Make agg_partials_pushdown test more predictable

So far, the parallel query plans of the agg_partials_pushdown test were
not deterministic since multiple workers were used and the parallel
session leader also participated in query processing. This makes it
impossible to predict which worker would process how many tuples. With
this patch, the number of parallel workers for the agg_partials_pushdown
test has been reduced to 1 and the parallel_leader_participation has
been disabled to ensure deterministic query plans.

tsl/test/expected/agg_partials_pushdown.out

@@ -2,6 +2,9 @@
 -- Please see the included NOTICE for copyright information and
 -- LICENSE-TIMESCALE for a copy of the license.
 \set PREFIX 'EXPLAIN (analyze, verbose, costs off, timing off, summary off)'
+-- Make parallel plans predictable
+SET max_parallel_workers_per_gather = 1;
+SET parallel_leader_participation = off;
 CREATE TABLE testtable(filter_1 int, filler_2 int, filler_3 int, time timestamptz NOT NULL, device_id int, v0 int, v1 int, v2 float, v3 float);
 SELECT create_hypertable('testtable', 'time');
    create_hypertable    
@@ -354,29 +357,34 @@ SELECT timeCustom t, min(series_0) FROM PUBLIC.testtable2 GROUP BY t ORDER BY t
 -- Test that we don't process groupingSets
 :PREFIX
 SELECT timeCustom t, min(series_0) FROM PUBLIC.testtable2 GROUP BY ROLLUP(t);
-                                              QUERY PLAN                                               
--------------------------------------------------------------------------------------------------------
- MixedAggregate (actual rows=7 loops=1)
-   Output: _hyper_3_5_chunk.timecustom, min(_hyper_3_5_chunk.series_0)
-   Hash Key: _hyper_3_5_chunk.timecustom
-   Group Key: ()
-   Batches: 1 
-   ->  Gather (actual rows=11 loops=1)
-         Output: _hyper_3_5_chunk.timecustom, _hyper_3_5_chunk.series_0
-         Workers Planned: 2
-         Workers Launched: 2
-         ->  Parallel Append (actual rows=4 loops=3)
-               Worker 0:  actual rows=0 loops=1
-               Worker 1:  actual rows=0 loops=1
-               ->  Parallel Seq Scan on _timescaledb_internal._hyper_3_5_chunk (actual rows=7 loops=1)
+                                          QUERY PLAN                                          
+----------------------------------------------------------------------------------------------
+ Gather (actual rows=7 loops=1)
+   Output: _hyper_3_5_chunk.timecustom, (min(_hyper_3_5_chunk.series_0))
+   Workers Planned: 1
+   Workers Launched: 1
+   Single Copy: true
+   ->  MixedAggregate (actual rows=7 loops=1)
+         Output: _hyper_3_5_chunk.timecustom, min(_hyper_3_5_chunk.series_0)
+         Hash Key: _hyper_3_5_chunk.timecustom
+         Group Key: ()
+         Worker 0:  actual rows=7 loops=1
+           Batches: 1 
+         ->  Append (actual rows=11 loops=1)
+               Worker 0:  actual rows=11 loops=1
+               ->  Seq Scan on _timescaledb_internal._hyper_3_5_chunk (actual rows=7 loops=1)
                      Output: _hyper_3_5_chunk.timecustom, _hyper_3_5_chunk.series_0
-               ->  Parallel Seq Scan on _timescaledb_internal._hyper_3_6_chunk (actual rows=1 loops=1)
+                     Worker 0:  actual rows=7 loops=1
+               ->  Seq Scan on _timescaledb_internal._hyper_3_6_chunk (actual rows=1 loops=1)
                      Output: _hyper_3_6_chunk.timecustom, _hyper_3_6_chunk.series_0
-               ->  Parallel Seq Scan on _timescaledb_internal._hyper_3_7_chunk (actual rows=2 loops=1)
+                     Worker 0:  actual rows=1 loops=1
+               ->  Seq Scan on _timescaledb_internal._hyper_3_7_chunk (actual rows=2 loops=1)
                      Output: _hyper_3_7_chunk.timecustom, _hyper_3_7_chunk.series_0
-               ->  Parallel Seq Scan on _timescaledb_internal._hyper_3_8_chunk (actual rows=1 loops=1)
+                     Worker 0:  actual rows=2 loops=1
+               ->  Seq Scan on _timescaledb_internal._hyper_3_8_chunk (actual rows=1 loops=1)
                      Output: _hyper_3_8_chunk.timecustom, _hyper_3_8_chunk.series_0
-(20 rows)
+                     Worker 0:  actual rows=1 loops=1
+(25 rows)
 
 -- Check parallel fallback into a non-partial aggregation
 SET timescaledb.enable_chunkwise_aggregation = OFF;

tsl/test/sql/agg_partials_pushdown.sql

@@ -4,6 +4,10 @@
 
 \set PREFIX 'EXPLAIN (analyze, verbose, costs off, timing off, summary off)'
 
+-- Make parallel plans predictable
+SET max_parallel_workers_per_gather = 1;
+SET parallel_leader_participation = off;
+
 CREATE TABLE testtable(filter_1 int, filler_2 int, filler_3 int, time timestamptz NOT NULL, device_id int, v0 int, v1 int, v2 float, v3 float);
 SELECT create_hypertable('testtable', 'time');
 ALTER TABLE testtable SET (timescaledb.compress, timescaledb.compress_orderby='time DESC', timescaledb.compress_segmentby='device_id');