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merge semi join cost calculation error - Mailing list pgsql-hackers

From	Pavel Stehule
Subject	merge semi join cost calculation error
Date	October 8, 2018 17:26:37
Msg-id	CAFj8pRDnixz78Pw-S_yACaA9hSXGGUDrmPX0G4U4Vaz8itF39g@mail.gmail.com Whole thread Raw
Responses	Re: merge semi join cost calculation error (Tom Lane <tgl@sss.pgh.pa.us>) Re: merge semi join cost calculation error (Tom Lane <tgl@sss.pgh.pa.us>)
List	pgsql-hackers

The user sent a plan:

QUERY PLAN

Merge Semi Join (cost=82.97..580.24 rows=580 width=8) (actual time=0.503..9557.396 rows=721 loops=1)

Merge Cond: (tips.users_id = follows.users_id_to)

-> Index Scan using tips_idx_users_id01 on tips (cost=0.43..8378397.19 rows=2491358 width=16) (actual time=0.009..9231.585 rows=2353914 loops=1)

-> Sort (cost=1.77..1.82 rows=22 width=8) (actual time=0.052..0.089 rows=28 loops=1)

Sort Key: follows.users_id_to

Sort Method: quicksort Memory: 26kB

-> Seq Scan on follows (cost=0.00..1.27 rows=22 width=8) (actual time=0.013..0.020 rows=28 loops=1)

Filter: (users_id_from = 1)

He has PostgreSQL 10.5. I cannot to understand to too low total cost of Merge Semi Join because subnode has very high cost 8378397.

I cannot to emulate this case on my comp - so it looks like maybe some build error. What do you think about?

Regards

Pavel

From: John Naylor
Date: 08 October 2018, 16:49:03
Subject: Re: WIP: Avoid creation of the free space map for small tables

From: Peter Eisentraut
Date: 08 October 2018, 17:27:19
Subject: Re: chained transactions