Mark partialize_agg as parallel safe

Postgres knows whether a given aggregate is parallel-safe, and creates
parallel aggregation plans based on that. The `partialize_agg` is a
wrapper we use to perform partial aggregation on data nodes. It is a
pure function that produces serialized aggregation state as a result.
Being pure, it doesn't influence parallel safety. This means we don't
need to mark it parallel-unsafe to artificially disable the parallel
plans for partial aggregation. They will be chosen as usual based on
the parallel-safety of the underlying aggregate function.

sql/partialize_finalize.sql

@@ -2,24 +2,30 @@
 -- Please see the included NOTICE for copyright information and
 -- LICENSE-APACHE for a copy of the license.
 
+-- These wrapper functions are used to push down aggregation to data nodes.
+-- They can be marked parallel-safe, and the parallel plan will be chosen
+-- depending on whether the underlying aggregate function itself is
+-- parallel-safe.
+
 CREATE OR REPLACE FUNCTION _timescaledb_internal.partialize_agg(arg ANYELEMENT)
-RETURNS BYTEA AS '@MODULE_PATHNAME@', 'ts_partialize_agg' LANGUAGE C VOLATILE;
+RETURNS BYTEA AS '@MODULE_PATHNAME@', 'ts_partialize_agg' LANGUAGE C STABLE PARALLEL SAFE;
 
 CREATE OR REPLACE FUNCTION _timescaledb_internal.finalize_agg_sfunc(
 tstate internal, aggfn TEXT, inner_agg_collation_schema NAME, inner_agg_collation_name NAME, inner_agg_input_types NAME[][], inner_agg_serialized_state BYTEA, return_type_dummy_val ANYELEMENT)
 RETURNS internal
 AS '@MODULE_PATHNAME@', 'ts_finalize_agg_sfunc'
-LANGUAGE C IMMUTABLE ;
+LANGUAGE C IMMUTABLE PARALLEL SAFE;
 
 CREATE OR REPLACE FUNCTION _timescaledb_internal.finalize_agg_ffunc(
 tstate internal, aggfn TEXT, inner_agg_collation_schema NAME, inner_agg_collation_name NAME, inner_agg_input_types NAME[][], inner_agg_serialized_state BYTEA, return_type_dummy_val ANYELEMENT)
 RETURNS anyelement
 AS '@MODULE_PATHNAME@', 'ts_finalize_agg_ffunc'
-LANGUAGE C IMMUTABLE ;
+LANGUAGE C IMMUTABLE PARALLEL SAFE;
 
 CREATE OR REPLACE AGGREGATE _timescaledb_internal.finalize_agg(agg_name TEXT,  inner_agg_collation_schema NAME,  inner_agg_collation_name NAME, inner_agg_input_types NAME[][], inner_agg_serialized_state BYTEA, return_type_dummy_val anyelement) (
     SFUNC = _timescaledb_internal.finalize_agg_sfunc,
     STYPE = internal,
     FINALFUNC = _timescaledb_internal.finalize_agg_ffunc,
-    FINALFUNC_EXTRA
+    FINALFUNC_EXTRA,
+    PARALLEL = SAFE
 );

test/runner_shared.sh

@@ -81,6 +81,7 @@ ${PSQL} -U ${TEST_PGUSER} \
           sed  -e '/<exclude_from_test>/,/<\/exclude_from_test>/d' \
                -e 's!_[0-9]\{1,\}_[0-9]\{1,\}_chunk!_X_X_chunk!g' \
                -e 's!^ \{1,\}QUERY PLAN \{1,\}$!QUERY PLAN!' \
+               -e 's!:  actual rows!: actual rows!' \
                -e '/^-\{1,\}$/d' \
                -e 's!\(_timescaledb_internal.chunks_in([^,]\{1,\}, ARRAY\[\)[^]]\{1,\}\]!\1..]!' \
                -e 's! Memory: [0-9]\{1,\}kB!!' \

