bitmap heap scan recheck on full text search with gin index - Mailing list pgsql-performance

Hi All,

I'm getting poor performance on full text searches that return lots of
entries from a table with about 7 million rows.  I think the cause is
rechecking the text match on all of the returned rows, even though I'm
using a @@ query on a gin index, which the docs say should not require
a recheck.

Here's the table:

****
mediacloud=> \d download_texts;
                                     Table "public.download_texts"
     Column       |  Type   |                                 Modifiers
-------------------+---------
+
----------------------------------------------------------------------------
download_texts_id | integer | not null default
nextval('download_texts_download_texts_id_seq'::regclass)
downloads_id      | integer | not null
download_text     | text    | not null
Indexes:
   "download_texts_pkey" PRIMARY KEY, btree (download_texts_id)
   "download_texts_downloads_id_index" UNIQUE, btree (downloads_id)
   "download_texts_textsearch_idx" gin
(to_tsvector('english'::regconfig, download_text)), tablespace
"large_table_space"
Foreign-key constraints:
   "download_texts_downloads_id_fkey" FOREIGN KEY (downloads_id)
REFERENCES downloads(downloads_id)
Tablespace: "large_table_space"
****

And here's the query:

****
mediacloud=> explain analyze select count(dt.download_texts_id) from
download_texts dt where to_tsvector('english', download_text) @@
to_tsquery('english', 'stimulus');
QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate  (cost=26161.16..26161.17 rows=1 width=4) (actual
time=153640.083..153640.083 rows=1 loops=1)
  ->  Bitmap Heap Scan on download_texts dt  (cost=3937.41..26146.11
rows=6018 width=4) (actual time=1957.074..153529.351 rows=72225 loops=1)
        Recheck Cond: (to_tsvector('english'::regconfig,
download_text) @@ '''stimulus'''::tsquery)
        ->  Bitmap Index Scan on download_texts_textsearch_idx
(cost=0.00..3935.90 rows=6018 width=0) (actual time=1048.556..1048.556
rows=72225 loops=1)
              Index Cond: (to_tsvector('english'::regconfig,
download_text) @@ '''stimulus'''::tsquery)
Total runtime: 153642.249 ms
****

Nearly all of the time is being spent in the bitmap heap scan, I
suspect because of the work of rereading and rechecking the text of
all the matched entries.  Is this indeed what's going on here?  Is
there any way to make postgres not do that recheck?


Thanks!

-hal