Re: pgstattuple documentation clarification

On 12/20/2016 10:01 AM, Tom Lane 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.

Well, the reason I put it like that was that in my experimentation,
after I vacuumed the table after a large delete the item pointer table
didn't seem to shrink (at least according to the pgstattuple output), so
we had a page with 0 dead tuples but some non-live line pointer space.
If that's not what's happening then something is going on that I don't
understand. (Wouldn't be a first.)

>
> 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.
>
>

Sure, sounds like a good idea. Meanwhile it would be nice to explain to
people exactly what we currently have. If you have a good formulation
I'm all ears.

cheers

andrew