASE 
    WHEN SUM (CASE WHEN i < 0 THEN -1 END) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(LN(ABS(i)))) multiplication2
FROM v;
The above yielding 
multiplication1      
--------------------
-23.999999999999993 


multiplication2     
-------------------
119.99999999999997
Close enough.

Caveat: Zero

Try running this: 

SELECT LN(0)
You’ll get: 
SQL Error [2201E]: ERROR: cannot take logarithm of zero
Zero is different from negative numbers. A product that has a zero operand is always zero, so we should be able to handle this. We’ll do it in two steps:
  • Exclude zero values from the actual aggregation that uses EXP() and LN()
  • Add an additional CASE expression that checks if any of the operands is zero
The first step might not be necessary depending on how your database optimiser executes the second step. 

WITH v(i) AS (VALUES (2), (3), (0))
SELECT 
  CASE 
    WHEN SUM (CASE WHEN i = 0 THEN 1 END) > 0
    THEN 0
    WHEN SUM (CASE WHEN i < 0 THEN -1 END) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(LN(ABS(NULLIF(i, 0))))) multiplication
FROM v;

Extension: DISTINCT

Calculating the product of all DISTINCT values requires to repeat the DISTINCT keyword in 2 out of the above 3 sums: 

WITH v(i) AS (VALUES (2), (3), (3))
SELECT 
  CASE 
    WHEN SUM (CASE WHEN i = 0 THEN 1 END) > 0
    THEN 0
    WHEN SUM (DISTINCT CASE WHEN i < 0 THEN -1 END) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(DISTINCT LN(ABS(NULLIF(i, 0))))) multiplication
FROM v;
The result is now: 
multiplication |
---------------|
6              |
Notice that the first SUM() that checks for the presence of NULL values doesn’t require a DISTINCT keyword, so we omit it to improve performance.

Extension: Window functions

Of course, if we are able to emulate a PRODUCT() aggregate function, we’d love to turn it into a window function as well. This can be done simply by transforming each individual SUM() into a window function: 

WITH v(i, j) AS (
  VALUES (1, 2), (2, -3), (3, 4), 
         (4, -5), (5, 0), (6, 0)
)
SELECT i, j, 
  CASE 
    WHEN SUM (CASE WHEN j = 0 THEN 1 END) 
      OVER (ORDER BY i) > 0
    THEN 0
    WHEN SUM (CASE WHEN j < 0 THEN -1 END) 
      OVER (ORDER BY i) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(LN(ABS(NULLIF(j, 0)))) 
    OVER (ORDER BY i)) multiplication
FROM v;
The result is now: 
i |j  |multiplication      |
--|---|--------------------|
1 | 2 |2                   |
2 |-3 |-6                  |
3 | 4 |-23.999999999999993 |
4 |-5 |119.99999999999997  |
5 | 0 |0                   |
6 | 1 |0                   |
So cool! The cumulative product gets bigger and bigger until it hits he first zero, from then on it stays zero.

jOOQ support

jOOQ 3.12 will support this as well and emulate it correctly on all databases: https://github.com/jOOQ/jOOQ/issues/5939

A note on Oracle performance

Do note that Oracle is very slow to calculate LN(number_type). It can be MUCH faster to calculate, instead LN(binary_double_type). An explicit type cast produced a 100x performance improvement in a simple test, documented here.

How to Reduce Syntactic Overhead Using the SQL WINDOW Clause

Posted on by lukaseder
SQL is a verbose language, and one of the most verbose features are window functions. In a stack overflow question that I’ve encountered recently, someone asked to calculate the difference between the first and the last value in a time series for any given day: Input 
volume  tstamp
---------------------------
29011   2012-12-28 09:00:00
28701   2012-12-28 10:00:00
28830   2012-12-28 11:00:00
28353   2012-12-28 12:00:00
28642   2012-12-28 13:00:00
28583   2012-12-28 14:00:00
28800   2012-12-29 09:00:00
28751   2012-12-29 10:00:00
28670   2012-12-29 11:00:00
28621   2012-12-29 12:00:00
28599   2012-12-29 13:00:00
28278   2012-12-29 14:00:00
Desired output 
first  last   difference  date
------------------------------------
29011  28583  428         2012-12-28
28800  28278  522         2012-12-29

How to write the query?

