On Tue, 2020-12-01 at 14:38 +0700, mobigroup wrote: > I have strange behaviour when EXPLAIN returns parallel execution plan but the query execution is not parallel. The query looks as > > SELECT > plpgsql_function(...parameters…) > FROM table as t > WHERE id=1 > UNION ALL > SELECT > plpgsql_function(...parameters…) > FROM table as t > WHERE id=2 > UNION ALL > ... > > EXPLAIN outputs looks ok: > > Append (cost=0.00..10.25 rows=8 width=32) > -> Gather (cost=0.00..1.27 rows=1 width=32) > Workers Planned: 8 > -> Parallel Seq Scan on table t (cost=0.00..1.27 rows=1 width=32) > Filter: (id = 1) > -> Gather (cost=0.00..1.27 rows=1 width=32) > Workers Planned: 8 > -> Parallel Seq Scan on table t_1 (cost=0.00..1.27 rows=1 width=32) > Filter: (id = 2) > -> Gather (cost=0.00..1.27 rows=1 width=32) > ... > > > But the query still uses just one core and I see sequential RAISE NOTICE outputs from the function plpgsql_function(). > > select version(); > version > --------------------------------------------------------------------------------------------------------------------------------- > PostgreSQL 12.4 (Ubuntu 12.4-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit The essential question here is when the function is executed, so you should use EXPLAIN (VERBOSE) to see that. Possible explanations: - The function is executed after the "Gather" node. Perhaps you didn't define it as PARALLEL SAFE. - Perhaps the tables are small. During a parallel sequential scan, each worker is assigned a range of blocks to scan, so all rows found in a single block are scanned by a single worker. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
