Support ALTER TABLE SET DISTRIBUTED BY for external tables

The external tables have distribution policy but it does not dictate
actual data distribution. It is only used when unloading data, to
compare with source table's distribution policy.

Therefore, when supporting ALTER TABLE SET DISTRIBUTED BY for external
tables, we don't really need to re-organize the table but just need
to make sure the catalog change happens.

Author: huansong

src/backend/commands/tablecmds.c

@@ -5563,7 +5563,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 				pass = AT_PASS_MISC;
 				break;
 			}
-			ATSimplePermissions(rel, ATT_TABLE | ATT_DIRECTORY_TABLE);
+			ATSimplePermissions(rel, ATT_TABLE | ATT_DIRECTORY_TABLE | ATT_FOREIGN_TABLE);
 
 			if (!recursing) /* MPP-5772, MPP-5784 */
 			{
@@ -18620,6 +18620,12 @@ ATExecSetDistributedBy(Relation rel, Node *node, AlterTableCmd *cmd)
 
 			lwith = nlist;
 		}
+		/* External tables cannot really be re-organized. Error out if we are instructed to do so.*/
+		if (force_reorg && rel_is_external_table(RelationGetRelid(rel)))
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot reorganize external table \"%s\"",
+							RelationGetRelationName(rel))));
 
 		if (ldistro)
 			change_policy = true;
@@ -18853,7 +18859,8 @@ ATExecSetDistributedBy(Relation rel, Node *node, AlterTableCmd *cmd)
 		{
 			need_reorg = true;
 		}
-		else if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		else if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE ||
+			rel_is_external_table(RelationGetRelid(rel)))
 			need_reorg = false;
 		else
 			elog(ERROR, "unexpected relkind '%c'", rel->rd_rel->relkind);

src/test/regress/input/external_table.source

@@ -3614,9 +3614,6 @@ ALTER TABLE ext_w_dist SET DISTRIBUTED BY (b);
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'ext_w_dist'::regclass;
 ALTER TABLE ext_w_dist SET DISTRIBUTED RANDOMLY;
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'ext_w_dist'::regclass;
-CREATE EXTERNAL WEB TABLE ext_r_dist(a int) EXECUTE 'printf ${GP_SEGMENT_ID}' FORMAT 'TEXT' DISTRIBUTED BY (a);
-CREATE EXTERNAL WEB TABLE ext_r_dist(a int) EXECUTE 'printf ${GP_SEGMENT_ID}' FORMAT 'TEXT';
-ALTER TABLE ext_r_dist SET DISTRIBUTED BY (a); -- should error out altering readable external tables' distribution policy
 
 -- Testing external table as the partition child.
 CREATE TABLE part_root(a int) PARTITION BY RANGE(a);
@@ -3629,7 +3626,7 @@ ALTER TABLE part_root ATTACH PARTITION part_ext_w FOR VALUES FROM (10) TO (20);
 ALTER TABLE part_root ADD COLUMN b int;
 INSERT INTO part_root SELECT i,i FROM generate_series(1,19)i;
 
--- altering distribution policy should work fine
+-- altering distribution policy should work fine 
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'part_ext_w'::regclass;
 ALTER TABLE part_root SET DISTRIBUTED BY (b);
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'part_ext_w'::regclass;

src/test/regress/output/external_table.source

@@ -4916,34 +4916,27 @@ DROP TABLE test_part_integrity;
 -- Testing altering the distribution policy of external tables.
 CREATE WRITABLE EXTERNAL WEB TABLE ext_w_dist(a int, b int) EXECUTE 'cat > @abs_srcdir@/data/ext_w_dist.tbl' FORMAT 'TEXT' (DELIMITER AS '|' NULL AS 'null' ESCAPE AS ' ') DISTRIBUTED BY (a);
 ALTER TABLE ext_w_dist SET WITH (reorganize=true); -- should error out if forcing reorganize
-ERROR:  "ext_w_dist" is not a table or directory table
+ERROR:  cannot reorganize external table "ext_w_dist"
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'ext_w_dist'::regclass;
  policytype | distkey 
 ------------+---------
  p          | 1
 (1 row)
 
 ALTER TABLE ext_w_dist SET DISTRIBUTED BY (b);
-ERROR:  "ext_w_dist" is not a table or directory table
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'ext_w_dist'::regclass;
  policytype | distkey 
 ------------+---------
- p          | 1
+ p          | 2
 (1 row)
 
 ALTER TABLE ext_w_dist SET DISTRIBUTED RANDOMLY;
-ERROR:  "ext_w_dist" is not a table or directory table
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'ext_w_dist'::regclass;
  policytype | distkey 
 ------------+---------
- p          | 1
+ p          | 
 (1 row)
 
-CREATE EXTERNAL WEB TABLE ext_r_dist(a int) EXECUTE 'printf ${GP_SEGMENT_ID}' FORMAT 'TEXT' DISTRIBUTED BY (a);
-ERROR:  readable external tables can't specify a DISTRIBUTED BY clause
-CREATE EXTERNAL WEB TABLE ext_r_dist(a int) EXECUTE 'printf ${GP_SEGMENT_ID}' FORMAT 'TEXT';
-ALTER TABLE ext_r_dist SET DISTRIBUTED BY (a); -- should error out altering readable external tables' distribution policy
-ERROR:  "ext_r_dist" is not a table or directory table
 -- Testing external table as the partition child.
 CREATE TABLE part_root(a int) PARTITION BY RANGE(a);
 CREATE TABLE part_child (LIKE part_root);
@@ -4953,19 +4946,18 @@ ALTER TABLE part_root ATTACH PARTITION part_ext_w FOR VALUES FROM (10) TO (20);
 -- Adding column should work fine
 ALTER TABLE part_root ADD COLUMN b int;
 INSERT INTO part_root SELECT i,i FROM generate_series(1,19)i;
--- altering distribution policy should work fine
+-- altering distribution policy should work fine 
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'part_ext_w'::regclass;
  policytype | distkey 
 ------------+---------
  p          | 1
 (1 row)
 
 ALTER TABLE part_root SET DISTRIBUTED BY (b);
-ERROR:  "part_ext_w" is not a table or directory table
 SELECT policytype, distkey FROM gp_distribution_policy WHERE localoid = 'part_ext_w'::regclass;
  policytype | distkey 
 ------------+---------
- p          | 1
+ p          | 2
 (1 row)
 
 DROP TABLE part_root;