[PATCH] Covering SPGiST index

Hi, hackers!

I'd like to propose a patch which introduces a functionality to include
additional columns to SPGiST index to increase speed of queries containing
them due to making the scans index only in this case. To date this
functionality was available in GiSt and btree, I suppose the same is useful
in SPGiST also.

A few words on realisaton:

1. The patch is intended to be fully compatible with previous SPGiSt
indexes so SpGist leaf tuple structure remains unchanged until the ending
of key attribute. All changes are introduced only after it. Internal tuples
remain unchanged at all.

2. Included data is added in the form very similar to heap tuple but unlike
the later it should not start from MAXALIGN boundary. I.e. nulls mask (if
exist) starts just after the key value (it doesn't need alignment). Each of
included attributes start from their own typealign boundary. The goal is to
make leaf tuples and therefore index more compact.

3. Leaf tuple header is modified to store additional per tuple flags:
a) is nullmask present - if there is at least one null value among included
attributes of a tuple
(Note that this nullmask apply only to include attributes as nulls
management for key attributes is already realised in SPGiSt by placing
leafs with null keys in separate list not in the main index tree.)
b) is there variable length values among included. If there is no and key
attribute is also fixed-length e.g. (kd-tree, quad-tree etc.) then leaf
tuple processing can be speed up using attcacheoff.

These bits are incorporated into unused higher bits of nextOffset in the
header SPGiST leaf tuple. Even if we have 64Kb pages and tuples of minimum
12 bytes (the length of the header on 32-bit architectures) + 4 bytes
ItemIdData 14 bit for nextOffset is more than enough.

All this changes only affect private index structures so all outside
behavior like WAL, vacuum etc will remain unchanged.

As usual I very much appreciate your feedback

--
Best regards,
Pavel Borisov

Postgres Professional: http://postgrespro.com <http://www.postgrespro.com>