Re: pgstattuple documentation clarification

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
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