Raise error on concurrent drop of partitioned index

We were already raising an error for DROP INDEX CONCURRENTLY on a
partitioned table, albeit a different and confusing one:
  ERROR:  DROP INDEX CONCURRENTLY must be first action in transaction

Change that to throw a more comprehensible error:
  ERROR:  cannot drop partitioned index \"%s\" concurrently

Michael Paquier authored the test case for indexes on temporary
partitioned tables.

Backpatch to 11, where indexes on partitioned tables were added.

Reported-by: Jan Mussler <jan.mussler@zalando.de>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/16594-d2956ca909585067@postgresql.org

doc/src/sgml/ref/drop_index.sgml

@@ -57,6 +57,8 @@ DROP INDEX [ CONCURRENTLY ] [ IF EXISTS ] <replaceable class="parameter">name</r
       Also, regular <command>DROP INDEX</command> commands can be
       performed within a transaction block, but
       <command>DROP INDEX CONCURRENTLY</command> cannot.
+      Lastly, indexes on partitioned tables cannot be dropped using this
+      option.
      </para>
      <para>
       For temporary tables, <command>DROP INDEX</command> is always

src/backend/commands/tablecmds.c

@@ -1347,6 +1347,17 @@ RemoveRelations(DropStmt *drop)
 			flags |= PERFORM_DELETION_CONCURRENTLY;
 		}
 
+		/*
+		 * Concurrent index drop cannot be used with partitioned indexes,
+		 * either.
+		 */
+		if ((flags & PERFORM_DELETION_CONCURRENTLY) != 0 &&
+			get_rel_relkind(relOid) == RELKIND_PARTITIONED_INDEX)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot drop partitioned index \"%s\" concurrently",
+							rel->relname)));
+
 		/* OK, we're ready to delete this one */
 		obj.classId = RelationRelationId;
 		obj.objectId = relOid;

src/test/regress/expected/indexing.out

@@ -173,6 +173,8 @@ create table idxpart1 partition of idxpart for values from (0) to (10);
 drop index idxpart1_a_idx;	-- no way
 ERROR:  cannot drop index idxpart1_a_idx because index idxpart_a_idx requires it
 HINT:  You can drop index idxpart_a_idx instead.
+drop index concurrently idxpart_a_idx;	-- unsupported
+ERROR:  cannot drop partitioned index "idxpart_a_idx" concurrently
 drop index idxpart_a_idx;	-- both indexes go away
 select relname, relkind from pg_class
   where relname like 'idxpart%' order by relname;
@@ -193,6 +195,24 @@ select relname, relkind from pg_class
 (2 rows)
 
 drop table idxpart;
+-- DROP behavior with temporary partitioned indexes
+create temp table idxpart_temp (a int) partition by range (a);
+create index on idxpart_temp(a);
+create temp table idxpart1_temp partition of idxpart_temp
+  for values from (0) to (10);
+drop index idxpart1_temp_a_idx; -- error
+ERROR:  cannot drop index idxpart1_temp_a_idx because index idxpart_temp_a_idx requires it
+HINT:  You can drop index idxpart_temp_a_idx instead.
+-- non-concurrent drop is enforced here, so it is a valid case.
+drop index concurrently idxpart_temp_a_idx;
+select relname, relkind from pg_class
+  where relname like 'idxpart_temp%' order by relname;
+   relname    | relkind 
+--------------+---------
+ idxpart_temp | p
+(1 row)
+
+drop table idxpart_temp;
 -- ALTER INDEX .. ATTACH, error cases
 create table idxpart (a int, b int) partition by range (a, b);
 create table idxpart1 partition of idxpart for values from (0, 0) to (10, 10);

src/test/regress/sql/indexing.sql

@@ -92,6 +92,7 @@ create table idxpart (a int) partition by range (a);
 create index on idxpart (a);
 create table idxpart1 partition of idxpart for values from (0) to (10);
 drop index idxpart1_a_idx;	-- no way
+drop index concurrently idxpart_a_idx;	-- unsupported
 drop index idxpart_a_idx;	-- both indexes go away
 select relname, relkind from pg_class
   where relname like 'idxpart%' order by relname;
@@ -101,6 +102,18 @@ select relname, relkind from pg_class
   where relname like 'idxpart%' order by relname;
 drop table idxpart;
 
+-- DROP behavior with temporary partitioned indexes
+create temp table idxpart_temp (a int) partition by range (a);
+create index on idxpart_temp(a);
+create temp table idxpart1_temp partition of idxpart_temp
+  for values from (0) to (10);
+drop index idxpart1_temp_a_idx; -- error
+-- non-concurrent drop is enforced here, so it is a valid case.
+drop index concurrently idxpart_temp_a_idx;
+select relname, relkind from pg_class
+  where relname like 'idxpart_temp%' order by relname;
+drop table idxpart_temp;
+
 -- ALTER INDEX .. ATTACH, error cases
 create table idxpart (a int, b int) partition by range (a, b);
 create table idxpart1 partition of idxpart for values from (0, 0) to (10, 10);