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Ask for disk type and set page_cost

See original GitHub issue

Based on this article it would be useful to ask for the disk (HDD, SSD,…) and set the random_page_cost, seq_page_cost accordingly.

https://amplitude.engineering/how-a-single-postgresql-config-change-improved-slow-query-performance-by-50x-85593b8991b0

Issue Analytics

State:
Created 6 years ago
Comments:5 (3 by maintainers)

Top GitHub Comments

2reactions

Komzpacommented, Dec 2, 2017

Hi @le0pard, a safe change for everyone would be this:

if SSD:
  random_page_cost = 1.1
else
  random_page_cost = 4

Technically the only thing it changes is Postgres using indexes more eagerly, affecting only planner, so nothing to benchmark - some plans query plans switch to using indexes. random_page_cost basically says “you’d better read 4x more data sequentially than read it by random index scan”.

Setting random_page_cost = 1.1 on SSD is pretty modest change. Further tweaking for system may get it lower, down to seq_page_cost.

Changing seq_page_cost is not normally necessary, what matters is ratio between random and seq scan - you need to get data from disk, and you’ll get it either randomly reading or sequentially reading 😃

This change is critical for anyone who uses PostGIS as their index operations are much faster than on-disk operations, and there is currently no way in Postgres to inform planner about it.

It’s currently number one reason for “why are my indexes not being used?!” when debugging someone’s query in https://t.me/pgsql. Setting 1.1 usually solves issues.

Postgres developers recommend going as low as 1.0: http://www.sai.msu.su/~megera/postgres/talks/Everything-Indexes-v4.pdf

0reactions

le0pardcommented, Dec 5, 2017

@Komzpa Thanks, added in pgtune. You can check with @dewey 😃