Re: pg_stat_*_columns?

On 06/24/2015 07:54 PM, Jeff Janes wrote:
>
> Were the stories (or the experience which lead to the stories) on 9.3 or
> later? Do they have a good way to reproduce it for testing purposes?

The per-db split can only improve things if there actually are multiple
databases, and if the objects are somehow evenly distributed among them.
If there's a single database, or if most of the objects are in a single
database (but stored in multiple schemas, for example), it does not help
much. So it's trivially to reproduce the previous issues.

> When I was testing the stat file split patch, one thing I noticed is
> that on the ext4 file system, when you atomically replace a file by
> renaming a file over the top of an existing file name, it will
> automatically fsync the file being renamed. This is exactly what we
> don't want in the case of the temp stats files. But there seems to be
> no way to circumvent it, other than unlinking the target file before the
> rename (which of course defeats the point of atomic replacement), or
> moving to a different filesystem for the stats files.

Interesting. I don't think unlinking is a good idea - my understanding
is that we do it this way because rename is supposed to be atomic or
something like that. I don't know whether addressing this filesystem
feature is worth it, or if pgstat.c is the right place to do that.

--
Tomas Vondra http://www.2ndQuadrant.com/
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