Обсуждение: index bloat estimation

Dear Colleagues,

What queries do you use to estimate index and table bloat?

I've researched some on the Net and found multiple scripts mentioned in
https://wiki.postgresql.org/wiki/Index_Maintenance#Index_Bloat, also
in https://github.com/pgexperts/pgx_scripts etc. 

Most of the stuff I've looked at is pretty old, much seems unsupported.
What is the current best practice?

I'd be grateful if you could share your personal favourite ways of
estimating bloat.

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49@fidonet http://vas.tomsk.ru/

On Fri, 12 Feb 2021, Victor Sudakov wrote:

> Most of the stuff I've looked at is pretty old, much seems unsupported.
> What is the current best practice?

None of the estimate queries ever really worked well.  They just gave a 
bit more information than zero in the days before PG's internal functions 
were really reliable and useful for bloat measurement, something that 
happened in version 9.5 development.

For any modern PG, it's worth the trouble to learn how to directly use 
pgstattuple https://www.postgresql.org/docs/current/pgstattuple.html to do 
this job.  You run and interpret the output from pgstattuple(relation) and 
its faster estimate version pgstattuple_approx.  It's not hard to run some 
simulations with deleted rows to see what bloat looks like when it builds 
up.

There's still some need for manual estimates if you want to account for 
fillfactor in all cases, but I see that as a niche topic, not where people 
should start at.

There are also wrapper scripts built on top of pgstattuple around, like 
the already mentioned https://github.com/keithf4/pg_bloat_check

A good bit of the work done in that script is around handling multiple 
versions of PG and building some long-term idea of bloat state on all 
tables.  Workloads that have a bloat problem are sometimes fixed, but in a 
lot of cases the best you can do is monitor them and rebuild things when 
it gets bad.  That's one context Keith's packaging of this feature aims 
at.

--
Greg Smith  greg.smith@crunchydata.com
Director of Open Source Strategy
Crunchy Data https://www.crunchydata.com/

Keith Fiske wrote:
> >
> > What queries do you use to estimate index and table bloat?

[dd]

> Why estimate when you can get the exact amount? At least for b-tree indexes
> anyway.
> 
> https://github.com/keithf4/pg_bloat_check

Hello Keith,

Thanks for this script and for reminding about pgstattuple.

The script has the drawback of requiring r/w access to the database, so
it cannot be run on a replica. But the idea is excellent. At least it
can show the top N bloated relations, and then I can further explore with
pgstattuple(), pgstatindex() etc.


-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49@fidonet http://vas.tomsk.ru/

Keith Fiske wrote:

[dd]
> 
> You cannot run the actual scan on the replica, no. But it can be set to run
> against the replica and just report the statistics so you can have a cron
> set up to always run on the given system in case of failover. The
> "--recovery_mode_norun" can be set so it will only run if the target system
> is actually a primary.
> 
> I could possibly see about letting this actually run against the replica,
> however this can be a rather long running transaction depending on the size
> of the tables involved. You can set the "--commit_rate" to avoid some of
> that, but if you have really large tables, it can still run quite long. So
> this sort of check is really best run against the primary to avoid issues
> around having to allow long running queries on the replica (delayed
> replication or even worse bloat buildup).

If you have a dedicated replica for OLAP, even one running from a WAL
archive (not from a replication slot), this is not an issue.

So running pg_bloat_check against a replica would be very useful for
some of us.

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49@fidonet http://vas.tomsk.ru/