Re: Is auto-analyze as thorough as manual analyze?

On Sat, Sep 23, 2017 at 6:10 PM, Jack Christensen <jack(at)jackchristensen(dot)com>
wrote:

> Just had an issue where a prepared query would occasionally choose a very
> bad plan in production. The same data set in a different environment
> consistently would choose the index scan. As would be expected, running
> analyze on that table in production resolved the issue.
>
> However, before I ran the analyze I checked pg_stat_user_tables to see
> last_autoanalyze for that table. It had run today. But the problem existed
> before that. I would have expected that the auto-analyze would have
> corrected this (or prevented it entirely if run enough).
>
> So that leaves me wondering: is an auto-analyze the same as manually
> running analyze or is a manual analyze more thorough? This is running
> version 9.6.3 on Heroku.
>
> Thanks,
>
> Jack
>
>
>
>
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>is an auto-analyze the same as manually running analyze or is a manual
analyze more thorough?

It's not that one is "more thorough" than the other, it's that
autovacuum_analyze will only kick in when it meets
one of the following conditions:

autovacuum_analyze_scale_factor 0.1 #Number of tuple inserts,
updates, or deletes prior to analyze as a fraction of reltuples.
autovacuum_analyze_threshold 50 #Minimum number of tuple
inserts, updates, or deletes prior to analyze.

https://www.postgresql.org/docs/9.6/static/runtime-config-autovacuum.html

Note: You can adjust the settings for individual tables.
EG:

*ALTER TABLE some_schema.your_table SET (autovacuum_vacuum_scale_factor = 0.5);
ALTER TABLE some_schema.your_table SET (autovacuum_vacuum_threshold = 1000);*

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.