Re: trgm regex index peculiarity

On Fri, June 21, 2013 05:25, Tom Lane wrote:
> "Erik Rijkers" <er@xs4all.nl> writes:
>> In a 112 MB test table (containing random generated text) with a trgm index (gin_trgm_ops), I consistently get
these
>> timings:
>> select txt from azjunk6 where txt ~ '^abcd';
>>    130 ms
>> select txt from azjunk6
>> where txt ~ 'abcd' and substr(txt,1,4) = 'abcd';
>>    3 ms
>
> Hm, could you provide a self-contained test case?
>

yes, sorry.   I tested on a 1M row table:

#!/bin/sh

# create table:
for power in 6;
do table=azjunk${power} index=${table}_trgm_re_idx perl -E' sub ss{ join"",@_[ map{rand @_} 1 .. shift ] };
say(ss(80,"a".."g","","h".."m"," ","n".."s"," ","t".."z")) for 1 .. 1e'"${power};" \ | psql -aqXc "   drop table if
exists$table;   create table $table(txt text);   copy $table from stdin;"; echo "set session
maintenance_work_mem='1GB';  create index $index on $table using gin (txt gin_trgm_ops);   analyze $table;" | psql
-qtAX;
done

# test:
echo "
\\timing on
explain analyze select txt from azjunk6 where txt ~ '^abcd';  -- slow (140 ms)
explain analyze select txt from azjunk6 where txt ~ 'abcd' and substr(txt,1,4) = 'abcd'; -- fast (5 ms)
" | psql -Xqa