Are there any plans to provide support for keyword indexes in Postgre? The
idea is to efficiently index every word in a long text column, in order to
support queries such as :
Select... from Products where ProductDescription like "%wool%" and
ProductDescription like "%black%"
This will work but it requires a table scan.
I found an article in the contrib directory by Maarten Boekhold
<maartenb@dutepp0.et.tudelft.nl> (see below) , it's a good interim solution
but it would be much more efficient if this functionality were built-in.
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An attempt at some sort of Full Text Indexing for PostgreSQL.
The included software is an attempt to add some sort of Full Text
Indexing
support to PostgreSQL. I mean by this that we can ask questions like:
Give me all rows that have 'still' and 'nash' in the 'artist' or
'title'fields.
Ofcourse we can write this as:
select * from cds where (artist ~* 'stills' or title ~*
'stills') and(artist ~* 'nash' or title ~* 'nash');
But this does not use any indices, and therefore, if your database
gets very large, it will not have very high performance (the above query
requires at least one sequential scan, it probably takes 2 due to the
self-join).
The approach used by this add-on is to define a trigger on the table and
columns you want to do this queries on. On every insert in the table, it
takes the value in the specified columns, breaks the text in these
columns
up into pieces, and stores all sub-strings into another table, together
with a reference to the row in the original table that contained this
sub-string (it uses the oid of that row).
By now creating an index over the 'fti-table', we can search for
substrings that occur in the original table. By making a join between
the fti-table and the orig-table, we can get the actual rows we want
(this can also be done by using subselects - but subselects are
currently
inefficient in Postgres, and maybe there're other ways too).
The trigger code also allows an array called StopWords, that prevents
certain words from being indexed.
As an example we take the previous query, where we assume we have all
sub-strings in the table 'cds-fti':
select c.*from cds c, cds-fti f1, cds-fti f2where f1.string ~ '^stills' and f2.string ~ '^nash' and f1.id =
c.oidand f2.id = c.oid ;
We can use the ~ (case-sensitive regular expression) here, because of
the way sub-strings are built: from right to left, ie. house -> 'se' +
'use' + 'ouse' + 'house'. If a ~ search starts with a ^ (match start of
string), btree indices can be used by PostgreSQL.
Now, how do we create the trigger that maintains the fti-table? First:
the
fti-table should have the following schema:
create cds-fti ( string varchar(N), id oid );
Don't change the *names* of the columns, the varchar() can in fact also
be of text-type. If you do use varchar, make sure the largest possible
sub-string will fit.
The create the function that contains the trigger::
create function fti() returns opaque as '/path/to/fti.so' language 'C';
And finally define the trigger on the 'cds' table:
create trigger cds-fti-trigger after update or insert or delete
on cds for each row execute procedure fti(cds-fti, artist, title);
Here, the trigger will be defined on table 'cds', it will create
sub-strings from the fields 'artist' and 'title', and it will place
those sub-strings in the table 'cds-fti'.
Now populate the table 'cds'. This will also populate the table
'cds-fti'.
It's fastest to populate the table *before* you create the indices. Use
the
supplied 'fti.pl' to assist you with this.
Before you start using the system, you should at least have the
following
indices:
create index cds-fti-idx on cds-fti (string); -- String matchingcreate index cds-fti-idx on cds-fti (id); -- For
deletinga
cds rowcreate index cds-oid-idx on cds (oid); -- For
joining cds to cds-fti
To get the most performance out of this, you should have 'cds-fti'
clustered on disk, ie. all rows with the same sub-strings should be
close to each other. There are 3 ways of doing this:
1. After you have created the indices, execute 'cluster cds-fti-idx on
cds-fti'.
2. Do a 'select * into tmp-table from cds-fti order by string' *before* you create the indices, then 'drop table
cds-fti'and 'alter table tmp-table rename to cds-fti'
3. *Before* creating indices, dump the contents of the cds-fti table
using 'pg_dump -a -t cds-fti dbase-name', remove the \connect from the beginning and the \. from the end, and sort it
usingthe UNIX 'sort' program, and reload the data.
Method 1 is very slow, 2 a lot faster, and for very large tables, 3 is
preferred.
