Function remove_distinct_sort_clause

src/backend/optimizer/plan/transform.c:551–606

* DISTINCT/DISTINCT ON/ORDER BY optimization. * Remove DISTINCT clause if possibile, ex: * select DISTINCT count(a) from t; to * select count(a) from t; * There is one row returned at most, DISTINCT and/or ON is pointless then. * The same with ORDER BY clause; */

 * The same with ORDER BY clause;
 */
Query *remove_distinct_sort_clause(Query *parse)
{
	if (parse->hasAggs &&
		parse->groupClause == NIL &&
		!contain_mutable_functions((Node *) parse) &&
		!tlist_has_srf(parse))
	{
		List	   *useless_tlist = NIL;
		List	   *tles;
		List	   *sortops;
		List	   *eqops;
		ListCell   *lc;

		if (parse->distinctClause != NIL)
		{
			get_sortgroupclauses_tles(parse->distinctClause, parse->targetList,
									  &tles, &sortops, &eqops);
			foreach(lc, tles)
			{
				TargetEntry *tle = lfirst(lc);
				if (tle->resjunk)
					useless_tlist = lappend(useless_tlist, tle);
			}
			parse->distinctClause = NIL;
			if (parse->hasDistinctOn)
				parse->hasDistinctOn = false;
		}

		if (parse->sortClause != NIL)
		{

			get_sortgroupclauses_tles(parse->sortClause, parse->targetList,
									  &tles, &sortops, &eqops);
			foreach(lc, tles)
			{
				TargetEntry *tle = lfirst(lc);
				/*
				 * For SELECT DISTINCT, ORDER BY expressions must appear in select list,
				 * Some tles may be already in the list.
				 */ 
				if (tle->resjunk)
					useless_tlist = list_append_unique(useless_tlist, tle);
			}
			parse->sortClause = NIL;
		}

		/*
		 * There is no groupClause, sortClause and distinctClause now .
		 * The junk TargetEntrys with ressortgroupref index are safe to be removed.
		 */
		if (useless_tlist != NIL)
			parse->targetList = list_difference(parse->targetList, useless_tlist);
	}

	return parse;
}