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Re: [HACKERS] full text search index scan query plan changed in 8.4.2?

From: Chris <dmagick(at)gmail(dot)com>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: xu fei <autofei(at)yahoo(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [HACKERS] full text search index scan query plan changed in 8.4.2?
Date: 2010-03-01 02:38:08
Message-ID: 4B8B2890.40000@gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Josh Berkus wrote:
> Xufei,
> 
> List changed to psql-performance, which is where this discussion belongs.
> 
>> I am testing the index used by full text search recently.
>>
>> I have install 8.3.9 and 8.4.2 separately. 
>>
>> In 8.3.9, the query plan is like:
>>
>> postgres=# explain SELECT s.name as source , t.name as target FROM element as s, element as t WHERE to_tsvector('testcfg',s.name)  @@ to_tsquery('testcfg',replace(t.name,':','|'));                                                                         QUERY PLAN                     ------------------------------------------------------------------------------------------------------------------------------------------------------------ 
>> Nested Loop  (cost=0.01..259.92 rows=491 width=18)   
>>  ->  Seq Scan on element t  (cost=0.00..13.01 rows=701 width=9)   
>>  ->  Index Scan using element_ftsidx_test on element s  (cost=0.01..0.33 rows=1 width=9)         
>>      Index Cond: (to_tsvector('testcfg'::regconfig, (s.name)::text) @@ to_tsquery('testcfg'::regconfig, replace((t.name)::text, ':'::text, '|'::text)))
>> (4 rows)
>>
>> I have index: "element_ftsidx_test" gin (to_tsvector('testcfg'::regconfig, name::text))
>>
>> The same index and query in 8.4.2: 
>>
>> postgres=# explain SELECT s.name as source , t.name as target FROM element as s, element as t WHERE to_tsvector('testcfg',s.name)  @@ to_tsquery('testcfg',replace(t.name,':','|')) ;                                                                            QUERY PLAN               ------------------------------------------------------------------------------------------------------------------------------------------------------------------ 
>> Nested Loop  (cost=0.32..3123.51 rows=2457 width=18)   
>>  ->  Seq Scan on element t  (cost=0.00..13.01 rows=701 width=9)   
>>  ->  Bitmap Heap Scan on element s  (cost=0.32..4.36 rows=4 width=9)              Recheck Cond: (to_tsvector('testcfg'::regconfig, (s.name)::text) @@ to_tsquery('testcfg'::regconfig, replace((t.name)::text, ':'::text, '|'::text)))         
>>         ->  Bitmap Index Scan on element_ftsidx_test  (cost=0.00..0.32 rows=4 width=0)
>>              Index Cond: (to_tsvector('testcfg'::regconfig, (s.name)::text) @@ to_tsquery('testcfg'::regconfig, replace((t.name)::text, ':'::text, '|'::text)))
>> (6 rows)
>>
>> Why the query plans are different and why? Thanks!
> 
> Because the row estimates changed, since 8.4 improved row estimation for
> TSearch.  The 2nd query is probably actually faster, no?  If not, you
> may need to increase your stats collection.  Or at least show us a
> VACUUM ANALYZE.

I'm sure you mean explain analyze :)

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