Handle MERGE command in reference join pushdown

At the moment, the MERGE command is not supported on distributed
hypertables. This patch ensures that the join pushdown code ignores the
invocation by the MERGE command.

tsl/src/fdw/data_node_scan_plan.c

@@ -1176,6 +1176,12 @@ data_node_generate_pushdown_join_paths(PlannerInfo *root, RelOptInfo *joinrel, R
 	if (joinrel->fdw_private)
 		return;
 
+	/* Distributed hypertables are not supported by MERGE at the moment. Ensure that
+	 * we perform our planning only on SELECTs.
+	 */
+	if (root->parse->commandType != CMD_SELECT)
+		return;
+
 #ifdef ENABLE_DEAD_CODE
 	/*
 	 * This code does not work for joins with lateral references, since those

tsl/test/expected/dist_ref_table_join-12.out

@@ -1954,6 +1954,25 @@ DEBUG:  join pushdown on reference table is not supported for the used query
                            Output: metric_name.name
 (30 rows)
 
+-------
+-- MERGE is supported in PG >= 15. Currently, it is not supported in TimescaleDB
+-- on distributed hypertables. Perform a MERGE here to check if the join pushdown
+-- can handle the MERGE command properly. ON_ERROR_STOP is disabled for this test.
+-- Older PostgreSQL versions report an error because MERGE is not supported. This
+-- will be ignored due to the setting.
+-------
+\set ON_ERROR_STOP 0
+MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+ERROR:  syntax error at or near "MERGE" at character 1
+\set ON_ERROR_STOP 1
 -------
 -- Tests without enable_per_data_node_queries (no pushdown supported)
 -------

tsl/test/expected/dist_ref_table_join-13.out

@@ -1954,6 +1954,25 @@ DEBUG:  join pushdown on reference table is not supported for the used query
                            Output: metric_name.name
 (30 rows)
 
+-------
+-- MERGE is supported in PG >= 15. Currently, it is not supported in TimescaleDB
+-- on distributed hypertables. Perform a MERGE here to check if the join pushdown
+-- can handle the MERGE command properly. ON_ERROR_STOP is disabled for this test.
+-- Older PostgreSQL versions report an error because MERGE is not supported. This
+-- will be ignored due to the setting.
+-------
+\set ON_ERROR_STOP 0
+MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+ERROR:  syntax error at or near "MERGE" at character 1
+\set ON_ERROR_STOP 1
 -------
 -- Tests without enable_per_data_node_queries (no pushdown supported)
 -------

tsl/test/expected/dist_ref_table_join-14.out

@@ -1954,6 +1954,25 @@ DEBUG:  join pushdown on reference table is not supported for the used query
                            Output: metric_name.name
 (30 rows)
 
+-------
+-- MERGE is supported in PG >= 15. Currently, it is not supported in TimescaleDB
+-- on distributed hypertables. Perform a MERGE here to check if the join pushdown
+-- can handle the MERGE command properly. ON_ERROR_STOP is disabled for this test.
+-- Older PostgreSQL versions report an error because MERGE is not supported. This
+-- will be ignored due to the setting.
+-------
+\set ON_ERROR_STOP 0
+MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+ERROR:  syntax error at or near "MERGE" at character 1
+\set ON_ERROR_STOP 1
 -------
 -- Tests without enable_per_data_node_queries (no pushdown supported)
 -------

tsl/test/expected/dist_ref_table_join-15.out

@@ -1998,6 +1998,34 @@ DEBUG:  join pushdown on reference table is not supported for the used query
                            Output: metric_name.name
 (30 rows)
 
+-------
+-- MERGE is supported in PG >= 15. Currently, it is not supported in TimescaleDB
+-- on distributed hypertables. Perform a MERGE here to check if the join pushdown
+-- can handle the MERGE command properly. ON_ERROR_STOP is disabled for this test.
+-- Older PostgreSQL versions report an error because MERGE is not supported. This
+-- will be ignored due to the setting.
+-------
+\set ON_ERROR_STOP 0
+MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+LOG:  statement: MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+ERROR:  The MERGE command does not support hypertables in this version
+\set ON_ERROR_STOP 1
 -------
 -- Tests without enable_per_data_node_queries (no pushdown supported)
 -------

tsl/test/sql/dist_ref_table_join.sql.in

@@ -406,6 +406,25 @@ FROM metric JOIN metric_name ON texteq(concat('cpu', textin(int4out(metric.id)))
 GROUP BY name
 ORDER BY name;
 
+-------
+-- MERGE is supported in PG >= 15. Currently, it is not supported in TimescaleDB
+-- on distributed hypertables. Perform a MERGE here to check if the join pushdown
+-- can handle the MERGE command properly. ON_ERROR_STOP is disabled for this test.
+-- Older PostgreSQL versions report an error because MERGE is not supported. This
+-- will be ignored due to the setting.
+-------
+\set ON_ERROR_STOP 0
+MERGE INTO metric as target_0
+USING metric as input_0
+    inner join (select id from metric_name as input_1) as subq_0
+      ON (TRUE)
+ON target_0.id = input_0.id
+WHEN MATCHED
+   THEN DO NOTHING
+WHEN NOT MATCHED
+   THEN DO NOTHING;
+\set ON_ERROR_STOP 1
+
 -------
 -- Tests without enable_per_data_node_queries (no pushdown supported)
 -------