WIP: Covering + unique indexes.

Hi hackers,

I'm working on a patch that allows to combine covering and unique 
functionality for btree indexes.

_Previous discussion was here:_
1) Proposal thread 
<http://www.postgresql.org/message-id/55F2CCD0.7040608@postgrespro.ru>
2) Message with proposal clarification 
<http://www.postgresql.org/message-id/55F84DF4.5030207@postgrespro.ru>

In a nutshell, the feature allows to create index with "key" columns and 
"included" columns.
"key" columns can be used as scan keys. Unique constraint relates only 
to "key" columns.
"included" columns may be used as scan keys if they have suitable opclass.
Both "key" and "included" columns can be returned from index by 
IndexOnlyScan.

Btree is the default index and it's used everywhere. So it requires 
properly testing. Volunteers are welcome)

_Use case:_
- We have a table (c1, c2, c3, c4);
- We need to have an unique index on (c1, c2).
- We would like to have a covering index on all columns to avoid reading 
of heap pages.

Old way:
CREATE UNIQUE INDEX olduniqueidx ON oldt USING btree (c1, c2);
CREATE INDEX oldcoveringidx ON oldt USING btree (c1, c2, c3, c4);

What's wrong?
Two indexes contain repeated data. Overhead to data manipulation 
operations and database size.

New way:
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);

The patch is attached.
In 'test.sql' you can find a test with detailed comments on each step, 
and comparison of old and new indexes.

New feature has following syntax:
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);
Keyword INCLUDING defines the "included" columns of index. These columns 
aren't concern to unique constraint.
Also, them are not stored in index inner pages. It allows to decrease 
index size.

_Results:_
1) Additional covering index is not required anymore.
2) New index can use IndexOnlyScan on queries, where old index can't.

For example,
explain analyze select c1, c2 from newt where c1<10000 and c3<20;

*more examples in 'test.sql'

_Future work:_
To do opclasses for "included" columns optional.

CREATE TABLE tbl (c1 int, c4 box);
CREATE UNIQUE INDEX idx ON tbl USING btree (c1) INCLUDING (c4);

If we don't need c4 as an index scankey, we don't need any btree opclass 
on it.
But we still want to have it in covering index for queries like

SELECT c4 FROM tbl WHERE c1=1000;
SELECT * FROM tbl WHERE c1=1000;

-- 
Anastasia Lubennikova
Postgres Professional:http://www.postgrespro.com
The Russian Postgres Company