From: Tom Lane <tgl@sss.pgh.pa.us>
To: =?UTF-8?B?5aSn5aGa5oay5Y+4?= <otsuka.kenji@nttcom.co.jp>
cc: pgsql-docs@postgresql.org, pgsql-hackers@postgresql.org
Subject: Re: The number of bytes is stored in index_size of pgstatindex() ?
In-reply-to: <56C5B2C7.7010604@nttcom.co.jp>
References: <56C5B2C7.7010604@nttcom.co.jp>
Comments: In-reply-to =?UTF-8?B?5aSn5aGa5oay5Y+4?= <otsuka.kenji@nttcom.co.jp>
	message dated "Thu, 18 Feb 2016 21:02:15 +0900"
Date: Thu, 18 Feb 2016 12:40:48 -0500
Message-ID: <31925.1455817248@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-docs-owner@postgresql.org

=?UTF-8?B?5aSn5aGa5oay5Y+4?= <otsuka.kenji@nttcom.co.jp> writes:
> It seems that description of index_size that is returned by pgstatindex() is wrong.
> The document says "Total number of pages in index".
> But it looks like the number of bytes is stored in index_size.

Hmm, you're right, because what the code does is

        values[j++] = psprintf(INT64_FORMAT,
                               (indexStat.root_pages +
                                indexStat.leaf_pages +
                                indexStat.internal_pages +
                                indexStat.deleted_pages +
                                indexStat.empty_pages) * BLCKSZ);

so the result is clearly measured in bytes not pages.

I was going to just change the docs to say "Total index size in bytes",
but then I noticed that this number is not that either: it does not
count the metapage and therefore is BLCKSZ less than the true file
size.  As an example:

regression=# select * from pgstatindex('tenk1_unique1');
 version | tree_level | index_size | root_block_no | internal_pages | leaf_pages
 | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation 
---------+------------+------------+---------------+----------------+-----------
-+-------------+---------------+------------------+--------------------
       2 |          1 |     237568 |             3 |              0 |         28
 |           0 |             0 |            87.91 |                  0
(1 row)

regression=# select relfilenode from pg_class where relname = 'tenk1_unique1';
 relfilenode 
-------------
       27449
(1 row)

$ ls -l 27449
-rw-------. 1 postgres postgres 245760 Feb 17 18:58 27449


I think this is a bug and we ought to fix the code to include the
metapage in the reported index_size.  Thoughts?

			regards, tom lane


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