The Ultimate Guide to High Availability with MariaDB
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Conversion of Big IN Predicates Into Subqueries

Starting from MariaDB 10.3, the optimizer converts 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 (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

Benefits of the Optimization

If column is a key-prefix, MariaDB optimizer will process the condition

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

by trying to construct a range access. If the list is large, the analysis may take a lot of memory and CPU time. The problem gets worse when column is a part of a multi-column index and the query has conditions on other parts of the index.

Conversion of IN predicates into a subqueries bypass the range analysis, which means the query optimization phase will use less CPU and memory.

Possible disadvantages of the conversion are are:

  • The optimization may convert 'IN LIST elements' key accesses to a table scan (if there is no other usable index for the table)
  • The estimates for the number of rows matching the IN (...) are less precise.

See Also

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

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