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

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 :)

--
Postgresql & php tutorials
http://www.designmagick.com/