COUNT(*) scalar subquery comparison
Applies to ✅ Open Source Edition ✅ Express Edition ✅ Professional Edition ✅ Enterprise Edition
When comparing a scalar subquery that calculates COUNT(*) with a single value, then chances are that weaker optimisers might be better off with an equivalent EXISTS predicate as can be seen in this blog post about COUNT(*) vs EXISTS.
This transformation is only applied under certain circumstances, including:
- In the absence of
UNIONand other set operations - In the absence of
GROUP BYandHAVING - Only with
COUNT(*), not withCOUNT(expr)(see COUNT(expr) scalar subquery comparison for that case)
Using Settings.transformPatternsScalarSubqueryCountAsteriskGtZero, the following transformations can be achieved:
-- With Settings.transformPatternsScalarSubqueryCountAsteriskGtZero active, this: SELECT (SELECT COUNT(*) FROM tab) > 0; -- ... is transformed into the equivalent expression: SELECT EXISTS (SELECT 1 FROM tab);
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