Conversion of Big IN Predicates Into Subqueries

Starting from MariaDB 10.3, the optimizer will convert certain big IN predicates into IN subqueries.

That is, an IN predicate in the form

column [NOT] IN (const1, const2, .... )

is converted into an equivalent IN-subquery:

column [NOT] IN (select ... from temporary_table)

which opens new opportunities for the query optimizer.

The conversion happens if the following conditions are met:

- the IN list has more than 1000 elements (The threshold number is hard-coded in the release builds, in debug builds, one can control it through the @@in_predicate_conversion_threshold parameter). - the [NOT] IN condition is at the top level of the WHERE/ON clause.

Controlling the Optimization

  • There is no optimizer_switch flag for this optimization.
  • Debug builds have @@in_predicate_conversion_threshold variable.

https://jira.mariadb.org/browse/MDEV-12176

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