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Thread: Temporary Files

Temporary Files

From
"Campbell, Lance"
Date:

PostgreSQL 12

I just increased my work_mem to 24 MB.  I cleared my statistics.  I then went back later and saw the following:

 

Temporary files: 21

Size of temporary files 451 MB.

 

If you divide 451 by 21 you get 21.4 .  I would have expected to see an average size greater than 24 MB.  This implies to me that PostgreSQL is creating temp files that are smaller than 24 MB. 

 

What am I not understanding?

 

Thanks,

 

 

LANCE CAMPBELL

Software Architect

 

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Re: Temporary Files

From
Peter Geoghegan
Date:
On Mon, Mar 29, 2021 at 11:10 AM Campbell, Lance <lance@illinois.edu> wrote:
> If you divide 451 by 21 you get 21.4 .  I would have expected to see an average size greater than 24 MB.  This
impliesto me that PostgreSQL is creating temp files that are smaller than 24 MB.
 
>
> What am I not understanding?

work_mem limits the in-memory space used by sort and hash operations.
It doesn't necessarily follow that the on-disk footprint is exactly
(or even approximately) the same as the in-memory footprint for any
given operation that barely can't fit in memory. The precise space
overheads will vary based on lots of factors, most of which are
implementation details.


-- 
Peter Geoghegan