čt 20. 8. 2020 v 0:04 odesílatel David Rowley <dgrowle...@gmail.com> napsal:
> On Wed, 19 Aug 2020 at 16:18, Pavel Stehule <pavel.steh...@gmail.com> > wrote: > > > > > > > > st 19. 8. 2020 v 5:48 odesílatel David Rowley <dgrowle...@gmail.com> > napsal: > >> Current method: > >> > >> regression=# explain (analyze, costs off, timing off, summary off) > >> select twenty, (select count(*) from tenk1 t2 where t1.twenty = > >> t2.twenty) from tenk1 t1; > >> QUERY PLAN > >> --------------------------------------------------------------------- > >> Seq Scan on tenk1 t1 (actual rows=10000 loops=1) > >> SubPlan 1 > >> -> Result Cache (actual rows=1 loops=10000) > >> Cache Key: t1.twenty > >> Hits: 9980 Misses: 20 Evictions: 0 Overflows: 0 > >> -> Aggregate (actual rows=1 loops=20) > >> -> Seq Scan on tenk1 t2 (actual rows=500 loops=20) > >> Filter: (t1.twenty = twenty) > >> Rows Removed by Filter: 9500 > >> (9 rows) > >> > >> Andres' suggestion: > >> > >> regression=# explain (analyze, costs off, timing off, summary off) > >> select twenty, (select count(*) from tenk1 t2 where t1.twenty = > >> t2.twenty) from tenk1 t1; > >> QUERY PLAN > >> --------------------------------------------------------------------- > >> Seq Scan on tenk1 t1 (actual rows=10000 loops=1) > >> SubPlan 1 > >> Cache Key: t1.twenty Hits: 9980 Misses: 20 Evictions: 0 > Overflows: 0 > >> -> Aggregate (actual rows=1 loops=20) > >> -> Seq Scan on tenk1 t2 (actual rows=500 loops=20) > >> Filter: (t1.twenty = twenty) > >> Rows Removed by Filter: 9500 > >> (7 rows) > > > I didn't do performance tests, that should be necessary, but I think > Andres' variant is a little bit more readable. > > Thanks for chiming in on this. I was just wondering about the > readability part and what makes the one with the Result Cache node > less readable? I can think of a couple of reasons you might have this > view and just wanted to double-check what it is. > It is more compact - less rows, less nesting levels > David >
