materialized_from_subquery
materialized_from_subquery shows the optimizer materializing a subquery into a temporary table to improve efficiency
materialized_from_subquery shows the optimizer materializing a subquery into a temporary table (typically in memory) to improve efficiency. The first run of the subquery builds the temporary table (possibly with an additional hash index). Subsequently, rows are are read from the temporary table rather than having to re-execute the subquery, reducing IO.
In order to be able to materialize subqueries, the [materialization](https://dev.mysql.com/doc/refman/8.0/en/switchable-optimizations.html#optflag_materialization) optimizer switch system variable needs to be enabled.
Additional Links​
- MySQL Documentation - Optimizing Subqueries with Materialization
- MySQL Documentation - optimizer_switch materialization
- MySQL Documentation - Explain select_type
- MySQL Documentation - Explain Output
- MySQL Documentation - Explain
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