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, 31 insertions, 14 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 4e93df26cc8..bd5428de97b 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -1259,9 +1259,21 @@ ExplainNode(PlanState *planstate, List *ancestors,
 			break;
 		case T_FunctionScan:
 			if (es->verbose)
-				show_expression(((FunctionScan *) plan)->funcexpr,
+			{
+				List	   *fexprs = NIL;
+				ListCell   *lc;
+
+				foreach(lc, ((FunctionScan *) plan)->functions)
+				{
+					RangeTblFunction *rtfunc = (RangeTblFunction *) lfirst(lc);
+
+					fexprs = lappend(fexprs, rtfunc->funcexpr);
+				}
+				/* We rely on show_expression to insert commas as needed */
+				show_expression((Node *) fexprs,
 								"Function Call", planstate, ancestors,
 								es->verbose, es);
+			}
 			show_scan_qual(plan->qual, "Filter", planstate, ancestors, es);
 			if (plan->qual)
 				show_instrumentation_count("Rows Removed by Filter", 1,
@@ -1984,26 +1996,31 @@ ExplainTargetRel(Plan *plan, Index rti, ExplainState *es)
 			break;
 		case T_FunctionScan:
 			{
-				Node	   *funcexpr;
+				FunctionScan *fscan = (FunctionScan *) plan;
 
 				/* Assert it's on a RangeFunction */
 				Assert(rte->rtekind == RTE_FUNCTION);
 
 				/*
-				 * If the expression is still a function call, we can get the
-				 * real name of the function.  Otherwise, punt (this can
-				 * happen if the optimizer simplified away the function call,
-				 * for example).
+				 * If the expression is still a function call of a single
+				 * function, we can get the real name of the function.
+				 * Otherwise, punt.  (Even if it was a single function call
+				 * originally, the optimizer could have simplified it away.)
 				 */
-				funcexpr = ((FunctionScan *) plan)->funcexpr;
-				if (funcexpr && IsA(funcexpr, FuncExpr))
+				if (list_length(fscan->functions) == 1)
 				{
-					Oid			funcid = ((FuncExpr *) funcexpr)->funcid;
-
-					objectname = get_func_name(funcid);
-					if (es->verbose)
-						namespace =
-							get_namespace_name(get_func_namespace(funcid));
+					RangeTblFunction *rtfunc = (RangeTblFunction *) linitial(fscan->functions);
+
+					if (IsA(rtfunc->funcexpr, FuncExpr))
+					{
+						FuncExpr   *funcexpr = (FuncExpr *) rtfunc->funcexpr;
+						Oid			funcid = funcexpr->funcid;
+
+						objectname = get_func_name(funcid);
+						if (es->verbose)
+							namespace =
+								get_namespace_name(get_func_namespace(funcid));
+					}
 				}
 				objecttag = "Function Name";
 			}