# Subquery Optimizations Map
Below is a map showing all types of subqueries allowed in the SQL language, and\
the optimizer strategies available to handle them.
* Uncolored areas represent different kinds of subqueries, for example:
* Subqueries that have form `x IN (SELECT ...)`
* Subqueries that are in the `FROM` clause
* .. and so forth
* The size of each uncolored area roughly corresponds to how important (i.e.\
frequently used) that kind of subquery is. For\
example, `x IN (SELECT ...)` queries are the most important,\
and `EXISTS (SELECT ...)` are relatively unimportant.
* Colored areas represent optimizations/execution strategies that are applied\
to handle various kinds of subqueries.
* The color of optimization indicates which version of MySQL/MariaDB it was\
available in (see legend below)
Some things are not on the map:
* MariaDB doesn't evaluate expensive subqueries when doing optimization\
(this means, EXPLAIN is always fast). MySQL 5.6 has made a progress in this regard, but its optimizer will still evaluate certain kinds of subqueries (for example, scalar-context subqueries used in range predicates)
## Links to pages about individual optimizations:
* [IN->EXISTS](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/non-semi-join-subquery-optimizations#the-in-to-exists-transformation)
* [Subquery Caching](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/subquery-optimizations/subquery-cache)
* [Semi-join optimizations](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/subquery-optimizations/semi-join-subquery-optimizations)
* [Table pullout](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/subquery-optimizations/table-pullout-optimization)
* [FirstMatch](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimization-strategies/firstmatch-strategy)
* [Materialization, +scan, +lookup](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimization-strategies/semi-join-materialization-strategy)
* [LooseScan](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimization-strategies/loosescan-strategy)
* [DuplicateWeedout execution strategy](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimization-strategies/duplicateweedout-strategy)
* Non-semi-join [Materialization](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/non-semi-join-subquery-optimizations#materialization-for-non-correlated-in-subqueries) (including NULL-aware and partial matching)
* Derived table optimizations
* [Derived table merge](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimizations-for-derived-tables/derived-table-merge-optimization)
* [Derived table with keys](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/query-optimizations/optimizations-for-derived-tables/derived-table-with-key-optimization)
## See also
* [Subquery optimizations in MariaDB 5.3](https://app.gitbook.com/s/aEnK0ZXmUbJzqQrTjFyb/community-server/old-releases/5.3/changes-improvements-in-mariadb-5-3#subquery-optimizations)
*This page is licensed: CC BY-SA / Gnu FDL*
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