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, 14 insertions, 12 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index bedcf040a7a..5a4034729cf 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1395,24 +1395,26 @@ typedef struct SubqueryScanState
  *		function appearing in FROM (typically a function returning set).
  *
  *		eflags				node's capability flags
- *		ordinal				column value for WITH ORDINALITY
- *		scan_tupdesc		scan tuple descriptor
- *		func_tupdesc		function tuple descriptor
- *		func_slot			function result slot, or null
- *		tuplestorestate		private state of tuplestore.c
- *		funcexpr			state for function expression being evaluated
+ *		ordinality			is this scan WITH ORDINALITY?
+ *		simple				true if we have 1 function and no ordinality
+ *		ordinal				current ordinal column value
+ *		nfuncs				number of functions being executed
+ *		funcstates			per-function execution states (private in
+ *							nodeFunctionscan.c)
  * ----------------
  */
+struct FunctionScanPerFuncState;
+
 typedef struct FunctionScanState
 {
 	ScanState	ss;				/* its first field is NodeTag */
 	int			eflags;
-	int64       ordinal;
-	TupleDesc	scan_tupdesc;
-	TupleDesc	func_tupdesc;
-	TupleTableSlot *func_slot;
-	Tuplestorestate *tuplestorestate;
-	ExprState  *funcexpr;
+	bool		ordinality;
+	bool		simple;
+	int64		ordinal;
+	int			nfuncs;
+	struct FunctionScanPerFuncState *funcstates;		/* array of length
+														 * nfuncs */
 } FunctionScanState;
 
 /* ----------------