Support XMLTABLE query expression

XMLTABLE is defined by the SQL/XML standard as a feature that allows
turning XML-formatted data into relational form, so that it can be used
as a <table primary> in the FROM clause of a query.

This new construct provides significant simplicity and performance
benefit for XML data processing; what in a client-side custom
implementation was reported to take 20 minutes can be executed in 400ms
using XMLTABLE.  (The same functionality was said to take 10 seconds
using nested PostgreSQL XPath function calls, and 5 seconds using
XMLReader under PL/Python).

The implemented syntax deviates slightly from what the standard
requires.  First, the standard indicates that the PASSING clause is
optional and that multiple XML input documents may be given to it; we
make it mandatory and accept a single document only.  Second, we don't
currently support a default namespace to be specified.

This implementation relies on a new executor node based on a hardcoded
method table.  (Because the grammar is fixed, there is no extensibility
in the current approach; further constructs can be implemented on top of
this such as JSON_TABLE, but they require changes to core code.)

Author: Pavel Stehule, Álvaro Herrera
Extensively reviewed by: Craig Ringer
Discussion: https://postgr.es/m/CAFj8pRAgfzMD-LoSmnMGybD0WsEznLHWap8DO79+-GTRAPR4qA@mail.gmail.com

1 files changed, 24 insertions, 13 deletions

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index ca0ae7883e7..1636a69dba4 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -69,17 +69,19 @@ create_upper_paths_hook_type create_upper_paths_hook = NULL;
 
 
 /* Expression kind codes for preprocess_expression */
-#define EXPRKIND_QUAL			0
-#define EXPRKIND_TARGET			1
-#define EXPRKIND_RTFUNC			2
-#define EXPRKIND_RTFUNC_LATERAL 3
-#define EXPRKIND_VALUES			4
-#define EXPRKIND_VALUES_LATERAL 5
-#define EXPRKIND_LIMIT			6
-#define EXPRKIND_APPINFO		7
-#define EXPRKIND_PHV			8
-#define EXPRKIND_TABLESAMPLE	9
-#define EXPRKIND_ARBITER_ELEM	10
+#define EXPRKIND_QUAL				0
+#define EXPRKIND_TARGET				1
+#define EXPRKIND_RTFUNC				2
+#define EXPRKIND_RTFUNC_LATERAL 	3
+#define EXPRKIND_VALUES				4
+#define EXPRKIND_VALUES_LATERAL 	5
+#define EXPRKIND_LIMIT				6
+#define EXPRKIND_APPINFO			7
+#define EXPRKIND_PHV				8
+#define EXPRKIND_TABLESAMPLE		9
+#define EXPRKIND_ARBITER_ELEM		10
+#define EXPRKIND_TABLEFUNC			11
+#define EXPRKIND_TABLEFUNC_LATERAL	12
 
 /* Passthrough data for standard_qp_callback */
 typedef struct
@@ -685,7 +687,15 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 		{
 			/* Preprocess the function expression(s) fully */
 			kind = rte->lateral ? EXPRKIND_RTFUNC_LATERAL : EXPRKIND_RTFUNC;
-			rte->functions = (List *) preprocess_expression(root, (Node *) rte->functions, kind);
+			rte->functions = (List *)
+				preprocess_expression(root, (Node *) rte->functions, kind);
+		}
+		else if (rte->rtekind == RTE_TABLEFUNC)
+		{
+			/* Preprocess the function expression(s) fully */
+			kind = rte->lateral ? EXPRKIND_TABLEFUNC_LATERAL : EXPRKIND_TABLEFUNC;
+			rte->tablefunc = (TableFunc *)
+				preprocess_expression(root, (Node *) rte->tablefunc, kind);
 		}
 		else if (rte->rtekind == RTE_VALUES)
 		{
@@ -844,7 +854,8 @@ preprocess_expression(PlannerInfo *root, Node *expr, int kind)
 	if (root->hasJoinRTEs &&
 		!(kind == EXPRKIND_RTFUNC ||
 		  kind == EXPRKIND_VALUES ||
-		  kind == EXPRKIND_TABLESAMPLE))
+		  kind == EXPRKIND_TABLESAMPLE ||
+		  kind == EXPRKIND_TABLEFUNC))
 		expr = flatten_join_alias_vars(root, expr);
 
 	/*