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Re: Tsearch2 Initial Search Speed

From: Howard Cole <howardnews(at)selestial(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Tsearch2 Initial Search Speed
Date: 2008-06-17 10:54:12
Message-ID: 485797D4.7010201@selestial.com (view raw or flat)
Thread:
Lists: pgsql-performance
I think I may have answered my own question partially, the problem may 
be how I structure the query.

I always structured my tsearch queries as follows following my initial 
read of the tsearch2 instructions...

select email_id from email, to_tsquery('default', 'howard') as q where 
q@@fts;

However if I construct them in the following way, as stipulated in the 
8.3 documentation....

select email_id from email where fts@@to_tsquery('default','howard')

Then the results are better due to the fact that the email table is not 
necessarily scanned as can be seen from the two analyse statements:

Original statement:

"Nested Loop  (cost=4.40..65.08 rows=16 width=8)"
"  ->  Function Scan on q  (cost=0.00..0.01 rows=1 width=32)"
"  ->  Bitmap Heap Scan on email  (cost=4.40..64.87 rows=16 width=489)"
"        Filter: (email.fts @@ q.q)"
"        ->  Bitmap Index Scan on email_fts_index  (cost=0.00..4.40 
rows=16 width=0)"
"              Index Cond: (email.fts @@ q.q)"

Second statement:

"Bitmap Heap Scan on email  (cost=4.40..64.91 rows=16 width=8)"
"  Filter: (fts @@ '''howard'''::tsquery)"
"  ->  Bitmap Index Scan on email_fts_index  (cost=0.00..4.40 rows=16 
width=0)"
"        Index Cond: (fts @@ '''howard'''::tsquery)"

This misses out the random access of the email table, turning my 27 
second query into 6 seconds.

I guess the construction of the first statement effectively stops the 
query optimisation from working.


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Subject: Re: Tsearch2 Initial Search Speed
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Subject: Re: Tsearch2 Initial Search Speed

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