- m(pk bigserial primary key, status text): with a single row
- s(pk bigserial primary key, status text, action_at date, m_fk bigint):
* 80% of the data has action_at between the current date and 1 year ago
and status of E or C
* 20% of the data has action_at between 5 days ago and 25 days into the
future and status of P, PD, or A
I have two partial indexes:
- s_pk_action_at on s(pk, action_at) where status in ('P', 'PD', 'A')
- s_action_at_pk on s(action_at, pk) where status in ('P', 'PD', 'A')
With the query:
INNER JOIN m ON
m.pk = s.m_fk
WHERE
s.status IN ('A', 'PD', 'P')
AND (action_at <= '2018-09-06')
AND s.status IN ('A', 'P')
AND m.status = 'A';
I generally expect the index s_action_at_pk to always be preferred over s_pk_action_at. And on stock Postgres it does in fact use that index (with a bitmap index scan).
We like to set random_page_cost = 2 since we use fast SSDs only. With that change Postgres strongly prefers the index s_pk_action_at unless I both disable the other index and turn off bitmap heap scans.
I'm attaching the following plans:
- base_plan.txt: default costs; both indexes available
- base_plan_rpc2.txt: random_page_cost = 2; both indexes available
- inddisabled_plan_rpc2.txt: random_page_cost = 2; only s_action_at_pk available
- inddisabled_bhsoff_plan_rpc2.txt: random_page_cost = 2; enable_bitmapscan = false; only s_action_at_pk available
A couple of questions:
- How is s_pk_action_at ever efficient to scan? Given that the highest cardinality (primary key) column is first, wouldn't an index scan effectively have to scan the entire index?
- Why does index scan on s_action_at_pk reads over 2x as many blocks as the bitmap heap scan with the same index?
- Would you expect Postgres to generally always prefer using the s_action_at_pk index over the s_pk_action_at index for this query? I realize changing the random page cost is part of what's driving this, but I still can't imagine reading the full s_pk_action_at index (assuming that's what it is doing) could ever be more valuable.
As a side note, the planner is very bad at understanding a query that happens (I realize you wouldn't write this by hand, but ORMs) when you have a where clause like:
s.status IN ('A', 'PD', 'P') AND s.status IN ('A', 'P')
the row estimates are significantly different from a where clause with only:
s.status IN ('A', 'P')
even though semantically those are identical.
