Reset attislocal on chunks attached to a hypertable

When attaching a chunk to a hypertable we would mark the relation as
inherited but leave the columns marked as local so a later DROP COLUMN
on the hypertable would not propagate to the chunk.

Clear the local flag on the inherited columns when a chunk is attached
so column changes on the hypertable propagate to it. Also add a
migration that fixes chunks already left in this state by an earlier
detach and attach round-trip.

Author: svenklemm

.unreleased/fix_attach_chunk_attislocal

@@ -0,0 +1 @@
+Fixes: #10352 Reset attislocal on chunks during attach_chunk

sql/updates/latest-dev.sql

@@ -0,0 +1,11 @@
+-- Reset attislocal on chunk columns left marked local by a
+-- detach_chunk/attach_chunk round-trip so a later hypertable DROP COLUMN
+-- propagates to them. Restricted to chunk relations via the catalog join.
+UPDATE pg_catalog.pg_attribute a
+SET attislocal = false
+FROM _timescaledb_catalog.chunk c
+WHERE a.attrelid = c.relid
+  AND a.attnum > 0
+  AND NOT a.attisdropped
+  AND a.attislocal
+  AND a.attinhcount > 0;

src/chunk.c

@@ -13,6 +13,7 @@
 #include <access/xact.h>
 #include <catalog/indexing.h>
 #include <catalog/namespace.h>
+#include <catalog/pg_attribute.h>
 #include <catalog/pg_class.h>
 #include <catalog/pg_constraint.h>
 #include <catalog/pg_inherits.h>
@@ -1116,6 +1117,47 @@ chunk_create_from_hypercube_after_lock(const Hypertable *ht, Hypercube *cube,
 	return chunk;
 }
 
+/*
+ * A chunk should have all columns inherited and none marked as local, so clear
+ * the attislocal flag that ALTER TABLE ... INHERIT leaves set when attaching
+ * a pre-existing table. Otherwise a later DROP COLUMN on the hypertable would
+ * not propagate to the chunk.
+ */
+static void
+chunk_reset_attislocal(Oid chunk_relid)
+{
+	Relation attrel = table_open(AttributeRelationId, RowExclusiveLock);
+	Relation chunkrel = table_open(chunk_relid, AccessShareLock);
+	TupleDesc tupdesc = RelationGetDescr(chunkrel);
+
+	for (int i = 0; i < tupdesc->natts; i++)
+	{
+		Form_pg_attribute att = TupleDescAttr(tupdesc, i);
+
+		/* Only touch inherited user columns that are still marked local */
+		if (att->attnum <= 0 || att->attisdropped || !att->attislocal || att->attinhcount == 0)
+		{
+			continue;
+		}
+
+		HeapTuple tuple = SearchSysCacheCopyAttNum(chunk_relid, att->attnum);
+		if (!HeapTupleIsValid(tuple))
+		{
+			elog(ERROR,
+				 "cache lookup failed for attribute %d of relation %u",
+				 att->attnum,
+				 chunk_relid);
+		}
+
+		((Form_pg_attribute) GETSTRUCT(tuple))->attislocal = false;
+		CatalogTupleUpdate(attrel, &tuple->t_self, tuple);
+		heap_freetuple(tuple);
+	}
+
+	table_close(chunkrel, NoLock);
+	table_close(attrel, RowExclusiveLock);
+}
+
 /*
  * Make a chunk table inherit a hypertable.
  *
@@ -1149,6 +1191,8 @@ chunk_add_inheritance(Chunk *chunk, const Hypertable *ht)
 	};
 
