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 <pry...@telsasoft.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_axsmpqw1scotnaqx8nfhgmjc6l87qzao3jezlibu...@mail.gmail.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 >
