Disallow LATERAL references to the target table of an UPDATE/DELETE.

On second thought, commit 0c051c9 was
over-hasty: rather than allowing this case, we ought to reject it for now.
That leaves the field clear for a future feature that allows the target
table to be re-specified in the FROM (or USING) clause, which will enable
left-joining the target table to something else.  We can then also allow
LATERAL references to such an explicitly re-specified target table.
But allowing them right now will create ambiguities or worse for such a
feature, and it isn't something we documented 9.3 as supporting.

While at it, add a convenience subroutine to avoid having several copies
of the ereport for disalllowed-LATERAL-reference cases.

src/backend/parser/analyze.c

@@ -367,8 +367,9 @@ transformDeleteStmt(ParseState *pstate, DeleteStmt *stmt)
 	/* there's no DISTINCT in DELETE */
 	qry->distinctClause = NIL;
 
-	/* subqueries in USING can see the result relation only via LATERAL */
+	/* subqueries in USING cannot access the result relation */
 	nsitem->p_lateral_only = true;
+	nsitem->p_lateral_ok = false;
 
 	/*
 	 * The USING clause is non-standard SQL syntax, and is equivalent in
@@ -378,8 +379,9 @@ transformDeleteStmt(ParseState *pstate, DeleteStmt *stmt)
 	 */
 	transformFromClause(pstate, stmt->usingClause);
 
-	/* remaining clauses can see the result relation normally */
+	/* remaining clauses can reference the result relation normally */
 	nsitem->p_lateral_only = false;
+	nsitem->p_lateral_ok = true;
 
 	qual = transformWhereClause(pstate, stmt->whereClause,
 								EXPR_KIND_WHERE, "WHERE");
@@ -1925,17 +1927,19 @@ transformUpdateStmt(ParseState *pstate, UpdateStmt *stmt)
 	/* grab the namespace item made by setTargetTable */
 	nsitem = (ParseNamespaceItem *) llast(pstate->p_namespace);
 
-	/* subqueries in FROM can see the result relation only via LATERAL */
+	/* subqueries in FROM cannot access the result relation */
 	nsitem->p_lateral_only = true;
+	nsitem->p_lateral_ok = false;
 
 	/*
 	 * the FROM clause is non-standard SQL syntax. We used to be able to do
 	 * this with REPLACE in POSTQUEL so we keep the feature.
 	 */
 	transformFromClause(pstate, stmt->fromClause);
 
-	/* remaining clauses can see the result relation normally */
+	/* remaining clauses can reference the result relation normally */
 	nsitem->p_lateral_only = false;
+	nsitem->p_lateral_ok = true;
 
 	qry->targetList = transformTargetList(pstate, stmt->targetList,
 										  EXPR_KIND_UPDATE_SOURCE);

src/backend/parser/parse_relation.c

@@ -37,6 +37,8 @@ static RangeTblEntry *scanNameSpaceForRefname(ParseState *pstate,
 						const char *refname, int location);
 static RangeTblEntry *scanNameSpaceForRelid(ParseState *pstate, Oid relid,
 					  int location);
+static void check_lateral_ref_ok(ParseState *pstate, ParseNamespaceItem *nsitem,
+					 int location);
 static void markRTEForSelectPriv(ParseState *pstate, RangeTblEntry *rte,
 					 int rtindex, AttrNumber col);
 static void expandRelation(Oid relid, Alias *eref,
@@ -170,14 +172,7 @@ scanNameSpaceForRefname(ParseState *pstate, const char *refname, int location)
 						 errmsg("table reference \"%s\" is ambiguous",
 								refname),
 						 parser_errposition(pstate, location)));
-			/* SQL:2008 demands this be an error, not an invisible item */
-			if (nsitem->p_lateral_only && !nsitem->p_lateral_ok)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-						 errmsg("invalid reference to FROM-clause entry for table \"%s\"",
-								refname),
-						 errdetail("The combining JOIN type must be INNER or LEFT for a LATERAL reference."),
-						 parser_errposition(pstate, location)));
+			check_lateral_ref_ok(pstate, nsitem, location);
 			result = rte;
 		}
 	}
@@ -221,14 +216,7 @@ scanNameSpaceForRelid(ParseState *pstate, Oid relid, int location)
 						 errmsg("table reference %u is ambiguous",
 								relid),
 						 parser_errposition(pstate, location)));
-			/* SQL:2008 demands this be an error, not an invisible item */
-			if (nsitem->p_lateral_only && !nsitem->p_lateral_ok)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-						 errmsg("invalid reference to FROM-clause entry for table \"%s\"",
-								rte->eref->aliasname),
-						 errdetail("The combining JOIN type must be INNER or LEFT for a LATERAL reference."),
-						 parser_errposition(pstate, location)));
+			check_lateral_ref_ok(pstate, nsitem, location);
 			result = rte;
 		}
 	}
@@ -410,6 +398,37 @@ checkNameSpaceConflicts(ParseState *pstate, List *namespace1,
 	}
 }
 