Notice that the value and timestamp progression do not correlate as it may appear. So, there is not a rule that if Timestamp2 > Timestamp1 then Value2 < Value1. Otherwise, this simple query would work (using PostgreSQL syntax): 

SELECT 
  max(volume)               AS first,
  min(volume)               AS last,
  max(volume) - min(volume) AS difference,
  CAST(tstamp AS DATE)      AS date
FROM t
GROUP BY CAST(tstamp AS DATE);
There are several ways to find the first and last values within a group that do not involve window functions. For example:
  • In Oracle, you can use the FIRST and LAST functions, which for some arcane reason are not written FIRST(...) WITHIN GROUP (ORDER BY ...) or LAST(...) WITHIN GROUP (ORDER BY ...), like other sorted set aggregate functions, but some_aggregate_function(...) KEEP (DENSE_RANK FIRST ORDER BY ...). Go figure
  • In PostgreSQL, you could use the DISTINCT ON syntax along with ORDER BY and LIMIT
More details about the various approaches can be found here: https://blog.jooq.org/2017/09/22/how-to-write-efficient-top-n-queries-in-sql The best performing approach would be to use an aggregate function like Oracle’s, but few databases have this function. So, we’ll resort to using the FIRST_VALUE and LAST_VALUE window functions: 

SELECT DISTINCT
  first_value(volume) OVER (
    PARTITION BY CAST(tstamp AS DATE) 
    ORDER BY tstamp
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) AS first,
  last_value(volume) OVER (
    PARTITION BY CAST(tstamp AS DATE) 
    ORDER BY tstamp
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) AS last,
  first_value(volume) OVER (
    PARTITION BY CAST(tstamp AS DATE) 
    ORDER BY tstamp
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) 
  - last_value(volume) OVER (
    PARTITION BY CAST(tstamp AS DATE) 
    ORDER BY tstamp
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) AS diff,
  CAST(tstamp AS DATE) AS date
FROM t
ORDER BY CAST(tstamp AS DATE)
Oops 🤔 That doesn’t look too readable. But it will yield the correct result. Granted, we could wrap the definition for the columns FIRST and LAST in a derived table, but that would still leave us with two repetitions of the window definition: 
PARTITION BY CAST(tstamp AS DATE) 
ORDER BY tstamp
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING

WINDOW clause to the rescue

Luckily, at least 3 databases have implemented the SQL standard WINDOW clause:
  • MySQL
  • PostgreSQL
  • Sybase SQL Anywhere
The above query can be refactored to this one: 

SELECT DISTINCT
  first_value(volume) OVER w AS first,
  last_value(volume) OVER w AS last,
  first_value(volume) OVER w 
    - last_value(volume) OVER w AS diff,
  CAST(tstamp AS DATE) AS date
FROM t
WINDOW w AS (
  PARTITION BY CAST(tstamp AS DATE) 
  ORDER BY tstamp
  ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
)
ORDER BY CAST(tstamp AS DATE)
Notice how I can specify a window name with a window specification in a similar way as I can define a common table expression (WITH clause): 
WINDOW 
    <window-name> AS (<window-specification>)
{  ,<window-name> AS (<window-specification>)... }
Not only can I reuse entire specifications, I could also build a specification from a partial specification, and reuse only parts. My previous query could have been rewritten as such: 

SELECT DISTINCT
  first_value(volume) OVER w3 AS first,
  last_value(volume) OVER w3 AS last,
  first_value(volume) OVER w3 
    - last_value(volume) OVER w3 AS diff,
  CAST(tstamp AS DATE) AS date
FROM t
WINDOW 
  w1 AS (PARTITION BY CAST(tstamp AS DATE)),
  w2 AS (w1 ORDER BY tstamp),
  w3 AS (w2 ROWS BETWEEN UNBOUNDED PRECEDING 
                     AND UNBOUNDED FOLLOWING)
ORDER BY CAST(tstamp AS DATE)
Each window specification can be created from scratch, or be based on a previously defined window specification. Note this is also true when referencing the window definition. If I wanted to reuse the PARTITION BY clause and the ORDER BY clause, but change the FRAME clause (ROWS ...), then I could have written this: 

