Steps of Select Execution MySQL

Every select is performed in these base steps:

  • JOIN::prepare
  • Initialization and linking JOIN structure to st_select_lex.
  • fix_fields() for all items (after fix_fields(), we know everything about item).
  • Moving HAVING to WHERE if possible.
  • Initialization procedure if there is one.
  • JOIN::optimize
  • Single select optimization.
  • Creation of first temporary table if needed.
  • JOIN::exec
  • Performing select (a second temporary table may be created).
  • JOIN::cleanup
  • Removing all temporary tables, other cleanup.
  • JOIN::reinit
  • Prepare all structures for execution of SELECT (with JOIN::exec).

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