On Tue, 12 Feb 2008, Mario Lopez wrote:
Hi!,
I optimized the LIKE 'keyword%' and LIKE '%keyword' with the following
results:
# time /Library/PostgreSQL8/bin/psql -U postgres -d testdb -c "select * from
table1 where varchar_reverse(data) like varchar_reverse('%keyword');"
real 0m0.055s
user 0m0.011s
sys 0m0.006s
# time /Library/PostgreSQL8/bin/psql -U postgres -d testdb -c "select * from
table1 where data like 'keyword%';"
real 0m0.026s
user 0m0.012s
sys 0m0.006s
It works flawlesly as you can see by the timings, take in consideration that
"table1" has 100 million records. The only problem is generating the reversed
index which takes like 20 minutes, I guess it has to do with the plperl
function, perhaps a C function for inverting would make it up in less time.
The problem is still with the LIKE '%keyword%', my problem is that I am not
searching for Words in a dictionary fashion, suppose my "data" is random
garbage, that it has common consecutive bytes. How could I generate a
dictionary from this random garbage to make it easier for indexing?
suffix tree (array) would speedup '%keyword%' query, but currently it doesn't
supported by GiST extension architecture (we have it in our TODO), so I see
several ways (not tested):
1. try contrib/pg_trgm to reduce a number of candidate words
2. generate all possible substrings and use your tested approach
On Mon, Feb 11, 2008 at 04:37:24PM +0100, Mario Lopez wrote:
SELECT * FROM names WHERE name LIKE keyword%
Or
SELECT * FROM names WHERE name LIKE %keyword%
check this:
http://www.depesz.com/index.php/2007/07/30/indexable-field-like-something/
and this:
http://www.depesz.com/index.php/2007/09/15/speeding-up-like-xxx/
depesz
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: [EMAIL PROTECTED], http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83
---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster
