Function transformRangeSubselect

src/backend/parser/parse_clause.c:547–605

* transformRangeSubselect --- transform a sub-SELECT appearing in FROM */

 * transformRangeSubselect --- transform a sub-SELECT appearing in FROM
 */
static ParseNamespaceItem *
transformRangeSubselect(ParseState *pstate, RangeSubselect *r)
{
	Query	   *query;

	/*
	 * We require user to supply an alias for a subselect, per SQL92. To relax
	 * this, we'd have to be prepared to gin up a unique alias for an
	 * unlabeled subselect.  (This is just elog, not ereport, because the
	 * grammar should have enforced it already.  It'd probably be better to
	 * report the error here, but we don't have a good error location here.)
	 */
	if (r->alias == NULL)
		elog(ERROR, "subquery in FROM must have an alias");

	/*
	 * Set p_expr_kind to show this parse level is recursing to a subselect.
	 * We can't be nested within any expression, so don't need save-restore
	 * logic here.
	 */
	Assert(pstate->p_expr_kind == EXPR_KIND_NONE);
	pstate->p_expr_kind = EXPR_KIND_FROM_SUBSELECT;

	/*
	 * If the subselect is LATERAL, make lateral_only names of this level
	 * visible to it.  (LATERAL can't nest within a single pstate level, so we
	 * don't need save/restore logic here.)
	 */
	Assert(!pstate->p_lateral_active);
	pstate->p_lateral_active = r->lateral;

	/*
	 * Analyze and transform the subquery.
	 */
	query = parse_sub_analyze(r->subquery, pstate, NULL,
							  getLockedRefname(pstate, r->alias->aliasname),
							  true);

	/* Restore state */
	pstate->p_lateral_active = false;
	pstate->p_expr_kind = EXPR_KIND_NONE;

	/*
	 * Check that we got a SELECT.  Anything else should be impossible given
	 * restrictions of the grammar, but check anyway.
	 */
	if (!IsA(query, Query) ||
		query->commandType != CMD_SELECT)
		elog(ERROR, "unexpected non-SELECT command in subquery in FROM");

	/*
	 * OK, build an RTE and nsitem for the subquery.
	 */
	return addRangeTableEntryForSubquery(pstate,
										 query,
										 r->alias,
										 r->lateral,
										 true);