Horton

Re: pgstattuple: fix free space calculation

Frédéric Yhuel <frederic.yhuel@dalibo.com>

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From: Frédéric Yhuel <frederic.yhuel@dalibo.com>
To: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-08-23T10:51:15Z
Lists: pgsql-hackers

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  1. Consistently use PageGetExactFreeSpace() in pgstattuple.


On 8/23/24 12:02, Rafia Sabih wrote:
> On the other hand, this got me thinking about the purpose of this space 
> information.
> If we want to understand that there's still some space for the tuples in 
> a page, then using PageGetExactFreeSpace is not doing justice in case of 
> heap page, because we will not be able to add any more tuples there if 
> there are already MaxHeapTuplesPerPage tuples there.

We won't be able to add, but we will be able to update a tuple in this 
page. It's hard to test, because I can't fit more than 226 tuples on a 
single page, while MaxHeapTuplesPerPage = 291 on my machine :-)

In any case, IMVHO, pgstattuple shouldn't answer to the question "can I 
add more tuples?". The goal is for educational, introspection or 
debugging purposes, and we want the exact amount of free space.

Best regards,
Frédéric