Constant subqueries marked as unmergeable

/*

  Check whether the merging algorithm can be used for this unit

  SYNOPSIS

    st_select_lex_unit::can_be_merged()

  DESCRIPTION

    We can apply merge algorithm for a unit if it is single SELECT with

    subqueries only in WHERE clauses or in ON conditions or in select list

    (we do not count SELECTs of underlying  views/derived tables/CTEs and

    second level subqueries) and we have no grouping, ordering, HAVING

    clause, aggregate functions, DISTINCT clause, LIMIT clause.

  RETURN

    FALSE - only temporary table algorithm can be used

    TRUE  - merge algorithm can be used

*/

bool st_select_lex_unit::can_be_merged()

{

  // TODO: do not forget implement case when select_lex.table_list.elements==0