Comments - optimizer_switch
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What does condition_pushdown_for_derived do, andn am I right in assuming that condition_pushdown_for_derived only has an effect when derived_merge=on ?
Thanks
No. derived_merge controls merging of derived tables. If a derived table is merged, it optimized together with its parent SELECT. However in some cases the optimizer cannot merge the derived table. This happens when derived table's top level operation is not a join: it is a GROUP BY, ORDER BY ... LIMIT, just LIMIT n, or window function computation. In that case, the optimizer will attempt to push a part of parent select's WHERE into the derived table. This is controlled by condition_pushdown_for_derived flag. I see that condition pushdown into derived tables is not documented. I will document it.