EXPLAIN generic_plan NOT supported in Citus (#7825)

We thought we provided support for this in

b8c493f2c4

However the use of parameters in SQL is not supported in Citus. Since
generic plan queries use parameters, we can't support for now.

Relevant PG16 commit https://github.com/postgres/postgres/commit/3c05284

Fixes #7813 with proper error message

(cherry picked from commit 0a6adf4ccc)

src/backend/distributed/planner/multi_explain.c

@@ -190,6 +190,14 @@ PG_FUNCTION_INFO_V1(worker_save_query_explain_analyze);
 void
 CitusExplainScan(CustomScanState *node, List *ancestors, struct ExplainState *es)
 {
+#if PG_VERSION_NUM >= PG_VERSION_16
+	if (es->generic)
+	{
+		ereport(ERROR, (errmsg(
+							"EXPLAIN GENERIC_PLAN is currently not supported for Citus tables")));
+	}
+#endif
+
 	CitusScanState *scanState = (CitusScanState *) node;
 	DistributedPlan *distributedPlan = scanState->distributedPlan;
 	EState *executorState = ScanStateGetExecutorState(scanState);
@@ -1021,18 +1029,12 @@ BuildRemoteExplainQuery(char *queryString, ExplainState *es)
 	appendStringInfo(explainQuery,
 					 "EXPLAIN (ANALYZE %s, VERBOSE %s, "
 					 "COSTS %s, BUFFERS %s, WAL %s, "
-#if PG_VERSION_NUM >= PG_VERSION_16
-					 "GENERIC_PLAN %s, "
-#endif
 					 "TIMING %s, SUMMARY %s, FORMAT %s) %s",
 					 es->analyze ? "TRUE" : "FALSE",
 					 es->verbose ? "TRUE" : "FALSE",
 					 es->costs ? "TRUE" : "FALSE",
 					 es->buffers ? "TRUE" : "FALSE",
 					 es->wal ? "TRUE" : "FALSE",
-#if PG_VERSION_NUM >= PG_VERSION_16
-					 es->generic ? "TRUE" : "FALSE",
-#endif
 					 es->timing ? "TRUE" : "FALSE",
 					 es->summary ? "TRUE" : "FALSE",
 					 formatStr,

src/test/regress/expected/pg16.out

@@ -81,29 +81,9 @@ SELECT create_distributed_table('tenk1', 'unique1');
 (1 row)
 
 SET citus.log_remote_commands TO on;
-EXPLAIN (GENERIC_PLAN) SELECT unique1 FROM tenk1 WHERE thousand = 1000;
+EXPLAIN (GENERIC_PLAN) SELECT unique1 FROM tenk1 WHERE thousand = $1;
-NOTICE:  issuing BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;SELECT assign_distributed_transaction_id(xx, xx, 'xxxxxxx');
+ERROR:  EXPLAIN GENERIC_PLAN is currently not supported for Citus tables
-DETAIL:  on server postgres@localhost:xxxxx connectionId: xxxxxxx
+EXPLAIN (GENERIC_PLAN, ANALYZE) SELECT unique1 FROM tenk1 WHERE thousand = $1;
-NOTICE:  issuing SAVEPOINT citus_explain_savepoint
-DETAIL:  on server postgres@localhost:xxxxx connectionId: xxxxxxx
-NOTICE:  issuing EXPLAIN (ANALYZE FALSE, VERBOSE FALSE, COSTS TRUE, BUFFERS FALSE, WAL FALSE, GENERIC_PLAN TRUE, TIMING FALSE, SUMMARY FALSE, FORMAT TEXT) SELECT unique1 FROM pg16.tenk1_950001 tenk1 WHERE (thousand OPERATOR(pg_catalog.=) 1000)
-DETAIL:  on server postgres@localhost:xxxxx connectionId: xxxxxxx
-NOTICE:  issuing ROLLBACK TO SAVEPOINT citus_explain_savepoint
-DETAIL:  on server postgres@localhost:xxxxx connectionId: xxxxxxx
-NOTICE:  issuing COMMIT
-DETAIL:  on server postgres@localhost:xxxxx connectionId: xxxxxxx
-                                   QUERY PLAN
----------------------------------------------------------------------
- Custom Scan (Citus Adaptive)  (cost=0.00..0.00 rows=0 width=0)
-   Task Count: 1
-   Tasks Shown: All
-   ->  Task
-         Node: host=localhost port=xxxxx dbname=regression
-         ->  Seq Scan on tenk1_950001 tenk1  (cost=0.00..35.50 rows=10 width=4)
-               Filter: (thousand = 1000)
-(7 rows)
-
-EXPLAIN (GENERIC_PLAN, ANALYZE) SELECT unique1 FROM tenk1 WHERE thousand = 1000;
 ERROR:  EXPLAIN options ANALYZE and GENERIC_PLAN cannot be used together
 SET citus.log_remote_commands TO off;
 -- Proper error when creating statistics without a name on a Citus table

src/test/regress/sql/pg16.sql

@@ -58,8 +58,8 @@ CREATE TABLE tenk1 (
 SELECT create_distributed_table('tenk1', 'unique1');
 
 SET citus.log_remote_commands TO on;
-EXPLAIN (GENERIC_PLAN) SELECT unique1 FROM tenk1 WHERE thousand = 1000;
+EXPLAIN (GENERIC_PLAN) SELECT unique1 FROM tenk1 WHERE thousand = $1;
-EXPLAIN (GENERIC_PLAN, ANALYZE) SELECT unique1 FROM tenk1 WHERE thousand = 1000;
+EXPLAIN (GENERIC_PLAN, ANALYZE) SELECT unique1 FROM tenk1 WHERE thousand = $1;
 SET citus.log_remote_commands TO off;
 
 -- Proper error when creating statistics without a name on a Citus table