Re: Scan buffercache for a table

On Mon, Jan 30, 2023 at 06:01:08PM -0800, Amin wrote:
> Hi,
>
> I am looking for function calls to scan the buffer cache for a table and
> find the cached pages. I want to find out which pages are cached and which
> of them are dirty. Having the relation id, how can I do that? I have gone
> through bufmgr.c and relcache.c, but could not find a way to get
> relation-specific pages from the buffer cache.

This looks like a re-post of the question you asked on Jan 13:
CAF-KA8_axSMpQW1scOTnAQx8NFHgmJc6L87QzAo3JezLiBU1HQ(at)mail(dot)gmail(dot)com
It'd be better not to start a new thread (or if you do that, it'd be
better to mention the old one and include its participants).

On Fri, Jan 13, 2023 at 05:28:31PM -0800, Amin wrote:
> Hi,
>
> Before scanning a relation, in the planner stage, I want to make a
> call to
> retrieve information about how many pages will be a hit for a specific
> relation. The module pg_buffercache seems to be doing a similar thing.

The planner is a *model* which (among other things) tries to guess how
many pages will be read/hit. It's not expected to be anywhere near
accurate.

pg_buffercache only checks for pages within postgres' own buffer cache.
It doesn't look for pages which are in the OS page cache, which require
a system call to access (but don't require device I/O).

Read about pgfincore for introinspection of the OS page cache.

> Also, pg_statio_all_tables seems to be having that information, but it
> is updated after execution. However, I want the information before
> execution. Also not sure how pg_statio_all_tables is created and how
> I can access it in the code.

But the view isn't omnicient. When you execute a plan, you don't know
how it's going to end. If you did, you wouldn't need to run it - you
could just print the answer.

Note that planning and execution are separate and independant. It's
possible to plan a query without ever running it, or to plan it once and
run it multiple times. The view reflects I/O requested by postgres; the
I/O normally comes primarily from execution.

You can look at how the view is defined:
\sv pg_statio_all_tables

And then you can look at how the functions that it calls are implemented
(\df+). Same for pg_buffercache. It seems like you'll want to learn
how to navigate the source code to find how things are connected.

--
Justin