Re: need clarification on CTE/join

On Tuesday, March 23, 2021, Marc Millas <marc(dot)millas(at)mokadb(dot)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.