Fast-forward to 2026 !

 I realize I had not published any blog post here since 2020, so an update is appropriate. In 2020, as COVID was hitting the world hard, I moved from Oracle to Huawei, still as remote worker based in France. At Huawei I worked on several different MySQL-based projects for six years (mainly Taurus and Hermes). And since mid-2026, I'm enjoying a pause from work. So I'm not anymore dealing every day with greedy search, semi-joins and anti-joins, derived tables, and what does the SQL:2023 Standard exactly mean at paragraph 243.27.1.a.ii !!!

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WITH RECURSIVE and MySQL

[2017 update: MySQL 8.0.1 now features SQL-standard CTE syntax; more information is here ; the entry below, from 2013, shows how to work around the absence of CTEs in older MySQL versions.] If you have been using certain DBMSs, or reading recent versions of the SQL standard, you are probably aware of the so-called "WITH clause" of SQL. Some call it Subquery Factoring . Others call it  Common Table Expression (CTE). In its simplest form, this feature is a kind of "boosted derived table". Assume that a table T1 has three columns: CREATE TABLE T1( YEAR INT, # 2000, 2001, 2002 ... MONTH INT, # January, February, ... SALES INT # how much we sold on that month of that year ); Now I want to know the sales trend (increase/decrease), year after year: SELECT D1.YEAR, (CASE WHEN D1.S>D2.S THEN 'INCREASE' ELSE 'DECREASE' END) AS TREND FROM (SELECT YEAR, SUM(SALES) AS S FROM T1 GROUP BY YEAR) AS D1, (SELECT YEAR, SUM(SALES) AS S FROM T1 GR...
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Faster subqueries with materialization

In a previous post , I analyzed how a query of the famous DBT3 benchmark was optimized by MySQL. It was this query, named "Q16" in the DBT3 jargon: select p_brand, p_type, p_size, count(distinct ps_suppkey) as supplier_cnt from partsupp, part where p_partkey = ps_partkey and p_brand <> 'Brand#23' and p_type not like 'LARGE PLATED%' and p_size in (43, 1, 25, 5, 35, 12, 42, 40) and ps_suppkey not in ( select s_suppkey from supplier where s_comment like '%Customer%Complaints%' ) group by p_brand, p_type, p_size order by supplier_cnt desc, p_brand, p_type, p_size; Here is a brief recap of conclusions I had drawn: for this query, MySQL tranforms the IN condition to EXISTS and then evaluates it with the "unique_subquery" technique, which does an index lookup into the subquery's table. IN is evaluated 120,000 times (once per combined row of the outer tables). The total execution time of query Q16...
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