Hi,
I have a question about parallel plans. I also posted it on the general list but perhaps it's a question for hackers. Here is my test case : select version(); version ---------------------------------------------------------------------------------------------------------------------------------- PostgreSQL 13.1 (Ubuntu 13.1-1.pgdg20.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, 64-bit create unlogged table drop_me as select generate_series(1,7e7) n1; SELECT 70000000 explain select count(*) from (select n1 from drop_me ) s; QUERY PLAN ---------------------------------------------------------------------------------------------- Finalize Aggregate (cost=675319.13..675319.14 rows=1 width=8) -> Gather (cost=675318.92..675319.13 rows=2 width=8) Workers Planned: 2 -> Partial Aggregate (cost=674318.92..674318.93 rows=1 width=8) -> Parallel Seq Scan on drop_me (cost=0.00..601402.13 rows=29166713 width=0) JIT: Functions: 4 Options: Inlining true, Optimization true, Expressions true, Deforming true Parallel plan, 1s explain select count(*) from (select n1 from drop_me union all select n1 from drop_me) ua; QUERY PLAN -------------------------------------------------------------------------------------------------------------- Finalize Aggregate (cost=1640315.00..1640315.01 rows=1 width=8) -> Gather (cost=1640314.96..1640314.99 rows=2 width=8) Workers Planned: 2 -> Partial Aggregate (cost=1640304.96..1640304.97 rows=1 width=8) -> Parallel Append (cost=0.00..1494471.40 rows=58333426 width=0) -> Parallel Seq Scan on drop_me (cost=0.00..601402.13 rows=29166713 width=0) -> Parallel Seq Scan on drop_me drop_me_1 (cost=0.00..601402.13 rows=29166713 width=0) JIT: Functions: 6 Options: Inlining true, Optimization true, Expressions true, Deforming true Parallel plan, 2s2 explain select count(*) from (select n1 from drop_me union all values(1)) ua; QUERY PLAN -------------------------------------------------------------------------------- Aggregate (cost=2934739.24..2934739.25 rows=1 width=8) -> Append (cost=0.00..2059737.83 rows=70000113 width=32) -> Seq Scan on drop_me (cost=0.00..1009736.12 rows=70000112 width=6) -> Subquery Scan on "*SELECT* 2" (cost=0.00..0.02 rows=1 width=32) -> Result (cost=0.00..0.01 rows=1 width=4) JIT: Functions: 4 Options: Inlining true, Optimization true, Expressions true, Deforming true No parallel plan, 2s6 I read the documentation but I don't get the reason of the "noparallel" seq scan of drop_me in the last case ? Best regards, Phil
