From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amin <amin.fallahi@gmail.com>
cc: pgsql-hackers@postgresql.org
Subject: Re: Get relid for a relation
In-reply-to: <CAF-KA89Ls-u+Ft+BD3R-jpHLxkoFhcuVNJ3UWTGbhkfj7P7ZbA@mail.gmail.com>
References: <CAF-KA89Ls-u+Ft+BD3R-jpHLxkoFhcuVNJ3UWTGbhkfj7P7ZbA@mail.gmail.com>
Comments: In-reply-to Amin <amin.fallahi@gmail.com>
	message dated "Thu, 12 Jan 2023 17:48:59 -0800"
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Date: Fri, 13 Jan 2023 09:45:26 -0500
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Amin <amin.fallahi@gmail.com> writes:
> In CustomScan cost estimator, where PlannerInfo and RelOptInfo are passed,
> I want to get access to the relation stats (for example pg_stat_all_tables)
> by calling pg_stat_fetch_stat_tabentry(). However, I don't have access to
> relid to pass to this function.

Sure you do.  The existing code, eg in selfuncs.c, does it about like
this:

        RangeTblEntry *rte = planner_rt_fetch(rel->relid, root);

        Assert(rte->rtekind == RTE_RELATION);
        relid = rte->relid;
        Assert(relid != InvalidOid);
        ...
        vardata.statsTuple = SearchSysCache3(STATRELATTINH,
                                             ObjectIdGetDatum(relid),
                                             Int16GetDatum(colnum),
                                             BoolGetDatum(rte->inh));

This is maybe a bit confusing, in that rel->relid is a range
table index but rte->relid is an OID.

FWIW, I seriously doubt that the numbers kept by the pg_stat mechanisms
are what you want for query planning purposes.

			regards, tom lane