To: pgsql-performance@postgresql.org
From: Ivan Voras <ivoras@freebsd.org>
Subject: Re: Understanding tsearch2 performance
Date: Wed, 14 Jul 2010 15:37:56 +0200
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Message-ID: <i1kej2$12t$1@dough.gmane.org>
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	<Pine.LNX.4.64.1007141629000.32129@sn.sai.msu.ru>
	<i1kc3m$ngr$1@dough.gmane.org>
	<Pine.LNX.4.64.1007141722480.32129@sn.sai.msu.ru>
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In-Reply-To: <Pine.LNX.4.64.1007141722480.32129@sn.sai.msu.ru>

On 07/14/10 15:25, Oleg Bartunov wrote:
> On Wed, 14 Jul 2010, Ivan Voras wrote:
> 
>>> Returning 8449 rows could be quite long.
>>
>> You are right, I didn't test this. Issuing a query which returns a
>> smaller result set is much faster.
>>
>> But, offtopic, why would returning 8500 records, each around 100 bytes
>> long so around 8.5 MB, over local unix sockets, be so slow? The machine
>> in question has a sustained memory bendwidth of nearly 10 GB/s. Does
>> PostgreSQL spend much time marshalling the data through the socket
>> stream?
> 
> It's disk access time.
> in the very bad case it could take  ~5 ms (for fast drive) to get one just
> one row.

No, it's not that. The table fits in RAM, I've verified there is no disk
IO involved. Something else is wrong:

cms=> explain analyze select id,title from forum where _fts_ @@
'fer'::tsquery limit 10;
                                                         QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..43.31 rows=10 width=35) (actual time=0.194..0.373
rows=10 loops=1)
   ->  Index Scan using forum_fts on forum  (cost=0.00..394.10 rows=91
width=35) (actual time=0.182..0.256 rows=10 loops=1)
         Index Cond: (_fts_ @@ '''fer'''::tsquery)
 Total runtime: 0.507 ms
(4 rows)

cms=> explain analyze select id,title from forum where _fts_ @@
'fer'::tsquery order by id limit 10;
                                                            QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=363.18..363.20 rows=10 width=35) (actual
time=118.358..118.516 rows=10 loops=1)
   ->  Sort  (cost=363.18..363.40 rows=91 width=35) (actual
time=118.344..118.396 rows=10 loops=1)
         Sort Key: id
         Sort Method:  top-N heapsort  Memory: 25kB
         ->  Bitmap Heap Scan on forum  (cost=29.21..361.21 rows=91
width=35) (actual time=3.066..64.091 rows=8449 loops=1)
               Recheck Cond: (_fts_ @@ '''fer'''::tsquery)
               ->  Bitmap Index Scan on forum_fts  (cost=0.00..29.19
rows=91 width=0) (actual time=2.106..2.106 rows=8449 loops=1)
                     Index Cond: (_fts_ @@ '''fer'''::tsquery)
 Total runtime: 118.689 ms
(9 rows)

See in the first query where I have a simple LIMIT, it fetches random 10
rows quickly, but in the second one, as soon as I give it to execute and
calculate the entire result set before I limit it, the performance is
horrible.