Support multi-argument UNNEST(), and TABLE() syntax for multiple functions.

This patch adds the ability to write TABLE( function1(), function2(), ...)
as a single FROM-clause entry.  The result is the concatenation of the
first row from each function, followed by the second row from each
function, etc; with NULLs inserted if any function produces fewer rows than
others.  This is believed to be a much more useful behavior than what
Postgres currently does with multiple SRFs in a SELECT list.

This syntax also provides a reasonable way to combine use of column
definition lists with WITH ORDINALITY: put the column definition list
inside TABLE(), where it's clear that it doesn't control the ordinality
column as well.

Also implement SQL-compliant multiple-argument UNNEST(), by turning
UNNEST(a,b,c) into TABLE(unnest(a), unnest(b), unnest(c)).

The SQL standard specifies TABLE() with only a single function, not
multiple functions, and it seems to require an implicit UNNEST() which is
not what this patch does.  There may be something wrong with that reading
of the spec, though, because if it's right then the spec's TABLE() is just
a pointless alternative spelling of UNNEST().  After further review of
that, we might choose to adopt a different syntax for what this patch does,
but in any case this functionality seems clearly worthwhile.

Andrew Gierth, reviewed by Zoltán Böszörményi and Heikki Linnakangas, and
significantly revised by me

1 files changed, 51 insertions, 0 deletions

diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index 032b2cdc133..9c8ede658f4 100644
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -502,6 +502,57 @@ build_index_pathkeys(PlannerInfo *root,
 }
 
 /*
+ * build_expression_pathkey
+ *	  Build a pathkeys list that describes an ordering by a single expression
+ *	  using the given sort operator.
+ *
+ * expr, nullable_relids, and rel are as for make_pathkey_from_sortinfo.
+ * We induce the other arguments assuming default sort order for the operator.
+ *
+ * Similarly to make_pathkey_from_sortinfo, the result is NIL if create_it
+ * is false and the expression isn't already in some EquivalenceClass.
+ */
+List *
+build_expression_pathkey(PlannerInfo *root,
+						 Expr *expr,
+						 Relids nullable_relids,
+						 Oid opno,
+						 Relids rel,
+						 bool create_it)
+{
+	List	   *pathkeys;
+	Oid			opfamily,
+				opcintype;
+	int16		strategy;
+	PathKey    *cpathkey;
+
+	/* Find the operator in pg_amop --- failure shouldn't happen */
+	if (!get_ordering_op_properties(opno,
+									&opfamily, &opcintype, &strategy))
+		elog(ERROR, "operator %u is not a valid ordering operator",
+			 opno);
+
+	cpathkey = make_pathkey_from_sortinfo(root,
+										  expr,
+										  nullable_relids,
+										  opfamily,
+										  opcintype,
+										  exprCollation((Node *) expr),
+									   (strategy == BTGreaterStrategyNumber),
+									   (strategy == BTGreaterStrategyNumber),
+										  0,
+										  rel,
+										  create_it);
+
+	if (cpathkey)
+		pathkeys = list_make1(cpathkey);
+	else
+		pathkeys = NIL;
+
+	return pathkeys;
+}
+
+/*
  * convert_subquery_pathkeys
  *	  Build a pathkeys list that describes the ordering of a subquery's
  *	  result, in the terms of the outer query.	This is essentially a