2.18.4 Index Hint and Optimizer Hint Limitations

• Index and optimizer hints. See Index Hints, and Optimizer Hints.

• Semijoin strategies other than FIRSTMATCH.

  MySQL attempts to enforce the FIRSTMATCH strategy and ignores all other semijoin strategies specified explicitly as subquery optimizer hints. However, MySQL may still select the DUPSWEEDOUT semijoin strategy during JOIN order optimization, even if an equivalent plan could be offered using the FIRSTMATCH strategy. A plan that uses the DUPSWEEDOUT semijoin strategy would produce incorrect results if executed on HeatWave.

  For general information about subquery optimizer hints, see Subquery Optimizer Hints.