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help / color / mirror / Atom feedFrom: Rucha Kulkarni <[email protected]>
To: [email protected]
Subject: Doubts regarding pg_freespacemap extension
Date: Mon, 17 Feb 2025 20:42:15 +0530
Message-ID: <CAPsneqqHWWb4RxBFeViZAVx-CBMECinVTvQzrc6H7169Zay9Bw@mail.gmail.com> (raw)
Hi PostgreSQL Community,
While exploring the pg_freespacemap extension, I came across a few doubts
and would appreciate any insights or clarifications:
1. In the code, it defines NonLeafNodesPerPage as (BLCKSZ / 2 - 1) as can
be seen here:
https://github.com/postgres/postgres/blob/master/src/include/storage/fsm_internals.h#L54
For a binary tree, (BLCKSZ - 1) / 2 seems more appropriate when calculating
the number of non-leaf nodes. Why does the code approximate it to (BLCKSZ /
2) - 1 ?
2. According to the comment mentioned here
https://github.com/postgres/postgres/blob/master/src/backend/storage/freespace/indexfsm.c#L19
, 0 is used to denote unused pages and (BLCKSZ - 1) for used pages for
indexes. This seems reversed, shouldn't 0 indicate space available for used
pages, and (BLCKSZ - 1) indicate space available for unused pages?
3. Whenever I check the values for B-tree indexes, I observe all values to
be 0.
postgres=# select * from pg_freespace('xyz_btree');
blkno | avail
-------+-------
0 | 0
1 | 0
2 | 0
3 | 0
4 | 0
(5 rows)
How can non-zero values be observed in B-tree indexes?
4. For BRIN indexes, I see a value of 8128 instead of 8191 (BLCKSZ - 1). Is
this due to the BRIN header occupying space, leading to category 254?
postgres=# select * from pg_freespace('xyz_brin');
blkno | avail
-------+-------
0 | 0
1 | 0
2 | 8128
(3 rows)
Regards,
Rucha Kulkarni
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