Single Select Engine MySQL

subselect_single_select_engine:

  • constructor allocate JOIN and store pointers on SELECT_LEX and JOIN.
  • prepare() call JOIN::prepare.
  • fix_length_and_dec() prepare cache and receive type and parameters of returning items (called only by Item_singlerow_subselect).
  • exec() drop 'assigned' flag of Item_subselect. If this is the first time, call JOIN::optimize and JOIN::exec(), else do nothing or JOIN::reinit() JOIN::exec() depending on type of subquery.

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