Re: bitmap heap scan recheck for gin/fts with no lossy blocks - Mailing list pgsql-performance

On Thu, Jul 23, 2015 at 9:58 AM, Laurent Debacker <debackerl@gmail.com> wrote:

Hi,

I have read that GIN indexes don't require a recheck cond for full text search as long as work_mem is big enough, otherwise you get lossy blocks, and the recheck cond.

In my case, I have no lossy blocks (from what I could tell), but I do have a recheck...

EXPLAIN (ANALYZE, BUFFERS) SELECT COUNT(1) FROM enterprises WHERE fts @@ 'activ'::tsquery

"Aggregate  (cost=264555.07..264555.08 rows=1 width=0) (actual time=25813.920..25813.921 rows=1 loops=1)"

"  Buffers: shared hit=1 read=178192"

"  ->  Bitmap Heap Scan on enterprises  (cost=5004.86..263202.54 rows=541014 width=0) (actual time=170.546..25663.048 rows=528376 loops=1)"

"        Recheck Cond: (fts @@ '''activ'''::tsquery)"

"        Heap Blocks: exact=178096"

"        Buffers: shared hit=1 read=178192"

"        ->  Bitmap Index Scan on enterprises_fts_idx  (cost=0.00..4869.61 rows=541014 width=0) (actual time=120.214..120.214 rows=528376 loops=1)"

"              Index Cond: (fts @@ '''activ'''::tsquery)"

"              Buffers: shared hit=1 read=96"

"Planning time: 2.383 ms"

"Execution time: 25824.476 ms"

Any advice would be greatly appreciated. I'm running PostgreSQL 9.4.1.

The Recheck Cond line is a plan-time piece of info, not a run-time piece.  It only tells you what condition is going to be rechecked if a recheck is found to be necessary.

It doesn't indicate how many times it was found it to be necessary to do the recheck.  Presumably that number was zero.

Cheers,

Jeff