Currently some of the subquery rewrites are done in check_and_do_in_subquery_rewrites().

This function is called from

• JOIN::prepare() // the "primary" call
• Item_exists_subselect::exists2in_processor // the secondary

This task is about moving subquery rewrites from JOIN::prepare() phase into JOIN::optimize().

check_and_do_in_subquery_rewrites() does several rewrites:

 - Convert subquery predicate into semi-join, or

 - Mark the subquery for execution using materialization, or

 - Perform IN->EXISTS transformation, or

 - Perform more/less ALL/ANY -> MIN/MAX rewrite

 - Substitute trivial scalar-context subquery with its value

Note that for semi-joins, check_and_do_in_subquery_rewrites() doesn't actually do the rewrites. It checks the condition and adds the subquery into SELECT_LEX::sj_subselects.

The actual conversion happens in JOIN::optimize_inner():

  if (select_lex->first_cond_optimization)

  {

    /* dump_TABLE_LIST_graph(select_lex, select_lex->leaf_tables); */

    if (convert_join_subqueries_to_semijoins(this))

      DBUG_RETURN(1); /* purecov: inspected */

    /* dump_TABLE_LIST_graph(select_lex, select_lex->leaf_tables); */

    select_lex->update_used_tables();

  }