On Wed, Aug 10, 2022 at 4:40 PM Alvaro Herrera <alvhe...@alvh.no-ip.org> wrote:
> On 2022-Aug-10, Richard Guo wrote: > > > The right-anti join plan has the same cost estimation with right join > > plan in this case. So would you please help to test what the right join > > plan looks like in your env for the query below? > > > > select * from foo left join bar on foo.a = bar.c; > > You're right, it does. > > 55432 16devel 475322=# explain (analyze, buffers) select * from foo left > join bar on foo.a = bar.c; > QUERY PLAN > > > ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── > Hash Right Join (cost=1.23..90875.24 rows=10 width=20) (actual > time=456.410..456.415 rows=10 loops=1) > Hash Cond: (bar.c = foo.a) > Buffers: shared hit=15852 read=6273 > -> Seq Scan on bar (cost=0.00..72124.00 rows=5000000 width=12) > (actual time=0.036..210.468 rows=5000000 loops=1) > Buffers: shared hit=15852 read=6272 > -> Hash (cost=1.10..1.10 rows=10 width=8) (actual time=0.037..0.038 > rows=10 loops=1) > Buckets: 1024 Batches: 1 Memory Usage: 9kB > Buffers: shared read=1 > -> Seq Scan on foo (cost=0.00..1.10 rows=10 width=8) (actual > time=0.022..0.026 rows=10 loops=1) > Buffers: shared read=1 > Planning: > Buffers: shared hit=92 read=13 > Planning Time: 1.077 ms > Execution Time: 456.458 ms > (14 filas) Thanks for help testing. Comparing the anti join plan and the right join plan, the estimated cost and the execution time mismatch a lot. Seems the cost estimate of hashjoin path is not that precise for this case even in the unpatched codes. Maybe this is something we need to improve. Thanks Richard