tsl/test/shared/expected/dist_parallel_agg.out

@@ -0,0 +1,104 @@
+-- This file and its contents are licensed under the Timescale License.
+-- Please see the included NOTICE for copyright information and
+-- LICENSE-TIMESCALE for a copy of the license.
+-- Test that for parallel-safe aggregate function a parallel plan is generated
+-- on data nodes, and for unsafe it is not. We use a manually created safe
+-- function and not a builtin one, to check that we can in fact create a
+-- function that is parallelized, to prevent a false negative (i.e. it's not
+-- parallelized, but for a different reason, not because it's unsafe).
+-- Create a relatively big table on one data node to test parallel plans and
+-- avoid flakiness.
+create table metrics_dist1(like metrics_dist);
+select table_name from create_distributed_hypertable('metrics_dist1', 'time', 'device_id',
+    data_nodes => '{"data_node_1"}');
+WARNING:  only one data node was assigned to the hypertable
+  table_name   
+ metrics_dist1
+(1 row)
+
+insert into metrics_dist1 select * from metrics_dist order by metrics_dist limit 20000;
+\set safe 'create or replace aggregate ts_debug_shippable_safe_count(*) (sfunc = int8inc, combinefunc=int8pl, stype = bigint, initcond = 0, parallel = safe);'
+\set unsafe 'create or replace aggregate ts_debug_shippable_unsafe_count(*) (sfunc = int8inc, combinefunc=int8pl, stype = bigint, initcond = 0, parallel = unsafe);'
+:safe
+call distributed_exec(:'safe');
+:unsafe
+call distributed_exec(:'unsafe');
+call distributed_exec($$ set parallel_tuple_cost = 0; $$);
+call distributed_exec($$ set parallel_setup_cost = 0; $$);
+call distributed_exec($$ set max_parallel_workers_per_gather = 1; $$);
+set timescaledb.enable_remote_explain = 1;
+set enable_partitionwise_aggregate = 1;
+\set analyze 'explain (analyze, verbose, costs off, timing off, summary off)'
+:analyze
+select count(*) from metrics_dist1;
+QUERY PLAN
+ Custom Scan (DataNodeScan) (actual rows=1 loops=1)
+   Output: (count(*))
+   Relations: Aggregate on (public.metrics_dist1)
+   Data node: data_node_1
+   Fetcher Type: Row by row
+   Chunks: _dist_hyper_X_X_chunk, _dist_hyper_X_X_chunk
+   Remote SQL: SELECT count(*) FROM public.metrics_dist1 WHERE _timescaledb_internal.chunks_in(public.metrics_dist1.*, ARRAY[..])
+   Remote EXPLAIN: 
+     Finalize Aggregate (actual rows=1 loops=1)
+       Output: count(*)
+       ->  Gather (actual rows=2 loops=1)
+             Output: (PARTIAL count(*))
+             Workers Planned: 1
+             Workers Launched: 1
+             ->  Partial Aggregate (actual rows=1 loops=2)
+                   Output: PARTIAL count(*)
+                   Worker 0: actual rows=1 loops=1
+                   ->  Parallel Append (actual rows=10000 loops=2)
+                         Worker 0: actual rows=0 loops=1
+                         ->  Parallel Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=17990 loops=1)
+                         ->  Parallel Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=2010 loops=1)
+
+(22 rows)
+
+:analyze
+select ts_debug_shippable_safe_count(*) from metrics_dist1;
+QUERY PLAN
+ Custom Scan (DataNodeScan) (actual rows=1 loops=1)
+   Output: (ts_debug_shippable_safe_count(*))
+   Relations: Aggregate on (public.metrics_dist1)
+   Data node: data_node_1
+   Fetcher Type: Row by row
+   Chunks: _dist_hyper_X_X_chunk, _dist_hyper_X_X_chunk
+   Remote SQL: SELECT public.ts_debug_shippable_safe_count(*) FROM public.metrics_dist1 WHERE _timescaledb_internal.chunks_in(public.metrics_dist1.*, ARRAY[..])
+   Remote EXPLAIN: 
+     Finalize Aggregate (actual rows=1 loops=1)
+       Output: public.ts_debug_shippable_safe_count(*)
+       ->  Gather (actual rows=2 loops=1)
+             Output: (PARTIAL public.ts_debug_shippable_safe_count(*))
+             Workers Planned: 1
+             Workers Launched: 1
+             ->  Partial Aggregate (actual rows=1 loops=2)
+                   Output: PARTIAL public.ts_debug_shippable_safe_count(*)
+                   Worker 0: actual rows=1 loops=1
+                   ->  Parallel Append (actual rows=10000 loops=2)
+                         Worker 0: actual rows=0 loops=1
+                         ->  Parallel Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=17990 loops=1)
+                         ->  Parallel Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=2010 loops=1)
+
+(22 rows)
+
+:analyze
+select ts_debug_shippable_unsafe_count(*) from metrics_dist1;
+QUERY PLAN
+ Custom Scan (DataNodeScan) (actual rows=1 loops=1)
+   Output: (ts_debug_shippable_unsafe_count(*))
+   Relations: Aggregate on (public.metrics_dist1)
+   Data node: data_node_1
+   Fetcher Type: Row by row
+   Chunks: _dist_hyper_X_X_chunk, _dist_hyper_X_X_chunk
+   Remote SQL: SELECT public.ts_debug_shippable_unsafe_count(*) FROM public.metrics_dist1 WHERE _timescaledb_internal.chunks_in(public.metrics_dist1.*, ARRAY[..])
+   Remote EXPLAIN: 
+     Aggregate (actual rows=1 loops=1)
+       Output: public.ts_debug_shippable_unsafe_count(*)
+       ->  Append (actual rows=20000 loops=1)
+             ->  Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=17990 loops=1)
+             ->  Seq Scan on _timescaledb_internal._dist_hyper_X_X_chunk (actual rows=2010 loops=1)
+
+(14 rows)
+

