contrib/bloom provides signature file based index.

Authors: Teodor Sigaev (teodor@sigaev.ru) and Oleg Bartunov (oleg@sai.msu.su)



Notes: This is a *working* prototype, which can be finishing up to production
in case of interest. Particularly, it misses wal support, because of
common  limitation of contrib modules.

This index is useful if table has many attributes and queries can include
their arbitary combinations. Traditional Btree index is faster than
bloom index , but it'd require too many indexes to support all possible 
queries, while one need only one bloom index. Bloom index supports only 
equality comparison. Since it's a signature file, not a tree, it always
should be readed fully, but sequentially, so search performance is 
constant and doesn't depends on a query. 
Implementation of Bloom filter (http://en.wikipedia.org/wiki/Bloom_filter)
allows fast exclusion of non-candidate tuples.
Since signature is a lossy representation of all indexed attributes, 
search results should be rechecked using heap information. 
User can specify signature length (in uint16, default is 5) and the number of 
bits, which can be setted, per attribute ( 1 < colN < 2048 ).

 For example:

CREATE INDEX bloomidx ON tbloom(i1,i2,i3) 
       WITH (length=5, col1=2, col2=2, col3=4);

Here, we create bloom index with signature length 80 bits and attributes
i1, i2  mapped to 2 bits, attribute i3 - to 4 bits.


Todo: 
* add more opclasses
* better configurability
* add support of arrays with  operations contains and intersection

Example of usage:

select
        (generate_series(1,1000)*random())::int as i1,
        (generate_series(1,1000)*random())::int as i2,
        (generate_series(1,1000)*random())::int as i3,
        (generate_series(1,1000)*random())::int as i4,
        (generate_series(1,1000)*random())::int as i5,
        (generate_series(1,1000)*random())::int as i6,
        (generate_series(1,1000)*random())::int as i7,
        (generate_series(1,1000)*random())::int as i8,
        (generate_series(1,1000)*random())::int as i9,
        (generate_series(1,1000)*random())::int as i10,
        (generate_series(1,1000)*random())::int as i11,
        (generate_series(1,1000)*random())::int as i12,
        (generate_series(1,1000)*random())::int as i13
into tbloom;
create index bloomidx on tbloom using
bloom(i1,i2,i3,i4,i5,i6,i7,i8,i9,i10,i11,i12);
select pg_relation_size('bloomidx');
create index btree_idx on tbloom(i1,i2,i3,i4,i5,i6,i7,i8,i9,i10,i11,i12);
select pg_relation_size('btree_idx');


=# explain analyze  select * from tbloom where i2=20 and i10=15;
                                                   QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on tbloom  (cost=1.50..5.52 rows=1 width=52) (actual time=0.057..0.057 rows=0 loops=1)
   Recheck Cond: ((i2 = 20) AND (i10 = 15))
   ->  Bitmap Index Scan on bloomidx  (cost=0.00..1.50 rows=1 width=0) (actual time=0.041..0.041 rows=9 loops=1)
         Index Cond: ((i2 = 20) AND (i10 = 15))
 Total runtime: 0.081 ms
(5 rows)

Seqscan is slow.

=# set enable_bitmapscan = off;
=# set enable_indexscan = off;
=# explain analyze  select * from tbloom where i2=20 and i10=15;
                                            QUERY PLAN
--------------------------------------------------------------------------------------------------
 Seq Scan on tbloom  (cost=0.00..25.00 rows=1 width=52) (actual time=0.162..0.162 rows=0 loops=1)
   Filter: ((i2 = 20) AND (i10 = 15))
 Total runtime: 0.181 ms
(3 rows)

Btree index will be not used.

=# drop index bloomidx;
=# create index btree_idx on tbloom(i1,i2,i3,i4,i5,i6,i7,i8,i9,i10,i11,i12);
=# explain analyze  select * from tbloom where i2=20 and i10=15;
                                            QUERY PLAN
--------------------------------------------------------------------------------------------------
 Seq Scan on tbloom  (cost=0.00..25.00 rows=1 width=52) (actual time=0.210..0.210 rows=0 loops=1)
   Filter: ((i2 = 20) AND (i10 = 15))
 Total runtime: 0.250 ms
(3 rows)