Re: Query Performance / Planner estimate off

On 10/21/20 2:38 PM, Sebastian Dressler wrote:

Hi Mats,

On 20. Oct 2020, at 11:37, Mats Julian Olsen <mats@duneanalytics.com> wrote:

[...]

1) Vanilla plan (16 min) : https://explain.depesz.com/s/NvDR

2) enable_nestloop=off (4 min): https://explain.depesz.com/s/buKK

3) enable_nestloop=off; enable_seqscan=off (2 min): https://explain.depesz.com/s/0WXx 

How can I get Postgres not to loop over 12M rows?

I looked at the plans and your config and there are some thoughts I'm having:

- The row estimate is off, as you possibly noticed. This can be possibly solved by raising `default_statistics_target` to e.g. 2500 (we typically use that) and run ANALYZE

I've `set default_statistics_target=2500` and ran analyze on both tables involved, unfortunately the plan is the same. The columns we use for joining here are hashes and we expect very few duplicates in the tables. Hence I think extended statistics (storing most common values and histogram bounds) aren't useful for this kind of data. Would you say the same thing?