Implement UPDATE tab SET (col1,col2,...) = (SELECT ...), ...

This SQL-standard feature allows a sub-SELECT yielding multiple columns
(but only one row) to be used to compute the new values of several columns
to be updated.  While the same results can be had with an independent
sub-SELECT per column, such a workaround can require a great deal of
duplicated computation.

The standard actually says that the source for a multi-column assignment
could be any row-valued expression.  The implementation used here is
tightly tied to our existing sub-SELECT support and can't handle other
cases; the Bison grammar would have some issues with them too.  However,
I don't feel too bad about this since other cases can be converted into
sub-SELECTs.  For instance, "SET (a,b,c) = row_valued_function(x)" could
be written "SET (a,b,c) = (SELECT * FROM row_valued_function(x))".

1 files changed, 23 insertions, 3 deletions

diff --git a/src/backend/parser/parse_target.c b/src/backend/parser/parse_target.c
index 2ee1270ec5d..328e0c67aca 100644
--- a/src/backend/parser/parse_target.c
+++ b/src/backend/parser/parse_target.c
@@ -113,9 +113,9 @@ transformTargetEntry(ParseState *pstate,
  * transformTargetList()
  * Turns a list of ResTarget's into a list of TargetEntry's.
  *
- * At this point, we don't care whether we are doing SELECT, UPDATE,
- * or RETURNING; we just transform the given expressions (the "val" fields).
- * However, our subroutines care, so we need the exprKind parameter.
+ * This code acts mostly the same for SELECT, UPDATE, or RETURNING lists;
+ * the main thing is to transform the given expressions (the "val" fields).
+ * The exprKind parameter distinguishes these cases when necesssary.
  */
 List *
 transformTargetList(ParseState *pstate, List *targetlist,
@@ -124,6 +124,9 @@ transformTargetList(ParseState *pstate, List *targetlist,
 	List	   *p_target = NIL;
 	ListCell   *o_target;
 
+	/* Shouldn't have any leftover multiassign items at start */
+	Assert(pstate->p_multiassign_exprs == NIL);
+
 	foreach(o_target, targetlist)
 	{
 		ResTarget  *res = (ResTarget *) lfirst(o_target);
@@ -172,6 +175,19 @@ transformTargetList(ParseState *pstate, List *targetlist,
 												false));
 	}
 
+	/*
+	 * If any multiassign resjunk items were created, attach them to the end
+	 * of the targetlist.  This should only happen in an UPDATE tlist.  We
+	 * don't need to worry about numbering of these items; transformUpdateStmt
+	 * will set their resnos.
+	 */
+	if (pstate->p_multiassign_exprs)
+	{
+		Assert(exprKind == EXPR_KIND_UPDATE_SOURCE);
+		p_target = list_concat(p_target, pstate->p_multiassign_exprs);
+		pstate->p_multiassign_exprs = NIL;
+	}
+
 	return p_target;
 }
 
@@ -234,6 +250,9 @@ transformExpressionList(ParseState *pstate, List *exprlist,
 						 transformExpr(pstate, e, exprKind));
 	}
 
+	/* Shouldn't have any multiassign items here */
+	Assert(pstate->p_multiassign_exprs == NIL);
+
 	return result;
 }
 
@@ -1691,6 +1710,7 @@ FigureColnameInternal(Node *node, char **name)
 					}
 					break;
 					/* As with other operator-like nodes, these have no names */
+				case MULTIEXPR_SUBLINK:
 				case ALL_SUBLINK:
 				case ANY_SUBLINK:
 				case ROWCOMPARE_SUBLINK: