Re: MinIndexTupleSize seems slightly wrong

At Thu, 12 Apr 2018 17:26:33 -0700, Peter Geoghegan <pg(at)bowt(dot)ie> wrote in <CAH2-WzkQmb54Kbx-YHXstRKXcNc+_87jwV3DRb54xcybLR7Oig(at)mail(dot)gmail(dot)com>
> itup.h says of MinIndexTupleSize/MaxIndexTuplesPerPage:
>
> /*
> * MaxIndexTuplesPerPage is an upper bound on the number of tuples that can
> * fit on one index page. An index tuple must have either data or a null
> * bitmap, so we can safely assume it's at least 1 byte bigger than a bare
> * IndexTupleData struct. We arrive at the divisor because each tuple
> * must be maxaligned, and it must have an associated item pointer.
> */
> #define MinIndexTupleSize MAXALIGN(sizeof(IndexTupleData) + 1)
> #define MaxIndexTuplesPerPage \
> ((int) ((BLCKSZ - SizeOfPageHeaderData) / \
> (MAXALIGN(sizeof(IndexTupleData) + 1) + sizeof(ItemIdData))))
>
> However, that at least seems questionable to me. See _bt_pgaddtup()
> for a simple example of this -- "minus infinity" items on internal
> pages are sized sizeof(IndexTupleData).
>
> The code still seems fine to me, since that only happens at most once
> per page. Is it worth noting the exception?

MinIndexTupleSize is not used at all. It doesn't harm as long as
MaxIndexTuplesPerPage is not less than the maximum number of
tuples in reality.

Applying values on my environment,

BLCKSZ = 8192
SizeOfPageHeaderData = 24
sizeof(IndexTupleData) = 8
sizeof(ItemIdData) = 4

MaxIndexTuplesPerPage is 408. If we have 407 normal and one
0-attr index tuple, they leave 16 byte spare space, in which no
normal tuple fits. In the case where we have BLCKSZ of 3 * 8192 =
24576 bytes, MaxIndexTuplesPerPage is 1227. 1226 normal and one
0-attr tuples uses 24556 bytes and it leaves spare 20 bytes, in
which one extra normal tuple can reside. This can cause
off-by-one error.

In reality, all types of index have opaque area with at least 8
bytes long and it prevent pages from having extra tuples. But it
doesn't seem to me stable enough.

reagards.

--
Kyotaro Horiguchi
NTT Open Source Software Center