+/*
+ * Complain if a namespace item is currently disallowed as a LATERAL reference.
+ * This enforces both SQL:2008's rather odd idea of what to do with a LATERAL
+ * reference to the wrong side of an outer join, and our own prohibition on
+ * referencing the target table of an UPDATE or DELETE as a lateral reference
+ * in a FROM/USING clause.
+ *
+ * Convenience subroutine to avoid multiple copies of a rather ugly ereport.
+ */
+static void
+check_lateral_ref_ok(ParseState *pstate, ParseNamespaceItem *nsitem,
+					 int location)
+{
+	if (nsitem->p_lateral_only && !nsitem->p_lateral_ok)
+	{
+		/* SQL:2008 demands this be an error, not an invisible item */
+		RangeTblEntry *rte = nsitem->p_rte;
+		char	   *refname = rte->eref->aliasname;
+
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
+			errmsg("invalid reference to FROM-clause entry for table \"%s\"",
+				   refname),
+				 (rte == pstate->p_target_rangetblentry) ?
+				 errhint("There is an entry for table \"%s\", but it cannot be referenced from this part of the query.",
+						 refname) :
+				 errdetail("The combining JOIN type must be INNER or LEFT for a LATERAL reference."),
+				 parser_errposition(pstate, location)));
+	}
+}
+
 /*
  * given an RTE, return RT index (starting with 1) of the entry,
  * and optionally get its nesting depth (0 = current).	If sublevels_up
@@ -632,15 +651,8 @@ colNameToVar(ParseState *pstate, char *colname, bool localonly,
 							(errcode(ERRCODE_AMBIGUOUS_COLUMN),
 							 errmsg("column reference \"%s\" is ambiguous",
 									colname),
-							 parser_errposition(orig_pstate, location)));
-				/* SQL:2008 demands this be an error, not an invisible item */
-				if (nsitem->p_lateral_only && !nsitem->p_lateral_ok)
-					ereport(ERROR,
-							(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-							 errmsg("invalid reference to FROM-clause entry for table \"%s\"",
-									rte->eref->aliasname),
-							 errdetail("The combining JOIN type must be INNER or LEFT for a LATERAL reference."),
-							 parser_errposition(orig_pstate, location)));
+							 parser_errposition(pstate, location)));
+				check_lateral_ref_ok(pstate, nsitem, location);
 				result = newresult;
 			}
 		}

src/include/parser/parse_node.h

@@ -186,7 +186,8 @@ struct ParseState
  * inside such a subexpression at the moment.)	If p_lateral_ok is not set,
  * it's an error to actually use such a namespace item.  One might think it
  * would be better to just exclude such items from visibility, but the wording
- * of SQL:2008 requires us to do it this way.
+ * of SQL:2008 requires us to do it this way.  We also use p_lateral_ok to
+ * forbid LATERAL references to an UPDATE/DELETE target table.
  *
  * At no time should a namespace list contain two entries that conflict
  * according to the rules in checkNameSpaceConflicts; but note that those

