Implement SQL-standard LATERAL subqueries.

This patch implements the standard syntax of LATERAL attached to a
sub-SELECT in FROM, and also allows LATERAL attached to a function in FROM,
since set-returning function calls are expected to be one of the principal
use-cases.

The main change here is a rewrite of the mechanism for keeping track of
which relations are visible for column references while the FROM clause is
being scanned.  The parser "namespace" lists are no longer lists of bare
RTEs, but are lists of ParseNamespaceItem structs, which carry an RTE
pointer as well as some visibility-controlling flags.  Aside from
supporting LATERAL correctly, this lets us get rid of the ancient hacks
that required rechecking subqueries and JOIN/ON and function-in-FROM
expressions for invalid references after they were initially parsed.
Invalid column references are now always correctly detected on sight.

In passing, remove assorted parser error checks that are now dead code by
virtue of our having gotten rid of add_missing_from, as well as some
comments that are obsolete for the same reason.  (It was mainly
add_missing_from that caused so much fudging here in the first place.)

The planner support for this feature is very minimal, and will be improved
in future patches.  It works well enough for testing purposes, though.

catversion bump forced due to new field in RangeTblEntry.

1 files changed, 26 insertions, 14 deletions

diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 670e0849936..13f745f6fa6 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -54,22 +54,25 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * p_joinlist: list of join items (RangeTblRef and JoinExpr nodes) that
  * will become the fromlist of the query's top-level FromExpr node.
  *
- * p_relnamespace: list of RTEs that represents the current namespace for
- * table lookup, ie, those RTEs that are accessible by qualified names.
- * This may be just a subset of the rtable + joinlist, and/or may contain
- * entries that are not yet added to the main joinlist.
- *
- * p_varnamespace: list of RTEs that represents the current namespace for
- * column lookup, ie, those RTEs that are accessible by unqualified names.
- * This is different from p_relnamespace because a JOIN without an alias does
- * not hide the contained tables (so they must still be in p_relnamespace)
- * but it does hide their columns (unqualified references to the columns must
- * refer to the JOIN, not the member tables).  Other special RTEs such as
- * NEW/OLD for rules may also appear in just one of these lists.
+ * p_relnamespace: list of ParseNamespaceItems that represents the current
+ * namespace for table lookup, ie, those RTEs that are accessible by
+ * qualified names.  (This may be just a subset of the whole rtable.)
+ *
+ * p_varnamespace: list of ParseNamespaceItems that represents the current
+ * namespace for column lookup, ie, those RTEs that are accessible by
+ * unqualified names.  This is different from p_relnamespace because a JOIN
+ * without an alias does not hide the contained tables (so they must be in
+ * p_relnamespace) but it does hide their columns (unqualified references to
+ * the columns must refer to the JOIN, not the member tables).	Other special
+ * RTEs such as NEW/OLD for rules may also appear in just one of these lists.
+ *
+ * p_lateral_active: TRUE if we are currently parsing a LATERAL subexpression
+ * of this parse level.  This makes p_lateral_only namespace items visible,
+ * whereas they are not visible when p_lateral_active is FALSE.
  *
  * p_ctenamespace: list of CommonTableExprs (WITH items) that are visible
- * at the moment.  This is different from p_relnamespace because you have
- * to make an RTE before you can access a CTE.
+ * at the moment.  This is entirely different from p_relnamespace because
+ * a CTE is not an RTE, rather "visibility" means you could make an RTE.
  *
  * p_future_ctes: list of CommonTableExprs (WITH items) that are not yet
  * visible due to scope rules.	This is used to help improve error messages.
@@ -93,6 +96,7 @@ struct ParseState
 								 * node's fromlist) */
 	List	   *p_relnamespace; /* current namespace for relations */
 	List	   *p_varnamespace; /* current namespace for columns */
+	bool		p_lateral_active;		/* p_lateral_only items visible? */
 	List	   *p_ctenamespace; /* current namespace for common table exprs */
 	List	   *p_future_ctes;	/* common table exprs not yet in namespace */
 	CommonTableExpr *p_parent_cte;		/* this query's containing CTE */
@@ -121,6 +125,14 @@ struct ParseState
 	void	   *p_ref_hook_state;		/* common passthrough link for above */
 };
 
+/* An element of p_relnamespace or p_varnamespace */
+typedef struct ParseNamespaceItem
+{
+	RangeTblEntry *p_rte;		/* The relation's rangetable entry */
+	bool		p_lateral_only; /* Is only visible to LATERAL expressions? */
+	bool		p_lateral_ok;	/* If so, does join type allow use? */
+} ParseNamespaceItem;
+
 /* Support for parser_errposition_callback function */
 typedef struct ParseCallbackState
 {