SELECT DISTINCT
  first_value(volume) OVER (
    w2 ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
  ) AS first,
  last_value(volume) OVER (
    w2 ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
  ) AS last,
  first_value(volume) OVER (
    w2 ROWS UNBOUNDED PRECEDING
  ) - last_value(volume) OVER (
    w2 ROWS BETWEEN 1 PRECEDING AND UNBOUNDED FOLLOWING
  ) AS diff,
  CAST(tstamp AS DATE) AS date
FROM t
WINDOW 
  w1 AS (PARTITION BY CAST(tstamp AS DATE)),
  w2 AS (w1 ORDER BY tstamp)
ORDER BY CAST(tstamp AS DATE)

What if my database doesn’t support the WINDOW clause?

In that case, you have to either manually write the window specification on each window function, or you use a SQL builder like jOOQ, which can emulate the window clause:
You can try this translation online on our website: https://www.jooq.org/translate

Beware of Hidden PL/SQL to SQL Context Switches

Posted on by lukaseder
I recently stumbled upon a curious query on a customer’s productive Oracle database: 

SELECT USER FROM SYS.DUAL
Two things caught my attention:
  • The query was executed many billions of times per month, accounting for about 0.3% of that system’s load. That’s 0.3% for something extremely silly!
  • I don’t think that customer would ever qualify the DUAL table as SYS.DUAL, which hints at some system functionality
I found it in Oracle Enterprise Manager, but you could also find it using a query like this one: 

SELECT 
  sql_id, 
  executions, 
  elapsed_time, 
  ratio_to_report(elapsed_time) over() p, 
  sql_text
FROM v$sql
ORDER BY p DESC;
Why was this query being run so often? In Enterprise Manager, the query’s statistics overview displayed that the query originated from a function called STANDARD.USER (I don’t know yet where I could find this information in the dictionary views, manually). Naively, I had always thought that the USER pseudo column or pseudo constant is some value from the context, but like many other functions, it’s really just a function in that package.

What does STANDARD.USER() do?

Now, I’m not 100% sure if that source code is something that I am allowed to reproduce from a legal perspective, this being Oracle and all. But if you run this query here, which I am freely allowing you to: 

WITH s AS (
  SELECT s.*,
    MIN(CASE 
      WHEN upper(text) LIKE '%FUNCTION USER%' 
      THEN line END
    ) OVER () s
  FROM all_source s
  WHERE owner = 'SYS' 
  AND name = 'STANDARD'
  AND type = 'PACKAGE BODY'
)
SELECT text
FROM s
WHERE line >= s AND line < s + 6;
Then you might be able to see something like this: 
  function USER return varchar2 is
  c varchar2(255);
  begin
        select user into c from sys.dual;
        return c;
  end;
This is just the result of some SQL query I’ve shown you. Any correspondence with actual source code is merely coincidental. Let’s assume this were the actual source code of the STANDARD.USER() function. We can now clearly see that this very SQL query that I’ve observed before is being executed! Want to verify this?

Let’s benchmark

As always, I’m using the benchmark technique described here. The full benchmark logic is at the end of the article. In essence, I’m comparing the performances of 500000 executions of this loop: 

FOR i IN 1 .. v_repeat LOOP
  v := USER;
END LOOP;
With this one: 

FOR i IN 1 .. v_repeat LOOP
  SELECT USER INTO v FROM dual;
END LOOP;
And this one: 

FOR i IN 1 .. v_repeat LOOP
  -- Note: According to the doc, use 'SESSION_USER' instead
  v := sys_context('USERENV', 'CURRENT_USER');
END LOOP;
The result of this benchmark is: 
Run 1, Statement 1 : 2.40509 (avg : 2.43158)
Run 1, Statement 2 : 2.13208 (avg : 2.11816)
Run 1, Statement 3 : 1.01452 (avg : 1.02081)

