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