Method explainable

sql/sql_lex.cc:13851–13870

bool SELECT_LEX_UNIT::explainable() const
{
  /*
    EXPLAIN/ANALYZE unit, when:
    (1) if it's a subquery - it's not part of eliminated WHERE/ON clause.
    (2) if it's a CTE - it's not hanging (needed for execution)
    (3) if it's a derived - it's not merged or eliminated
    if it's not 1/2/3 - it's some weird internal thing, ignore it
  */

  return item ?
           !item->eliminated :                        // (1)
           with_element ?
             derived && derived->derived_result &&
               !with_element->is_hanging_recursive(): // (2)
             derived ?
               derived->is_materialized_derived() && // (3)
                 !is_derived_eliminated() :
               false;
}

/**
  Find the real table in prepared SELECT tree