GSoC proposal. Index-only scans for GIST

Hello!
Here is the text of my proposal which I've applied to GSoC.
(and link
http://www.google-melange.com/gsoc/proposal/public/google/gsoc2014/lubennikovaav/5629499534213120)
Any suggestions and comments are welcome.

*Project name*

Support for index-only scans for GIST index

*Brief review*

Currently GiST index don't have index-only scan functionality. Index-only
scans are a major performance feature added to Postgres 9.2. They allow
certain types of queries to be satisfied just by retrieving data from
indexes, and not from tables. This feature for B-trees (implemented since
PostgreSQL-9.2) allows doing certain types of queries significantly faster.
This is achieved by reduction in the amount of I/O necessary to satisfy
queries. I think it will be useful to implement this feature for user
defined data types that use GiST index.

*Benefits to the PostgreSQL Community*

Faster GiST index search would be actual for many PostgreSQL applications
(for example some GIS systems).

*Quantifiable results*

Acceleration of GiST index search.

*Project details*

1. The GiST is a balanced tree structure like a B-tree, containing <key,
pointer> pairs. GiST key is a member of a user-defined class, and
represents some property that is true of all data items reachable from the
pointer associated with the key. The GiST provides a possibility to create
custom data types with indexed access methods and extensible set of queries.

There are seven methods that an index operator class for GiST must provide,
and an eighth that is optional.

-consistent

-union

-compress

-decompress

-penalty

-picksplit

-equal

-distance (optional)

I'm going to create new optional method fetch() in addition to existing.
Thus user can create a method of retrieving data from the index without
loss. It will be used when performing search queries to speed data
retrieving.

2. gistget algorithm (several parts omitted):

Check the key
gistindex_keytest() - does this index tuple satisfy the scan key(s)?

Scan all items on the GiST index page and insert them into the queue (or
directly to output areas)

plain scan

Heap tuple TIDs are returned into so->pageData[]

ordered scan

Heap tuple TIDs are pushed into individual search queue items

If the fetch() is specified by the developer, then using it, algorithm can
retrieve the data directly to output areas at this stage, without reference
to the heap.

3. Create function gistcanreturn to check whether fetch() is specified by
user.

Amcanreturn - Function to check whether index supports index-only scans, or
zero if none

There is the question of support index-only scans for multicolumn indexes.
Probably it would require extend the interface to add separate columns
checking.

To solve this question I need the community's help.

4. Add details for index only scans into gistcostestimate function

*Links*

1) Hellerstein J. M., Naughton J. F., Pfeffer A. Generalized search
trees for database systems. - September, 1995.

2) http://www.sai.msu.su/~megera/postgres/gist/

3) PostgreSQL 9.3.3 Documentation: chapters 11. Indexes, 55. GiST
Indexes.

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Best regards,
Lubennikova Anastasia