Re: pgstattuple: fix free space calculation

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: Rafia Sabih <rafia.pghackers@gmail.com>

Cc: Frédéric Yhuel <frederic.yhuel@dalibo.com>, Andreas Karlsson <andreas@proxel.se>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2024-09-07T20:45:34Z

Lists: pgsql-hackers

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Consistently use PageGetExactFreeSpace() in pgstattuple.
- de239d01e7cc 18.0 landed

I wrote:
> Now alternatively you could argue that a "new" page isn't usable free
> space yet and so we should count it as zero, just as we don't count
> dead tuples as usable free space.  You need VACUUM to turn either of
> those things into real free space.  But that'd be a bigger definitional
> change, and I'm not sure we want it.  Thoughts?

On the third hand: the code in question is in statapprox_heap, which
is presumably meant to deliver numbers comparable to pgstat_heap.
And pgstat_heap takes no special care for "new" pages, it just applies
PageGetHeapFreeSpace (or PageGetExactFreeSpace after this patch).
So that leaves me feeling pretty strongly that this whole stanza
is wrong and we should just do PageGetExactFreeSpace here.

			regards, tom lane