 	AlterTable(&alterstmt, lockmode, &atcontext);
+
+	chunk_reset_attislocal(atcontext.relid);
 }
 
 static Chunk *

tsl/test/expected/attach_chunk.out

@@ -235,3 +235,38 @@ DROP TABLE regular_table_to_attach;
 DROP TABLE attach_test_ref;
 DROP TABLE attach_test;
 DROP TABLE devices CASCADE;
+-- Test dropping columns after detach/attach
+CREATE TABLE drop_after_attach(time timestamptz NOT NULL, x int);
+SELECT create_hypertable('drop_after_attach', 'time', chunk_time_interval => interval '1 day');
+       create_hypertable        
+--------------------------------
+ (3,public,drop_after_attach,t)
+
+INSERT INTO drop_after_attach VALUES ('2026-01-01', 1), ('2026-01-05', 1);
+SELECT schema_name || '.' || table_name AS "ROUNDTRIP_CHUNK", slices AS "ROUNDTRIP_SLICES"
+FROM _timescaledb_functions.show_chunk((SELECT show_chunks('drop_after_attach', older_than => '2026-01-02') LIMIT 1)); \gset
+             ROUNDTRIP_CHUNK             |                ROUNDTRIP_SLICES                
+-----------------------------------------+------------------------------------------------
+ _timescaledb_internal._hyper_3_15_chunk | {"time": [1767225600000000, 1767312000000000]}
+
+CALL detach_chunk(:'ROUNDTRIP_CHUNK');
+CALL attach_chunk('drop_after_attach', :'ROUNDTRIP_CHUNK', :'ROUNDTRIP_SLICES');
+-- The re-attached chunk inherits without any locally-defined columns.
+SELECT attname, attislocal, attinhcount
+FROM pg_attribute
+WHERE attrelid = :'ROUNDTRIP_CHUNK'::regclass AND attnum > 0 AND NOT attisdropped
+ORDER BY attnum;
+ attname | attislocal | attinhcount 
+---------+------------+-------------
+ time    | f          |           1
+ x       | f          |           1
+
+ALTER TABLE drop_after_attach DROP COLUMN x;
+-- The column is gone from the round-tripped chunk, not left orphaned.
+SELECT attname, attislocal, attinhcount
+FROM pg_attribute
+WHERE attrelid = :'ROUNDTRIP_CHUNK'::regclass AND attname = 'x' AND NOT attisdropped;
+ attname | attislocal | attinhcount 
+---------+------------+-------------
+
+DROP TABLE drop_after_attach;

tsl/test/sql/attach_chunk.sql

@@ -186,3 +186,29 @@ DROP TABLE regular_table_to_attach;
 DROP TABLE attach_test_ref;
 DROP TABLE attach_test;
 DROP TABLE devices CASCADE;
+
+-- Test dropping columns after detach/attach
+CREATE TABLE drop_after_attach(time timestamptz NOT NULL, x int);
+SELECT create_hypertable('drop_after_attach', 'time', chunk_time_interval => interval '1 day');
+INSERT INTO drop_after_attach VALUES ('2026-01-01', 1), ('2026-01-05', 1);
+
+SELECT schema_name || '.' || table_name AS "ROUNDTRIP_CHUNK", slices AS "ROUNDTRIP_SLICES"
+FROM _timescaledb_functions.show_chunk((SELECT show_chunks('drop_after_attach', older_than => '2026-01-02') LIMIT 1)); \gset
+
+CALL detach_chunk(:'ROUNDTRIP_CHUNK');
+CALL attach_chunk('drop_after_attach', :'ROUNDTRIP_CHUNK', :'ROUNDTRIP_SLICES');
+
+-- The re-attached chunk inherits without any locally-defined columns.
+SELECT attname, attislocal, attinhcount
+FROM pg_attribute
+WHERE attrelid = :'ROUNDTRIP_CHUNK'::regclass AND attnum > 0 AND NOT attisdropped
+ORDER BY attnum;
+
+ALTER TABLE drop_after_attach DROP COLUMN x;
+
+-- The column is gone from the round-tripped chunk, not left orphaned.
+SELECT attname, attislocal, attinhcount
+FROM pg_attribute
+WHERE attrelid = :'ROUNDTRIP_CHUNK'::regclass AND attname = 'x' AND NOT attisdropped;
+
+DROP TABLE drop_after_attach;