Run 2, Statement 1 : 2.41889 (avg : 2.43158)
Run 2, Statement 2 : 2.09753 (avg : 2.11816)
Run 2, Statement 3 : 1.00203 (avg : 1.02081)

Run 3, Statement 1 : 2.45384 (avg : 2.43158)
Run 3, Statement 2 : 2.09060 (avg : 2.11816)
Run 3, Statement 3 : 1.02239 (avg : 1.02081)

Run 4, Statement 1 : 2.39516 (avg : 2.43158)
Run 4, Statement 2 : 2.14140 (avg : 2.11816)
Run 4, Statement 3 : 1.06512 (avg : 1.02081)

Run 5, Statement 1 : 2.48493 (avg : 2.43158)
Run 5, Statement 2 : 2.12922 (avg : 2.11816)
Run 5, Statement 3 : 1.00000 (avg : 1.02081)
How to read this benchmark result? These aren’t actual times, which are not interesting, but relative times compared to the fastest run (run 5, statement 3 = 1). The explicit SELECT USER FROM DUAL is about half as fast as the SYS_CONTEXT call, and the USER call is a bit slower, even. When re-running this query: 

SELECT 
  sql_id, 
  executions, 
  ratio_to_report(elapsed_time) over() p, 
  sql_text
FROM v$sql
ORDER BY p DESC;
We can see: 
SQL_ID          EXECUTIONS  P     SQL_TEXT
6r9s58qfu339c   1           0.26  DECLARE ...
1v717nvrhgbn9   2500000     0.14  SELECT USER FROM SYS.DUAL
...
So, this query has definitely been run way too many times, including the PL/SQL to SQL context switch that is involved. I’m running this benchmark in Oracle 18.0.0.0.0 in Docker on a Windows machine. More close-to-the-metal and less virtualised setups might achieve more drastic results. See, e.g. Connor McDonald got a much better improvement from using SYS_CONTEXT:
In this particular case, The STANDARD.USER() reference was used very often in triggers to fill in audit columns of many tables. Very easy to fix. Just use sys_context('USERENV', 'CURRENT_USER') instead.

Full benchmark logic


SET SERVEROUTPUT ON

ALTER SYSTEM FLUSH SHARED_POOL;
ALTER SYSTEM FLUSH BUFFER_CACHE;

CREATE TABLE results (
  run     NUMBER(2),
  stmt    NUMBER(2),
  elapsed NUMBER
);

DECLARE
  v_ts TIMESTAMP WITH TIME ZONE;
  v_repeat CONSTANT NUMBER := 500000;
  v NUMBER;
BEGIN

  -- Repeat the whole benchmark several times to 
  -- avoid warmup penalty
  FOR r IN 1..5 LOOP
    v_ts := SYSTIMESTAMP;
      
    FOR i IN 1 .. v_repeat LOOP
      v := v + length(USER);
    END LOOP;
  
    INSERT INTO results VALUES (r, 1, 
      SYSDATE + ((SYSTIMESTAMP - v_ts) * 86400) - SYSDATE);
    v_ts := SYSTIMESTAMP;
      
    FOR i IN 1 .. v_repeat LOOP
      SELECT v + length(USER) INTO v FROM dual;
    END LOOP;
      
    INSERT INTO results VALUES (r, 2, 
      SYSDATE + ((SYSTIMESTAMP - v_ts) * 86400) - SYSDATE);
    v_ts := SYSTIMESTAMP;
      
    FOR i IN 1 .. v_repeat LOOP
      -- Note: According to the doc, use 'SESSION_USER' instead
      v := v + length(sys_context('USERENV', 'CURRENT_USER'));
    END LOOP;
      
    INSERT INTO results VALUES (r, 3, 
      SYSDATE + ((SYSTIMESTAMP - v_ts) * 86400) - SYSDATE);
  END LOOP;
  
