Add debugging GUC for reduced compression locking

This GUC is used to check if reducing the locking during
compression can help with locking issues around rolling
up compressed chunks and cagg refresh.

src/guc.c

@@ -125,6 +125,7 @@ DebugRequireOption ts_guc_debug_require_vector_agg = DRO_Allow;
 #endif
 
 bool ts_guc_debug_compression_path_info = false;
+bool ts_guc_debug_reduce_compression_locking = false;
 
 static bool ts_guc_enable_hypertable_create = true;
 static bool ts_guc_enable_hypertable_compression = true;
@@ -832,6 +833,17 @@ _guc_init(void)
 							 /* assign_hook= */ NULL,
 							 /* show_hook= */ NULL);
 
+	DefineCustomBoolVariable(/* name= */ MAKE_EXTOPTION("debug_reduce_compression_locking"),
+							 /* short_desc= */ "Reduce locking behavior for compression operations",
+							 /* long_desc= */ "this is for debugging purposes",
+							 /* valueAddr= */ &ts_guc_debug_reduce_compression_locking,
+							 /* bootValue= */ false,
+							 /* context= */ PGC_USERSET,
+							 /* flags= */ 0,
+							 /* check_hook= */ NULL,
+							 /* assign_hook= */ NULL,
+							 /* show_hook= */ NULL);
+
 #ifdef USE_TELEMETRY
 	DefineCustomStringVariable(/* name= */ "timescaledb_telemetry.cloud",
 							   /* short_desc= */ "cloud provider",

src/guc.h

@@ -92,6 +92,7 @@ extern TSDLLEXPORT DebugRequireOption ts_guc_debug_require_vector_agg;
 #endif
 
 extern TSDLLEXPORT bool ts_guc_debug_compression_path_info;
+extern TSDLLEXPORT bool ts_guc_debug_reduce_compression_locking;
 
 extern TSDLLEXPORT bool ts_guc_debug_require_batch_sorted_merge;
 

tsl/src/compression/compression.c

@@ -266,8 +266,16 @@ compress_chunk(Oid in_table, Oid out_table, int insert_options)
 	 * a RowExclusive lock, and let other operations read and write this table
 	 * as we work. However, we currently compress each table as a oneshot, so
 	 * we're taking the stricter lock to prevent accidents.
+	 *
+	 * Putting RowExclusiveMode behind a GUC so we can try this out with
+	 * rollups during compression.
 	 */
-	Relation out_rel = relation_open(out_table, ExclusiveLock);
+	int out_rel_lockmode = ExclusiveLock;
+	if (ts_guc_debug_reduce_compression_locking)
+	{
+		out_rel_lockmode = RowExclusiveLock;
+	}
+	Relation out_rel = relation_open(out_table, out_rel_lockmode);
 
 	/* Sanity check we are dealing with relations */
 	Ensure(in_rel->rd_rel->relkind == RELKIND_RELATION, "compress_chunk called on non-relation");

tsl/test/expected/compression_merge.out

@@ -820,3 +820,28 @@ NOTICE:  chunk "_hyper_17_344_chunk" is already compressed
 (1 row)
 
 ROLLBACK;
+-- Test RowExclusiveLock on compressed chunk during chunk rollup using a GUC
+CREATE TABLE test10 ("Time" timestamptz, i integer, value integer);
+SELECT table_name from create_hypertable('test10', 'Time', chunk_time_interval=> INTERVAL '1 hour');
+NOTICE:  adding not-null constraint to column "Time"
+ table_name 
+------------
+ test10
+(1 row)
+
+INSERT INTO test10
+SELECT t, i, gen_rand_minstd()
+FROM generate_series('2018-03-02 1:00'::TIMESTAMPTZ, '2018-03-02 3:59', '10 minutes') t
+CROSS JOIN generate_series(1, 3, 1) i;
+ALTER TABLE test10 set (timescaledb.compress, timescaledb.compress_segmentby='i', timescaledb.compress_orderby='"Time"', timescaledb.compress_chunk_time_interval='2 hours');
+SET timescaledb.debug_reduce_compression_locking to on;
+SELECT compress_chunk(show_chunks('test10'));
+              compress_chunk               
+-------------------------------------------
+ _timescaledb_internal._hyper_19_351_chunk
+ _timescaledb_internal._hyper_19_351_chunk
+ _timescaledb_internal._hyper_19_353_chunk
+(3 rows)
+
+RESET timescaledb.debug_reduce_compression_locking;
+DROP TABLE test10;

tsl/test/sql/compression_merge.sql

@@ -307,3 +307,20 @@ BEGIN;
   -- should be rolled up
   SELECT hypertable_name, range_start, range_end FROM timescaledb_information.chunks WHERE hypertable_name = 'test9' ORDER BY 2;
 ROLLBACK;
+
+-- Test RowExclusiveLock on compressed chunk during chunk rollup using a GUC
+CREATE TABLE test10 ("Time" timestamptz, i integer, value integer);
+SELECT table_name from create_hypertable('test10', 'Time', chunk_time_interval=> INTERVAL '1 hour');
+
+INSERT INTO test10
+SELECT t, i, gen_rand_minstd()
+FROM generate_series('2018-03-02 1:00'::TIMESTAMPTZ, '2018-03-02 3:59', '10 minutes') t
+CROSS JOIN generate_series(1, 3, 1) i;
+
+ALTER TABLE test10 set (timescaledb.compress, timescaledb.compress_segmentby='i', timescaledb.compress_orderby='"Time"', timescaledb.compress_chunk_time_interval='2 hours');
+SET timescaledb.debug_reduce_compression_locking to on;
+
+SELECT compress_chunk(show_chunks('test10'));
+
+RESET timescaledb.debug_reduce_compression_locking;
+DROP TABLE test10;