Thanks for the answer!
On Tue, Dec 3, 2019 at 8:39 AM Jeff Janes <jeff.janes@xxxxxxxxx> wrote:
> What happens if you set enable_sort to off before running it?
Turning enable_sort to off makes the first query to not sort[1]. It
does run much slower though compared to the original query[2]. This
time I do VACUUM ANALYZE first so even the slow query is much faster,
but still much slower than the fast query[3].
> It thinks it will find 1 row, and actually finds 89,222. I don't know exactly why that would be, I suppose tag_id has an extremely skewed distribution. But yeah, that is going to cause some problems. For one thing, if there was actually just one qualifying row, then it wouldn't get to stop early, as the LIMIT would never be satisfied. So it thinks that if it choose to walk the index backwards, it would have to walk the **entire** index.
I'm not really sure what skewed distribution is. If by skewed you mean
that for a particular tag_id there are many entity and other tag_id
there might be low amount entity then yes, this particular key value
covers 80% of the entity. For this kind of dataset, is there any way
that I can do to improve it or is it just impossible?
> With this query, it can use the join condition to transfer the knowledge of tag.id=24 to become entity_tag.tag_id=24, and then look up stats on entity_tag.tag_id for the value 24. When you specify the single row of tag indirectly, it can't do that as it doesn't know what specific value of tag.id is going to be the one it finds (until after the query is done being planned and starts executing, at which point it is too late). But the row with id=24 doesn't seem to be the same one with "tag.key = 'status' AND tag.value = 'SUCCEEDED'", so you have basically changed the query entirely on us.
Apologies, I used the query for database on another environment
previously. The correct one uses tag_id=18 [3]. So it becomes like
this:
SELECT entity.id
FROM (
SELECT entity_tag.entity_id
FROM tag
JOIN entity_tag ON tag.id = entity_tag.tag_id
WHERE tag.id = 18
) matched
JOIN entity ON matched.entity_id = entity.id
WHERE entity.type = 'execution'
ORDER BY entity.id DESC
LIMIT 10;
It's still very fast and the query plan looks similar to me.
> If you replanned this query with ORDER BY entity.id+0 DESC, (and with the true value of tag_id) that might give you some more insight into the hidden "thought process" behind the planner.
I tried this on the fast query and it becomes very slow [4]. I guess
because it cannot consult the index for the ordering anymore so it
can't do LIMIT? I'm not so sure.
[1] https://explain.depesz.com/s/aEmR
[2] https://explain.depesz.com/s/kmNY
[3] https://explain.depesz.com/s/pD5v
[4] https://explain.depesz.com/s/4s7Q
--
Best regards,
Aufar Gilbran
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