src/test/regress/expected/join.out

@@ -4105,17 +4105,7 @@ LINE 1: select 1 from tenk1 a, lateral (select max(a.unique1) from i...
                                                ^
 -- check behavior of LATERAL in UPDATE/DELETE
 create temp table xx1 as select f1 as x1, -f1 as x2 from int4_tbl;
-select * from xx1;
-     x1      |     x2      
--------------+-------------
-           0 |           0
-      123456 |     -123456
-     -123456 |      123456
-  2147483647 | -2147483647
- -2147483647 |  2147483647
-(5 rows)
-
--- error, can't do this without LATERAL:
+-- error, can't do this:
 update xx1 set x2 = f1 from (select * from int4_tbl where f1 = x1) ss;
 ERROR:  column "x1" does not exist
 LINE 1: ... set x2 = f1 from (select * from int4_tbl where f1 = x1) ss;
@@ -4126,28 +4116,28 @@ ERROR:  invalid reference to FROM-clause entry for table "xx1"
 LINE 1: ...t x2 = f1 from (select * from int4_tbl where f1 = xx1.x1) ss...
                                                              ^
 HINT:  There is an entry for table "xx1", but it cannot be referenced from this part of the query.
--- OK:
+-- can't do it even with LATERAL:
 update xx1 set x2 = f1 from lateral (select * from int4_tbl where f1 = x1) ss;
-select * from xx1;
-     x1      |     x2      
--------------+-------------
-           0 |           0
-      123456 |      123456
-     -123456 |     -123456
-  2147483647 |  2147483647
- -2147483647 | -2147483647
-(5 rows)
-
--- error:
+ERROR:  invalid reference to FROM-clause entry for table "xx1"
+LINE 1: ...= f1 from lateral (select * from int4_tbl where f1 = x1) ss;
+                                                                ^
+HINT:  There is an entry for table "xx1", but it cannot be referenced from this part of the query.
+-- we might in future allow something like this, but for now it's an error:
+update xx1 set x2 = f1 from xx1, lateral (select * from int4_tbl where f1 = x1) ss;
+ERROR:  table name "xx1" specified more than once
+-- also errors:
 delete from xx1 using (select * from int4_tbl where f1 = x1) ss;
 ERROR:  column "x1" does not exist
 LINE 1: ...te from xx1 using (select * from int4_tbl where f1 = x1) ss;
                                                                 ^
 HINT:  There is a column named "x1" in table "xx1", but it cannot be referenced from this part of the query.
--- OK:
+delete from xx1 using (select * from int4_tbl where f1 = xx1.x1) ss;
+ERROR:  invalid reference to FROM-clause entry for table "xx1"
+LINE 1: ...from xx1 using (select * from int4_tbl where f1 = xx1.x1) ss...
+                                                             ^
+HINT:  There is an entry for table "xx1", but it cannot be referenced from this part of the query.
 delete from xx1 using lateral (select * from int4_tbl where f1 = x1) ss;
-select * from xx1;
- x1 | x2 
-----+----
-(0 rows)
-
+ERROR:  invalid reference to FROM-clause entry for table "xx1"
+LINE 1: ...xx1 using lateral (select * from int4_tbl where f1 = x1) ss;
+                                                                ^
+HINT:  There is an entry for table "xx1", but it cannot be referenced from this part of the query.

src/test/regress/sql/join.sql

@@ -1151,17 +1151,16 @@ select 1 from tenk1 a, lateral (select max(a.unique1) from int4_tbl b) ss;
 -- check behavior of LATERAL in UPDATE/DELETE
 
 create temp table xx1 as select f1 as x1, -f1 as x2 from int4_tbl;
-select * from xx1;
 
--- error, can't do this without LATERAL:
+-- error, can't do this:
 update xx1 set x2 = f1 from (select * from int4_tbl where f1 = x1) ss;
 update xx1 set x2 = f1 from (select * from int4_tbl where f1 = xx1.x1) ss;
--- OK:
+-- can't do it even with LATERAL:
 update xx1 set x2 = f1 from lateral (select * from int4_tbl where f1 = x1) ss;
-select * from xx1;
+-- we might in future allow something like this, but for now it's an error:
+update xx1 set x2 = f1 from xx1, lateral (select * from int4_tbl where f1 = x1) ss;
 
--- error:
+-- also errors:
 delete from xx1 using (select * from int4_tbl where f1 = x1) ss;
--- OK:
+delete from xx1 using (select * from int4_tbl where f1 = xx1.x1) ss;
 delete from xx1 using lateral (select * from int4_tbl where f1 = x1) ss;
-select * from xx1;