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, 41 insertions, 34 deletions

diff --git a/src/backend/access/common/tupdesc.c b/src/backend/access/common/tupdesc.c
index bf47640092a..d766ae72873 100644
--- a/src/backend/access/common/tupdesc.c
+++ b/src/backend/access/common/tupdesc.c
@@ -158,40 +158,6 @@ CreateTupleDescCopy(TupleDesc tupdesc)
 }
 
 /*
- * CreateTupleDescCopyExtend
- *		This function creates a new TupleDesc by copying from an existing
- *		TupleDesc, but adding space for more columns. The new tupdesc is
- *      not regarded as the same record type as the old one (and therefore
- *      does not inherit its typeid/typmod, which instead are left as an
- *      anonymous record type).
- *
- *      The additional column slots are not initialized in any way;
- *      callers must do their own TupleDescInitEntry on each.
- *
- * !!! Constraints and defaults are not copied !!!
- */
-TupleDesc
-CreateTupleDescCopyExtend(TupleDesc tupdesc, int moreatts)
-{
-	TupleDesc	desc;
-	int			i;
-	int         src_natts = tupdesc->natts;
-
-	Assert(moreatts >= 0);
-
-	desc = CreateTemplateTupleDesc(src_natts + moreatts, tupdesc->tdhasoid);
-
-	for (i = 0; i < src_natts; i++)
-	{
-		memcpy(desc->attrs[i], tupdesc->attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
-		desc->attrs[i]->attnotnull = false;
-		desc->attrs[i]->atthasdef = false;
-	}
-
-	return desc;
-}
-
-/*
  * CreateTupleDescCopyConstr
  *		This function creates a new TupleDesc by copying from an existing
  *		TupleDesc (including its constraints and defaults).
@@ -251,6 +217,47 @@ CreateTupleDescCopyConstr(TupleDesc tupdesc)
 }
 
 /*
+ * TupleDescCopyEntry
+ *		This function copies a single attribute structure from one tuple
+ *		descriptor to another.
+ *
+ * !!! Constraints and defaults are not copied !!!
+ */
+void
+TupleDescCopyEntry(TupleDesc dst, AttrNumber dstAttno,
+				   TupleDesc src, AttrNumber srcAttno)
+{
+	/*
+	 * sanity checks
+	 */
+	AssertArg(PointerIsValid(src));
+	AssertArg(PointerIsValid(dst));
+	AssertArg(srcAttno >= 1);
+	AssertArg(srcAttno <= src->natts);
+	AssertArg(dstAttno >= 1);
+	AssertArg(dstAttno <= dst->natts);
+
+	memcpy(dst->attrs[dstAttno - 1], src->attrs[srcAttno - 1],
+		   ATTRIBUTE_FIXED_PART_SIZE);
+
+	/*
+	 * Aside from updating the attno, we'd better reset attcacheoff.
+	 *
+	 * XXX Actually, to be entirely safe we'd need to reset the attcacheoff of
+	 * all following columns in dst as well.  Current usage scenarios don't
+	 * require that though, because all following columns will get initialized
+	 * by other uses of this function or TupleDescInitEntry.  So we cheat a
+	 * bit to avoid a useless O(N^2) penalty.
+	 */
+	dst->attrs[dstAttno - 1]->attnum = dstAttno;
+	dst->attrs[dstAttno - 1]->attcacheoff = -1;
+
+	/* since we're not copying constraints or defaults, clear these */
+	dst->attrs[dstAttno - 1]->attnotnull = false;
+	dst->attrs[dstAttno - 1]->atthasdef = false;
+}
+
+/*
  * Free a TupleDesc including all substructure
  */
 void