  FOR rec IN (
    SELECT 
      run, stmt, 
      CAST(elapsed / MIN(elapsed) OVER() AS NUMBER(10, 5)) ratio,
      CAST(AVG(elapsed) OVER (PARTITION BY stmt) / 
           MIN(elapsed) OVER() AS NUMBER(10, 5)) avg_ratio
    FROM results
    ORDER BY run, stmt
  )
  LOOP
    dbms_output.put_line('Run ' || rec.run || 
      ', Statement ' || rec.stmt || 
      ' : ' || rec.ratio || ' (avg : ' || rec.avg_ratio || ')');
  END LOOP;
  
  dbms_output.put_line('');
  dbms_output.put_line('Copyright Data Geekery GmbH');
  dbms_output.put_line('https://www.jooq.org/benchmark');
END;
/

DROP TABLE results;

Find the Next Non-NULL Row in a Series With SQL

Posted on by lukaseder
I’ve stumbled across this fun SQL question on reddit, recently. The question was looking at a time series of data points where some events happened. For each event, we have the start time and the end time 
timestamp             start    end
-----------------------------------
2018-09-03 07:00:00   1        null
2018-09-03 08:00:00   null     null
2018-09-03 09:00:00   null     null
2018-09-03 10:00:00   null     1
2018-09-03 12:00:00   null     null
2018-09-03 13:00:00   null     null
2018-09-03 14:00:00   1        null
2018-09-03 15:00:00   null     1
The desired output of the query should be this additional count column: 
timestamp             start    end    count
-------------------------------------------
2018-09-03 07:00:00   1        null   4
2018-09-03 08:00:00   null     null   null
2018-09-03 09:00:00   null     null   null
2018-09-03 10:00:00   null     1      null
2018-09-03 12:00:00   null     null   null
2018-09-03 13:00:00   null     null   null
2018-09-03 14:00:00   1        null   2
2018-09-03 15:00:00   null     1      null
So, the rule is simple. Whenever an event starts, we would like to know how many consecutive entries it takes until the event stops again. We can visually see how that makes sense: 
timestamp             start    end    count
-------------------------------------------
2018-09-03 07:00:00   1        null   4     -- 4 Rows in this event
2018-09-03 08:00:00   null     null   null
2018-09-03 09:00:00   null     null   null
2018-09-03 10:00:00   null     1      null

2018-09-03 12:00:00   null     null   null  -- No event here
2018-09-03 13:00:00   null     null   null

2018-09-03 14:00:00   1        null   2     -- 2 Rows in this event
2018-09-03 15:00:00   null     1      null
Some observations and assumptions about the problem at hand:
  • No two events will ever overlap
  • The time series does not progress monotonously, i.e. even if most data points are 1h apart, there can be larger or smaller gaps between data points
  • There are, however, no two identical timestamps in the series
How can we solve this problem?

Create the data set, first

We’re going to be using PostgreSQL for this example, but it will work with any database that supports window functions, which are most databases these days. In PostgreSQL, we can use the VALUES() clause to generate data in memory easily. For the sake of simplicity, we’re not going to use timestamps, but integer representations of the timestamps. I’ve included the same out-of-the-ordinary gap between 4 and 6: 

values (1, 1, null),
       (2, null, null),
       (3, null, null),
       (4, null, 1),
       (6, null, null),
       (7, null, null),
       (8, 1, null),
       (9, null, 1)
If we run this statement (yes, this is a standalone statement in PostgreSQL!), then the database will simply echo back the values we’ve sent it: 
column1 |column2 |column3 |
--------|--------|--------|
1       |1       |        |
2       |        |        |
3       |        |        |
4       |        |1       |
6       |        |        |
7       |        |        |
8       |1       |        |
9       |        |1       |

How to deal with non-monotonously growing series

The fact that column1 is not growing monotonously means that we cannot use it / trust it as a means to calculate the length of an event. We need to calculate an additional column that has a guaranteed monotonously growing set of integers in it. The ROW_NUMBER() window function is perfect for that. Consider this SQL statement: 

with 
  d(a, b, c) as (
	values (1, 1, null),
	       (2, null, null),
	       (3, null, null),
	       (4, null, 1),
	       (6, null, null),
	       (7, null, null),
	       (8, 1, null),
	       (9, null, 1)
  ),
  t as (
    select 
      row_number() over (order by a) as rn, a, b, c
    from d
  )
select * from t;
The new rn column is a row number calculated for each row based on the ordering of a. For simplicity, I’ve aliased:
  • a = timestamp
  • b = start
  • c = end
The result of this query is: 
rn |a |b |c |
---|--|--|--|
1  |1 |1 |  |
2  |2 |  |  |
3  |3 |  |  |
4  |4 |  |1 |
5  |6 |  |  |
6  |7 |  |  |
7  |8 |1 |  |
8  |9 |  |1 |
Nothing fancy yet.

