Re: BUG #16324: bad cost estimates for generic query plans

PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> With PG 11.7, we're seeing bad cost estimates for generic query plans where
> the cost of a very expensive InitPlan is not included in the total cost.

> test=# prepare s1 as SELECT EXISTS(SELECT 1 FROM audit_event WHERE id > $1
> AND event_name IN ($2,$3,$4,$5,$6,$7,$8,$9,$10)) ;
> PREPARE

> Then, after 5 invocations, PG switches to using a cached, generic query plan
> that is very slow:

> Result (cost=0.47..0.48 rows=1 width=1) (actual time=28314.960..28314.961
> rows=1 loops=1)
> InitPlan 1 (returns $0)
> -> Seq Scan on audit_event (cost=0.00..6796877.67 rows=14532272
> width=0) (actual time=28314.953..28314.953 rows=0 loops=1)
> Filter: ((id > $1) AND (event_name = ANY (ARRAY[$2, $3, $4, $5, $6,
> $7, $8, $9, $10])))
> Rows Removed by Filter: 82349547
> Planning Time: 0.377 ms
> Execution Time: 28315.003 ms
> (7 rows)

> It looks like the total cost of the plan is not including the substantial
> cost of the InitPlan.

It's not "ignoring" the cost. What it is doing, since this is an EXISTS
subplan, is assuming that it will fetch the first tuple and stop, the same
as if there'd been a LIMIT 1 in the subquery. Since the estimated number
of rows is (wrongly) very high, that results in a low estimated cost to
obtain the EXISTS result, specifically 6796877.67 / 14532272 or about
0.47. Then in reality there are *no* tuples in the result, so that the
seqscan has to run to completion to find that out. Ooops.

So in general this is just an instance of the well-known difficulty of
estimating costs with small LIMIT accurately. On the other hand, since
we know the context is EXISTS, maybe we could do better? There's an
argument that the user wouldn't be bothering to test EXISTS if there
weren't a chance of a false result, hence we ought to assume that the
subquery might need to run to completion; which would lead to taking the
cost as being the full run cost not the estimated-time-to-first-tuple.
On the other hand that seems like it would discourage use of fast-start
plans for this purpose, which is probably a net loss. On the third hand,
it looks like we have already settled on the subplan's plan by this
point so maybe that objection is bogus. If you want to experiment you
could try changing this bit in cost_subplan:

if (subplan->subLinkType == EXISTS_SUBLINK)
{
/* we only need to fetch 1 tuple; clamp to avoid zero divide */
sp_cost.per_tuple += plan_run_cost / clamp_row_est(plan->plan_rows);
}

The comment above that suggests that this logic needs to match
make_subplan, but I'm thinking we would want to intentionally make them
not match, since the other code is what drives picking a fast-start plan
for the subplan.

You could make an argument for charging full run cost, or maybe just
half of that as we do for ALL/ANY cases, depending on whether you
think we should be taking worst-case estimates or not.

regards, tom lane