Postgres Pro
Downloads
Home   >   mailing lists

Re: Choosing parallel_degree - Mailing list pgsql-hackers

From James Sewell
Subject Re: Choosing parallel_degree
Date
Msg-id CANkGpBsBhNJkdD97fv-U4eG3kE0aRQpM7m8XtdHNUL31XNnM=Q@mail.gmail.com
Whole thread Raw
In response to Re: Choosing parallel_degree  (Julien Rouhaud <julien.rouhaud@dalibo.com>)
Responses Re: Choosing parallel_degree
List pgsql-hackers
Tree view
On Wed, Mar 16, 2016 at 11:26 AM, Julien Rouhaud <julien.rouhaud@dalibo.com> wrote:

I'm not too familiar with parallel planning, but I tried to implement
both in attached patch. I didn't put much effort into the
parallel_threshold GUC documentation, because I didn't really see a good
way to explain it. I'd e happy to improve it if needed. Also, to make
this parameter easier to tune for users, perhaps we could divide the
default value by 3 and use it as is in the first iteration in
create_parallel_path() ?

Also, global max_parallel_degree still needs to be at least 1 for the
per table value to be considered.


All applies and works from my end.

Is the max_parallel_degree per table of much use here? It allows the max number of workers per table to be set - but it's still bound by the same formula (now from the GUC). So in reality it's only really useful for limiting the number of workers, not raising it.

Would there be a common use case for limiting parallelism on a subset of tables in a database you've explicitly set to have a higher amount of parallel operations via the GUC? I struggle to think of one?

I think in practicality the reverse would be more common, you'd want to set certain tables to a starting point of a certain number of workers (and ramp up to more if the formula allowed it). You could set this to 0 for never use parallel agg on this table.

Another option is to allow access to the the threshold multiplier (currently hard coded to 3) per table - but this might become pretty hard to explain succinctly in the documentation.

Cheers,
James

 

The contents of this email are confidential and may be subject to legal or professional privilege and copyright. No representation is made that this email is free of viruses or other defects. If you have received this communication in error, you may not copy or distribute any part of it or otherwise disclose its contents to anyone. Please advise the sender of your incorrect receipt of this correspondence.

pgsql-hackers by date:

Previous
From: Craig Ringer
Date:
Subject: Re: Soliciting Feedback on Improving Server-Side Programming Documentation
Next
From: Amit Langote
Date:
Subject: Typo in monitoring.sgml