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    • Igor Babaev's avatar
      MDEV-17871 Crash when running explain with CTE · 46960365
      Igor Babaev authored
      When the with clause of a query contains a recursive CTE that is not used
      then processing of EXPLAIN for this query does not require optimization
      of the unit specifying this CTE. In this case if 'derived' is the
      TABLE_LIST object created for this CTE then derived->derived_result is NULL
      and any assignment to derived->derived_result->table causes a crash.
      After fixing this problem in the code of st_select_lex_unit::prepare()
      EXPLAIN for such a query worked without crashes. Yet an execution
      plan for the recursive CTE appeared there. The cause of this problem was
      an incorrect condition used in JOIN::save_explain_data_intern() that
      determined whether CTE was to be optimized or not. A similar condition was
      used in select_describe() and this patch has corrected it as well.
      46960365