Add tab completion for partition MERGE/SPLIT operations

This commit implements psql tab completion for ALTER TABLE ... SPLIT PARTITION
and ALTER TABLE ... MERGE PARTITIONS commands.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/5dee3937-8e9f-cca4-11fb-737709a92b37%40gmail.com
Author: Dagfinn Ilmari Mannsåker, Pavel Borisov

src/bin/psql/tab-complete.c

@@ -2353,6 +2353,7 @@ psql_completion(const char *text, int start, int end)
 					  "OWNER TO", "SET", "VALIDATE CONSTRAINT",
 					  "REPLICA IDENTITY", "ATTACH PARTITION",
 					  "DETACH PARTITION", "FORCE ROW LEVEL SECURITY",
+					  "SPLIT PARTITION", "MERGE PARTITIONS (",
 					  "OF", "NOT OF");
 	/* ALTER TABLE xxx ADD */
 	else if (Matches("ALTER", "TABLE", MatchAny, "ADD"))
@@ -2609,10 +2610,10 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH("FROM (", "IN (", "WITH (");
 
 	/*
-	 * If we have ALTER TABLE <foo> DETACH PARTITION, provide a list of
+	 * If we have ALTER TABLE <foo> DETACH|SPLIT PARTITION, provide a list of
 	 * partitions of <foo>.
 	 */
-	else if (Matches("ALTER", "TABLE", MatchAny, "DETACH", "PARTITION"))
+	else if (Matches("ALTER", "TABLE", MatchAny, "DETACH|SPLIT", "PARTITION"))
 	{
 		set_completion_reference(prev3_wd);
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_partition_of_table);
@@ -2620,6 +2621,19 @@ psql_completion(const char *text, int start, int end)
 	else if (Matches("ALTER", "TABLE", MatchAny, "DETACH", "PARTITION", MatchAny))
 		COMPLETE_WITH("CONCURRENTLY", "FINALIZE");
 
+	/* ALTER TABLE <name> SPLIT PARTITION <name> */
+	else if (Matches("ALTER", "TABLE", MatchAny, "SPLIT", "PARTITION", MatchAny))
+		COMPLETE_WITH("INTO ( PARTITION");
+
+	/* ALTER TABLE <name> MERGE PARTITIONS ( */
+	else if (Matches("ALTER", "TABLE", MatchAny, "MERGE", "PARTITIONS", "("))
+	{
+		set_completion_reference(prev4_wd);
+		COMPLETE_WITH_SCHEMA_QUERY(Query_for_partition_of_table);
+	}
+	else if (Matches("ALTER", "TABLE", MatchAny, "MERGE", "PARTITIONS", "(*)"))
+		COMPLETE_WITH("INTO");
+
 	/* ALTER TABLE <name> OF */
 	else if (Matches("ALTER", "TABLE", MatchAny, "OF"))
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_composite_datatypes);