Having “constant” columns in foreign keys

I was asked a very interesting question on Twitter just now: https://twitter.com/connolly_s/status/1303957373107818497 Can we have "constant" foreign key columns in (PostgreSQL) tables? Luckily, yes, we can. Using a nice standard feature that is "computed columns" or "generated columns" Sometimes, you cannot completely normalise your schema for whatever reason. There may be a case where you … Continue reading Having “constant” columns in foreign keys

Using SQL Server FOR XML and FOR JSON Syntax on Other RDBMS With jOOQ

SQL Server supports transforming flat tabular SQL result sets into hierarchical structures by convention using the convenient FOR XML or FOR JSON syntaxes. This is really convenient and less verbose than the standard SQL/XML or SQL/JSON APIs - although the standard ones are more powerful. In this blog post, I'd like to show a few … Continue reading Using SQL Server FOR XML and FOR JSON Syntax on Other RDBMS With jOOQ

What’s Faster? COUNT(*) or COUNT(1)?

One of the biggest and undead myths in SQL is that COUNT(*) is faster than COUNT(1). Or was it that COUNT(1) is faster than COUNT(*)? Impossible to remember, because there's really no reason at all why one should be faster than the other. But is the myth justified? Let's measure! How does COUNT(...) work? But … Continue reading What’s Faster? COUNT(*) or COUNT(1)?

Oracle’s BINARY_DOUBLE Can Be Much Faster Than NUMBER

Using the right data type for some calculation sounds like some obvious advice. There are many blogs about using temporal data types for temporal data, instead of strings. An obvious reason is data integrity and correctness. We don't gain much in storing dates as 2019-09-10 in one record, and as Nov 10, 2019 in the … Continue reading Oracle’s BINARY_DOUBLE Can Be Much Faster Than NUMBER

How to Fetch All Current Identity Values in Oracle

Oracle 12c has introduced the useful SQL standard IDENTITY feature, which is essentially just syntax sugar for binding a sequence to a column default. We can use it like this: create table t1 (col1 number generated always as identity); create table t2 (col2 number generated always as identity); insert into t1 values (default); insert into … Continue reading How to Fetch All Current Identity Values in Oracle

Using IGNORE NULLS With SQL Window Functions to Fill Gaps

I found a very interesting SQL question on Twitter recently: https://twitter.com/vikkiarul/status/1120669222672261120 Rephrasing the question: We have a set of sparse data points: +------------+-------+ | VALUE_DATE | VALUE | +------------+-------+ | 2019-01-01 | 100 | | 2019-01-02 | 120 | | 2019-01-05 | 125 | | 2019-01-06 | 128 | | 2019-01-10 | 130 | +------------+-------+ … Continue reading Using IGNORE NULLS With SQL Window Functions to Fill Gaps

Calling an Oracle Function with PL/SQL BOOLEAN Type from SQL

One of the most wanted features in the Oracle database is the BOOLEAN type. The SQL standard specified it a while ago, and RDBMS like PostgreSQL show how powerful it can be, e.g. when using the EVERY() aggregate function. The PL/SQL language already has support for boolean types. We can write: CREATE OR REPLACE FUNCTION … Continue reading Calling an Oracle Function with PL/SQL BOOLEAN Type from SQL

Calculate Percentiles to Learn About Data Set Skew in SQL

B-Tree indexes are perfect when your data is uniformly distributed. They are not really useful, when you have skewed data. I'll explain later why this is the case, but let's first learn how to detect "skew" What is skew? Skew is a term from statistics when a normal distribution is not symmetric. The example given … Continue reading Calculate Percentiles to Learn About Data Set Skew in SQL

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

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 … Continue reading How to Use SQL UPDATE .. RETURNING to Run DML More Efficiently

Beware of Hidden PL/SQL to SQL Context Switches

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 … Continue reading Beware of Hidden PL/SQL to SQL Context Switches