MySQL attempts to enforce the
FIRSTMATCHstrategy and ignores all other semijoin strategies specified explicitly as subquery optimizer hints. However, MySQL may still select the
DUPSWEEDOUTsemijoin strategy during
JOINorder optimization, even if an equivalent plan could be offered using the
FIRSTMATCHstrategy. A plan that uses the
DUPSWEEDOUTsemijoin strategy would produce incorrect results if executed on HeatWave.
For general information about subquery optimizer hints, see Subquery Optimizer Hints.