Re: GIN improvements part2: fast scan

On Sat, Jun 15, 2013 at 2:55 AM, Alexander Korotkov <aekorotkov(at)gmail(dot)com>wrote:

> attached patch implementing "fast scan" technique for GIN. This is second
> patch of GIN improvements, see the 1st one here:
>
> http://www.postgresql.org/message-id/CAPpHfduxv-iL7aedwPW0W5fXrWGAKfxijWM63_hZujaCRxnmFQ@mail.gmail.com
> This patch allow to skip parts of posting trees when their scan is not
> necessary. In particular, it solves "frequent_term & rare_term" problem of
> FTS.
> It introduces new interface method pre_consistent which behaves like
> consistent, but:
> 1) allows false positives on input (check[])
> 2) allowed to return false positives
>
> Some example: "frequent_term & rare_term" becomes pretty fast.
>
> create table test as (select to_tsvector('english', 'bbb') as v from
> generate_series(1,1000000));
> insert into test (select to_tsvector('english', 'ddd') from
> generate_series(1,10));
> create index test_idx on test using gin (v);
>
> postgres=# explain analyze select * from test where v @@
> to_tsquery('english', 'bbb & ddd');
> QUERY PLAN
>
> -----------------------------------------------------------------------------------------------------------------------
> Bitmap Heap Scan on test (cost=942.75..7280.63 rows=5000 width=17)
> (actual time=0.458..0.461 rows=10 loops=1)
> Recheck Cond: (v @@ '''bbb'' & ''ddd'''::tsquery)
> -> Bitmap Index Scan on test_idx (cost=0.00..941.50 rows=5000
> width=0) (actual time=0.449..0.449 rows=10 loops=1)
> Index Cond: (v @@ '''bbb'' & ''ddd'''::tsquery)
> Total runtime: 0.516 ms
> (5 rows)
>

Attached version of patch has some refactoring and bug fixes.

------
With best regards,
Alexander Korotkov.