tsl/test/shared/sql/CMakeLists.txt

@@ -22,8 +22,8 @@ if((${PG_VERSION_MAJOR} GREATER_EQUAL "14"))
 endif()
 
 if(CMAKE_BUILD_TYPE MATCHES Debug)
-  list(APPEND TEST_FILES_SHARED dist_remote_error.sql timestamp_limits.sql
-       with_clause_parser.sql)
+  list(APPEND TEST_FILES_SHARED dist_parallel_agg.sql dist_remote_error.sql
+       timestamp_limits.sql with_clause_parser.sql)
   list(APPEND TEST_TEMPLATES_SHARED constify_now.sql.in)
 endif(CMAKE_BUILD_TYPE MATCHES Debug)
 

tsl/test/shared/sql/dist_parallel_agg.sql

@@ -0,0 +1,42 @@
+-- This file and its contents are licensed under the Timescale License.
+-- Please see the included NOTICE for copyright information and
+-- LICENSE-TIMESCALE for a copy of the license.
+
+-- Test that for parallel-safe aggregate function a parallel plan is generated
+-- on data nodes, and for unsafe it is not. We use a manually created safe
+-- function and not a builtin one, to check that we can in fact create a
+-- function that is parallelized, to prevent a false negative (i.e. it's not
+-- parallelized, but for a different reason, not because it's unsafe).
+
+-- Create a relatively big table on one data node to test parallel plans and
+-- avoid flakiness.
+create table metrics_dist1(like metrics_dist);
+select table_name from create_distributed_hypertable('metrics_dist1', 'time', 'device_id',
+    data_nodes => '{"data_node_1"}');
+insert into metrics_dist1 select * from metrics_dist order by metrics_dist limit 20000;
+
+\set safe 'create or replace aggregate ts_debug_shippable_safe_count(*) (sfunc = int8inc, combinefunc=int8pl, stype = bigint, initcond = 0, parallel = safe);'
+\set unsafe 'create or replace aggregate ts_debug_shippable_unsafe_count(*) (sfunc = int8inc, combinefunc=int8pl, stype = bigint, initcond = 0, parallel = unsafe);'
+
+:safe
+call distributed_exec(:'safe');
+:unsafe
+call distributed_exec(:'unsafe');
+
+call distributed_exec($$ set parallel_tuple_cost = 0; $$);
+call distributed_exec($$ set parallel_setup_cost = 0; $$);
+call distributed_exec($$ set max_parallel_workers_per_gather = 1; $$);
+
+set timescaledb.enable_remote_explain = 1;
+set enable_partitionwise_aggregate = 1;
+
+\set analyze 'explain (analyze, verbose, costs off, timing off, summary off)'
+
+:analyze
+select count(*) from metrics_dist1;
+
+:analyze
+select ts_debug_shippable_safe_count(*) from metrics_dist1;
+
+:analyze
+select ts_debug_shippable_unsafe_count(*) from metrics_dist1;