Re: select_parallel test fails with nonstandard block size

I wrote:
> OK, I'll take care of it (since I now realize that the inconsistency
> is my own fault --- I committed that GUC not you). It's unclear what
> this will do for Peter's complaint though.

On closer inspection, the answer is "nothing", because the select_parallel
test overrides the default value of min_parallel_relation_size anyway.
(Without that, I don't think tenk1 is large enough to trigger
consideration of parallel scan at all.)

I find that at BLCKSZ 8K, the planner thinks the best plan is

HashAggregate (cost=5320.28..7920.28 rows=10000 width=12)
Group Key: parallel_restricted(unique1)
-> Index Only Scan using tenk1_unique1 on tenk1 (cost=0.29..2770.28 rows=10000 width=8)

which is what the regression test script expects. Forcing the parallel
plan to be chosen, we get this using the cost parameters set up by
select_parallel:

HashAggregate (cost=5433.00..8033.00 rows=10000 width=12)
Group Key: parallel_restricted(unique1)
-> Gather (cost=0.00..2883.00 rows=10000 width=8)
Workers Planned: 4
-> Parallel Seq Scan on tenk1 (cost=0.00..383.00 rows=2500 width=4)

However, at BLCKSZ 16K, we get these numbers instead:

HashAggregate (cost=5264.28..7864.28 rows=10000 width=12)
Group Key: parallel_restricted(unique1)
-> Index Only Scan using tenk1_unique1 on tenk1 (cost=0.29..2714.28 rows=10000 width=8)

HashAggregate (cost=5251.00..7851.00 rows=10000 width=12)
Group Key: parallel_restricted(unique1)
-> Gather (cost=0.00..2701.00 rows=10000 width=8)
Workers Planned: 4
-> Parallel Seq Scan on tenk1 (cost=0.00..201.00 rows=2500 width=4)

so the planner goes for the second one.

I don't think there's anything particularly broken here. The seqscan
cost estimate is largely dependent on the number of blocks, and there's
half as many blocks at 16K. The indexscan estimate is also reduced,
but not as much, so it stops looking like the cheaper alternative.

We could maybe twiddle the cost parameters select_parallel uses so that
the same plan is chosen at both block sizes, but it seems like it would
be very fragile, and I'm not sure there's much point.

regards, tom lane