On 12/20/2016 11:41 PM, Robert Haas wrote:
On Tue, Dec 20, 2016 at 10:01 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
Andrew Dunstan <and...@dunslane.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.
Yes, I agree. In any case, before we change anything can we agree on a
description of what we currently do?
Here's a second attempt:
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
fixed page overhead, the per-page table of pointers to tuples, and
padding to ensure that tuples are correctly aligned.
I don't think any of that is weaselish :-)
cheers
andrew
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