Condition Pushdown into Derived Table Optimization
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If a query uses a derived table, the first action that the query optimizer will attempt is to apply the derived-table-merge-optimization and merge the derived table into its parent select. However, that optimization is only applicable when the select inside the derived table has a join as the top-level operation. If it has a GROUP-BY, DISTINCT, or uses window functions, then derived-table-merge-optimization is not applicable.
In that case, Condition Pushdown is applicable.
Condition Pushdown
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