From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Andrew Dunstan <andrew(at)dunslane(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: pgstattuple documentation clarification |
Date: | 2016-12-21 04:41:57 |
Message-ID: | CA+TgmobHxTq1rj3otGGO_4EwG7MV3eAZ1n0Bp5vaDYcydsbMng@mail.gmail.com |
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Lists: | pgsql-hackers |
On Tue, Dec 20, 2016 at 10:01 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>> Recently a client was confused because there was a substantial
>> difference between the reported table_len of a table and the sum of the
>> corresponding tuple_len, dead_tuple_len and free_space. The docs are
>> fairly silent on this point, and I agree that in the absence of
>> explanation it is confusing, so I propose that we add a clarification
>> note along the lines of:
>
>> The table_len will always be greater than the sum of the tuple_len,
>> dead_tuple_len and free_space. The difference is accounted for by
>> page overhead and space that is not free but cannot be attributed to
>> any particular tuple.
>
>> Or perhaps we should be more explicit and refer to the item pointers on
>> the page.
>
> I find "not free but cannot be attributed to any particular tuple"
> to be entirely useless weasel wording, not to mention wrong with
> respect to item pointers in particular.
>
> Perhaps we should start counting the item pointers in tuple_len.
> We'd still have to explain about page header overhead, but that
> would be a pretty small and fixed-size discrepancy.
It's pretty weird to count unused or dead line pointers as part of
tuple_len, and it would screw things up for anybody trying to
calculate the average width of their tuples, which is an entirely
reasonable thing to want to do. I think if we're going to count item
pointers as anything, it needs to be some new category -- either item
pointers specifically, or an "other stuff" bucket.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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