Re: Scan buffercache for a table

Thank you Justin. I started a new thread because the context is a little
bit different. I am no longer interested in statistics anymore. I want to
find exact individual pages of a table which are cached and are/aren't
dirty. pg_buffercache implements the loop, but it goes over all the
buffers. However, I want to scan a specific table cache pages.

On Mon, Jan 30, 2023 at 6:43 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> 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
>