Tom Lane <tgl@sss.pgh.pa.us> writes:
> You seem to be using a rather wacko value of cpu_tuple_cost; those
> Result nodes ought to be costed at 0.01 not 1.00. With the default
oops yes, thanks. that was left over from other experimentation.
> However, it looks to me like the subquery-scan-outside plan probably
> is the faster one, on both my machine and yours. I get
>
> regression=# explain analyze select * from tenk1, (select 1 union all select 2) as x;
> QUERY PLAN
>
----------------------------------------------------------------------------------------------------------------------------
> Nested Loop (cost=0.00..858.00 rows=20000 width=248) (actual time=0.42..3648.61 rows=20000 loops=1)
> -> Seq Scan on tenk1 (cost=0.00..458.00 rows=10000 width=244) (actual time=0.23..199.97 rows=10000 loops=1)
> -> Subquery Scan x (cost=0.00..0.02 rows=2 width=0) (actual time=0.07..0.24 rows=2 loops=10000)
...
> Total runtime: 3807.39 msec
> Nested Loop (cost=0.00..40718.00 rows=20000 width=248) (actual time=0.39..1214.42 rows=20000 loops=1)
> -> Subquery Scan x (cost=0.00..2.00 rows=2 width=0) (actual time=0.10..0.31 rows=2 loops=1)
> -> Seq Scan on tenk1 (cost=0.00..10358.00 rows=10000 width=244) (actual time=0.17..188.37 rows=10000 loops=2)
> Total runtime: 1371.17 msec
Woah, that's pretty whacky. It seems like it ought to be way faster to do a
single sequential scan and return two records for each tuple read rather than
do an entire unnecessary sequential scan, even if most or even all of the
second one is cached.
--
greg