Function check_output_expressions

src/backend/optimizer/path/allpaths.c:4210–4266

* check_output_expressions - check subquery's output expressions for safety * * There are several cases in which it's unsafe to push down an upper-level * qual if it references a particular output column of a subquery. We check * each output column of the subquery and set unsafeColumns[k] to true if * that column is unsafe for a pushed-down qual to reference. The conditions * checked here

 * subquery_is_pushdown_safe handles that.)
 */
static void
check_output_expressions(Query *subquery, pushdown_safety_info *safetyInfo)
{
	ListCell   *lc;

	foreach(lc, subquery->targetList)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(lc);

		if (tle->resjunk)
			continue;			/* ignore resjunk columns */

		/* We need not check further if output col is already known unsafe */
		if (safetyInfo->unsafeColumns[tle->resno])
			continue;

		/* Functions returning sets are unsafe (point 1) */
		if (subquery->hasTargetSRFs &&
			expression_returns_set((Node *) tle->expr))
		{
			safetyInfo->unsafeColumns[tle->resno] = true;
			continue;
		}

		/* Volatile functions are unsafe (point 2) */
		if (contain_volatile_functions((Node *) tle->expr))
		{
			safetyInfo->unsafeColumns[tle->resno] = true;
			continue;
		}

		/* If subquery uses DISTINCT ON, check point 3 */
		if (subquery->hasDistinctOn &&
			!targetIsInSortList(tle, InvalidOid, subquery->distinctClause))
		{
			/* non-DISTINCT column, so mark it unsafe */
			safetyInfo->unsafeColumns[tle->resno] = true;
			continue;
		}

		/* If subquery uses window functions, check point 4 */
		if (subquery->hasWindowFuncs &&
			!targetIsInAllPartitionLists(tle, subquery))
		{
			/* not present in all PARTITION BY clauses, so mark it unsafe */
			safetyInfo->unsafeColumns[tle->resno] = true;
			continue;
		}

		/* Refuse subplans */
		if (contain_subplans((Node *) tle->expr))
		{
			safetyInfo->unsafeColumns[tle->resno] = true;
			continue;
		}
	}
}