Now, how to use this rn column to find the length of an event?

Visually, we can get the idea quickly, seeing that an event’s length can be calculated using the formula RN2 - RN1 + 1: 
rn |a |b |c |
---|--|--|--|
1  |1 |1 |  | RN1 = 1
2  |2 |  |  |
3  |3 |  |  |
4  |4 |  |1 | RN2 = 4

5  |6 |  |  |
6  |7 |  |  |

7  |8 |1 |  | RN1 = 7
8  |9 |  |1 | RN2 = 8
We have two events:
  • 4 – 1 + 1 = 4
  • 8 – 7 + 1 = 2
So, all we have to do is for each starting point of an event at RN1, find the corresponding RN2, and run the arithmetic. This is quite a bit of syntax, but it isn’t so hard, so bear with me while I explain: 

with 
  d(a, b, c) as (
	values (1, 1, null),
	       (2, null, null),
	       (3, null, null),
	       (4, null, 1),
	       (6, null, null),
	       (7, null, null),
	       (8, 1, null),
	       (9, null, 1)
  ),
  t as (
    select 
      row_number() over (order by a) as rn, a, b, c
    from d
  )

-- Interesting bit here:
select
  a, b, c,
  case 
    when b is not null then 
      min(case when c is not null then rn end) 
        over (order by rn 
          rows between 1 following and unbounded following) 
      - rn + 1 
  end cnt
from t;
Let’s look at this new cnt column, step by step. First, the easy part: The CASE expression There’s a case expression that goes like this: 
case 
  when b is not null then 
    ...
end cnt
All this does is check if b is not null and if this is true, then calculate something. Remember, b = start, so we’re putting a calculated value in the row where an event started. That was the requirement. The new window function So, what do we calculate there? 
min(...) over (...) ...
A window function that finds the minimum value over a window of data. That minimum value is RN2, the next row number value where the event ends. So, what do we put in the min() function to get that value? 
min(case when c is not null then rn end) 
over (...) 
...
Another case expression. When c is not null, we know the event has ended (remember, c = end). And if the event has ended, we want to find that row’s rn value. So that would be the minimum value of that case expression for all the rows after the row that started the event. Visually: 
rn |a |b |c | case expr | minimum "next" value
---|--|--|--|-----------|---------------------
1  |1 |1 |  | null      | 4
2  |2 |  |  | null      | null
3  |3 |  |  | null      | null
4  |4 |  |1 | 4         | null

5  |6 |  |  | null      | null
6  |7 |  |  | null      | null

7  |8 |1 |  | null      | 8
8  |9 |  |1 | 8         | null
Now, we only need to specify that OVER() clause to form a window of all rows that follow the current row. 
min(case when c is not null then rn end) 
  over (order by rn 
    rows between 1 following and unbounded following) 
...
The window is ordered by rn and it starts 1 row after the current row (1 following) and ends in infinity (unbounded following). The only thing left to do now is do the arithmetic: 
min(case when c is not null then rn end) 
  over (order by rn 
    rows between 1 following and unbounded following) 
- rn + 1
This is a verbose way of calculating RN2 - RN1 + 1, and we’re doing that only in those columns that start an event. The result of the complete query above is now: 
a |b |c |cnt |
--|--|--|----|
1 |1 |  |4   |
2 |  |  |    |
3 |  |  |    |
4 |  |1 |    |
6 |  |  |    |
7 |  |  |    |
8 |1 |  |2   |
9 |  |1 |    |
Read more about window functions on this blog.