On Tuesday, March 23, 2021, Marc Millas <marc.mil...@mokadb.com> wrote:
> Hi, > > I cannot agree. > I did an explain analyze with and without the cast: its > extremely different: > > postgres=# explain analyze with numb as(select ceiling(2582*random())::int > rand, generate_series(1,5) as monnum) select monnum, prenom from > numb,prenoms where numb.rand=prenoms.id; > QUERY PLAN > ------------------------------------------------------------ > ----------------------------------------------------------------- > Nested Loop (cost=0.33..37.65 rows=5 width=11) (actual time=0.059..0.147 > rows=5 loops=1) > CTE numb > -> ProjectSet (cost=0.00..0.05 rows=5 width=8) (actual > time=0.021..0.022 rows=5 loops=1) > -> Result (cost=0.00..0.01 rows=1 width=0) (actual > time=0.000..0.000 rows=1 loops=1) > -> CTE Scan on numb (cost=0.00..0.10 rows=5 width=8) (actual > time=0.023..0.025 rows=5 loops=1) > -> Index Scan using prenoms_pkey on prenoms (cost=0.28..7.50 rows=1 > width=11) (actual time=0.024..0.024 rows=1 loops=5) > Index Cond: (id = numb.rand) > Planning Time: 0.111 ms > Execution Time: 0.201 ms > (9 lignes) > > > postgres=# explain analyze with numb as(select ceiling(2582*random()) > rand, generate_series(1,5) as monnum) select monnum, prenom from > numb,prenoms where numb.rand=prenoms.id; > QUERY PLAN > ------------------------------------------------------------ > --------------------------------------------------- > Hash Join (cost=0.21..54.59 rows=65 width=11) (actual time=0.105..0.291 > rows=5 loops=1) > Hash Cond: ((prenoms.id)::double precision = numb.rand) > CTE numb > -> ProjectSet (cost=0.00..0.05 rows=5 width=12) (actual > time=0.002..0.003 rows=5 loops=1) > -> Result (cost=0.00..0.01 rows=1 width=0) (actual > time=0.000..0.000 rows=1 loops=1) > -> Seq Scan on prenoms (cost=0.00..40.82 rows=2582 width=11) (actual > time=0.011..0.131 rows=2582 loops=1) > -> Hash (cost=0.10..0.10 rows=5 width=12) (actual time=0.012..0.012 > rows=5 loops=1) > Buckets: 1024 Batches: 1 Memory Usage: 9kB > -> CTE Scan on numb (cost=0.00..0.10 rows=5 width=12) (actual > time=0.004..0.005 rows=5 loops=1) > Planning Time: 0.070 ms > Execution Time: 0.313 ms > (11 lignes) > > If I’m reading that correctly since prenoms.id is an integer if you don’t cast the ceiling(random) away from double you cannot use the index since its not the same type - the integer has to become double, not the reverse. So you get a different execution and thus different result ordering since the executor doesn’t have to care about row order. David J.
