Fix segfault in UNION queries with top-level ordering

We can't just filter the equivalence member based on em_is_child as our
hypertable might not be top-level equivalence member.

.unreleased/pr_6957

@@ -0,0 +1 @@
+Fixes: #6957 Fix segfault in UNION queries with ordering on compressed chunks

tsl/src/nodes/decompress_chunk/decompress_chunk.c

@@ -195,7 +195,7 @@ build_compressed_scan_pathkeys(SortInfo *sort_info, PlannerInfo *root, List *chu
 			 * already refers a compressed column, it is a bug. See
 			 * build_sortinfo().
 			 */
-			Assert(compressed_em != NULL);
+			Ensure(compressed_em, "corresponding equivalence member not found");
 
 			required_compressed_pathkeys = lappend(required_compressed_pathkeys, pk);
 
@@ -1482,17 +1482,6 @@ add_segmentby_to_equivalence_class(PlannerInfo *root, EquivalenceClass *cur_ec,
 		Var *var;
 		Assert(!bms_overlap(cur_em->em_relids, info->compressed_rel->relids));
 
-		/*
-		 * We want to base our equivalence member on the hypertable equivalence
-		 * member, not on the uncompressed chunk one, because the latter is
-		 * marked as child itself. This is mostly relevant for PG16 where we
-		 * have to specify a parent for the newly created equivalence member.
-		 */
-		if (cur_em->em_is_child)
-		{
-			continue;
-		}
-
 		/* only consider EquivalenceMembers that are Vars, possibly with RelabelType, of the
 		 * uncompressed chunk */
 		var = (Var *) cur_em->em_expr;
@@ -1501,6 +1490,13 @@ add_segmentby_to_equivalence_class(PlannerInfo *root, EquivalenceClass *cur_ec,
 		if (!(var && IsA(var, Var)))
 			continue;
 
+		/*
+		 * We want to base our equivalence member on the hypertable equivalence
+		 * member, not on the uncompressed chunk one. We can't just check for
+		 * em_is_child though because the hypertable might be a child itself and not
+		 * a top-level EquivalenceMember. This is mostly relevant for PG16+ where
+		 * we have to specify a parent for the newly created equivalence member.
+		 */
 		if ((Index) var->varno != info->ht_rel->relid)
 			continue;
 

tsl/test/shared/expected/transparent_decompress_chunk-14.out

@@ -1040,3 +1040,27 @@ EXECUTE param_prep (1);
 
 DEALLOCATE param_prep;
 RESET plan_cache_mode;
+-- test hypertable being non-toplevel equivalence member #6925
+CREATE TABLE i6925_t1(observed timestamptz not null, queryid int8, total_exec_time int8);
+CREATE TABLE i6925_t2(LIKE i6925_t1);
+SELECT table_name FROM create_hypertable('i6925_t1', 'observed');
+ table_name 
+ i6925_t1
+(1 row)
+
+ALTER TABLE i6925_t1 SET (timescaledb.compress, timescaledb.compress_segmentby = 'queryid');
+NOTICE:  default order by for hypertable "i6925_t1" is set to "observed DESC"
+INSERT INTO i6925_t1 SELECT '2020-01-01', 1, 1;
+SELECT count(compress_chunk(chunk_name)) FROM show_chunks('i6925_t1') chunk_name;
+ count 
+     1
+(1 row)
+
+SELECT queryid, lag(total_exec_time) OVER (PARTITION BY queryid) FROM (SELECT * FROM i6925_t1 UNION ALL SELECT * FROM i6925_t2) q;
+ queryid | lag 
+---------+-----
+       1 |    
+(1 row)
+
+DROP TABLE i6925_t1;
+DROP TABLE i6925_t2;

tsl/test/shared/expected/transparent_decompress_chunk-15.out

@@ -1042,3 +1042,27 @@ EXECUTE param_prep (1);
 
 DEALLOCATE param_prep;
 RESET plan_cache_mode;
+-- test hypertable being non-toplevel equivalence member #6925
+CREATE TABLE i6925_t1(observed timestamptz not null, queryid int8, total_exec_time int8);
+CREATE TABLE i6925_t2(LIKE i6925_t1);
+SELECT table_name FROM create_hypertable('i6925_t1', 'observed');
+ table_name 
+ i6925_t1
+(1 row)
+
+ALTER TABLE i6925_t1 SET (timescaledb.compress, timescaledb.compress_segmentby = 'queryid');
+NOTICE:  default order by for hypertable "i6925_t1" is set to "observed DESC"
+INSERT INTO i6925_t1 SELECT '2020-01-01', 1, 1;
+SELECT count(compress_chunk(chunk_name)) FROM show_chunks('i6925_t1') chunk_name;
+ count 
+     1
+(1 row)
+
+SELECT queryid, lag(total_exec_time) OVER (PARTITION BY queryid) FROM (SELECT * FROM i6925_t1 UNION ALL SELECT * FROM i6925_t2) q;
+ queryid | lag 
+---------+-----
+       1 |    
+(1 row)
+
+DROP TABLE i6925_t1;
+DROP TABLE i6925_t2;

tsl/test/shared/expected/transparent_decompress_chunk-16.out

@@ -1042,3 +1042,27 @@ EXECUTE param_prep (1);
 
 DEALLOCATE param_prep;
 RESET plan_cache_mode;
+-- test hypertable being non-toplevel equivalence member #6925
+CREATE TABLE i6925_t1(observed timestamptz not null, queryid int8, total_exec_time int8);
+CREATE TABLE i6925_t2(LIKE i6925_t1);
+SELECT table_name FROM create_hypertable('i6925_t1', 'observed');
+ table_name 
+ i6925_t1
+(1 row)
+
+ALTER TABLE i6925_t1 SET (timescaledb.compress, timescaledb.compress_segmentby = 'queryid');
+NOTICE:  default order by for hypertable "i6925_t1" is set to "observed DESC"
+INSERT INTO i6925_t1 SELECT '2020-01-01', 1, 1;
+SELECT count(compress_chunk(chunk_name)) FROM show_chunks('i6925_t1') chunk_name;
+ count 
+     1
+(1 row)
+
+SELECT queryid, lag(total_exec_time) OVER (PARTITION BY queryid) FROM (SELECT * FROM i6925_t1 UNION ALL SELECT * FROM i6925_t2) q;
+ queryid | lag 
+---------+-----
+       1 |    
+(1 row)
+
+DROP TABLE i6925_t1;
+DROP TABLE i6925_t2;

tsl/test/shared/sql/transparent_decompress_chunk.sql.in

@@ -331,3 +331,18 @@ EXECUTE param_prep (2);
 EXECUTE param_prep (1);
 DEALLOCATE param_prep;
 RESET plan_cache_mode;
+
+-- test hypertable being non-toplevel equivalence member #6925
+CREATE TABLE i6925_t1(observed timestamptz not null, queryid int8, total_exec_time int8);
+CREATE TABLE i6925_t2(LIKE i6925_t1);
+
+SELECT table_name FROM create_hypertable('i6925_t1', 'observed');
+ALTER TABLE i6925_t1 SET (timescaledb.compress, timescaledb.compress_segmentby = 'queryid');
+INSERT INTO i6925_t1 SELECT '2020-01-01', 1, 1;
+SELECT count(compress_chunk(chunk_name)) FROM show_chunks('i6925_t1') chunk_name;
+
+SELECT queryid, lag(total_exec_time) OVER (PARTITION BY queryid) FROM (SELECT * FROM i6925_t1 UNION ALL SELECT * FROM i6925_t2) q;
+
+DROP TABLE i6925_t1;
+DROP TABLE i6925_t2;
+