Skip 'already exists' in CREATE TABLE IF NOT EXISTS PARTITION OF (#4507)

* Just skip 'already exists' in CT IF NOT EXISTS PARTITION OF

* Generalize to tables that are not already distributed partitions

src/backend/distributed/commands/table.c

@@ -283,13 +283,40 @@ PostprocessCreateTableStmtPartitionOf(CreateStmt *createStatement, const
 
 	Assert(parentRelationId != InvalidOid);
 
+	Oid relationId = RangeVarGetRelid(createStatement->relation, NoLock, missingOk);
+
+	/*
+	 * In case of an IF NOT EXISTS statement, Postgres lets it pass through the
+	 * standardProcess_Utility, and gets into this Post-process hook by
+	 * ignoring the statement if the table already exists. Thus, we need to make
+	 * sure Citus behaves like plain PG in case the relation already exists.
+	 */
+	if (createStatement->if_not_exists)
+	{
+		if (IsCitusTable(relationId))
+		{
+			/*
+			 * Ignore if the relation is already distributed.
+			 */
+			return;
+		}
+		else if (!PartitionTable(relationId) ||
+				 PartitionParentOid(relationId) != parentRelationId)
+		{
+			/*
+			 * Ignore if the relation is not a partition, or if that
+			 * partition's parent is not the current parent from parentRelationId
+			 */
+			return;
+		}
+	}
+
 	/*
 	 * If a partition is being created and if its parent is a distributed
 	 * table, we will distribute this table as well.
 	 */
 	if (IsCitusTable(parentRelationId))
 	{
-		Oid relationId = RangeVarGetRelid(createStatement->relation, NoLock, missingOk);
 		Var *parentDistributionColumn = DistPartitionKeyOrError(parentRelationId);
 		char parentDistributionMethod = DISTRIBUTE_BY_HASH;
 		char *parentRelationName = generate_qualified_relation_name(parentRelationId);

src/test/regress/expected/multi_partitioning.out

@@ -1958,12 +1958,43 @@ SELECT parent_table, partition_column, partition, from_value, to_value FROM time
  public.non_distributed_partitioned_table | a                | public.non_distributed_partitioned_table_1 | 0          | 10
 (6 rows)
 
+-- create the same partition to verify it behaves like in plain PG
+CREATE TABLE partitioning_test_2011 PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+ERROR:  relation "partitioning_test_2011" already exists
+CREATE TABLE IF NOT EXISTS partitioning_test_2011 PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+NOTICE:  relation "partitioning_test_2011" already exists, skipping
+-- verify we can create a partition that doesn't already exist with IF NOT EXISTS
+CREATE TABLE IF NOT EXISTS partitioning_test_2013 PARTITION OF partitioning_test FOR VALUES FROM ('2013-01-01') TO ('2014-01-01');
+SELECT logicalrelid FROM pg_dist_partition WHERE logicalrelid IN ('partitioning_test', 'partitioning_test_2013') ORDER BY 1;
+      logicalrelid
+---------------------------------------------------------------------
+ partitioning_test
+ partitioning_test_2013
+(2 rows)
+
+-- create the same table but that is not a partition and verify it behaves like in plain PG
+CREATE TABLE not_partition(time date);
+CREATE TABLE not_partition PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+ERROR:  relation "not_partition" already exists
+CREATE TABLE IF NOT EXISTS not_partition PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+NOTICE:  relation "not_partition" already exists, skipping
+DROP TABLE not_partition;
+-- verify it skips when the partition with the same name belongs to another table
+CREATE TABLE another_table(id int, time date) PARTITION BY RANGE (time);
+CREATE TABLE partition_of_other_table PARTITION OF another_table FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+CREATE TABLE partition_of_other_table PARTITION OF partitioning_test FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+ERROR:  relation "partition_of_other_table" already exists
+CREATE TABLE IF NOT EXISTS partition_of_other_table PARTITION OF partitioning_test FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+NOTICE:  relation "partition_of_other_table" already exists, skipping
+ALTER TABLE another_table DETACH PARTITION partition_of_other_table;
+DROP TABLE another_table, partition_of_other_table;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2008;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2009;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2010;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2011;
+ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2013;
 DROP TABLE partitioning_test, partitioning_test_2008, partitioning_test_2009,
-           partitioning_test_2010, partitioning_test_2011,
+           partitioning_test_2010, partitioning_test_2011, partitioning_test_2013,
            reference_table, reference_table_2;
 RESET SEARCH_PATH;
 -- not timestamp partitioned

src/test/regress/sql/multi_partitioning.sql

@@ -1155,13 +1155,36 @@ ALTER TABLE partitioning_test ATTACH PARTITION partitioning_test_2011
 
 SELECT parent_table, partition_column, partition, from_value, to_value FROM time_partitions;
 
+-- create the same partition to verify it behaves like in plain PG
+CREATE TABLE partitioning_test_2011 PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+CREATE TABLE IF NOT EXISTS partitioning_test_2011 PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+
+-- verify we can create a partition that doesn't already exist with IF NOT EXISTS
+CREATE TABLE IF NOT EXISTS partitioning_test_2013 PARTITION OF partitioning_test FOR VALUES FROM ('2013-01-01') TO ('2014-01-01');
+SELECT logicalrelid FROM pg_dist_partition WHERE logicalrelid IN ('partitioning_test', 'partitioning_test_2013') ORDER BY 1;
+
+-- create the same table but that is not a partition and verify it behaves like in plain PG
+CREATE TABLE not_partition(time date);
+CREATE TABLE not_partition PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+CREATE TABLE IF NOT EXISTS not_partition PARTITION OF partitioning_test FOR VALUES FROM ('2011-01-01') TO ('2012-01-01');
+DROP TABLE not_partition;
+
+-- verify it skips when the partition with the same name belongs to another table
+CREATE TABLE another_table(id int, time date) PARTITION BY RANGE (time);
+CREATE TABLE partition_of_other_table PARTITION OF another_table FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+CREATE TABLE partition_of_other_table PARTITION OF partitioning_test FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+CREATE TABLE IF NOT EXISTS partition_of_other_table PARTITION OF partitioning_test FOR VALUES FROM ('2014-01-01') TO ('2015-01-01');
+ALTER TABLE another_table DETACH PARTITION partition_of_other_table;
+DROP TABLE another_table, partition_of_other_table;
+
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2008;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2009;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2010;
 ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2011;
+ALTER TABLE partitioning_test DETACH PARTITION partitioning_test_2013;
 
 DROP TABLE partitioning_test, partitioning_test_2008, partitioning_test_2009,
-           partitioning_test_2010, partitioning_test_2011,
+           partitioning_test_2010, partitioning_test_2011, partitioning_test_2013,
            reference_table, reference_table_2;
 
 RESET SEARCH_PATH;