Is there a way to index in postgres for fast substring searches

Options for text search and indexing include:

• full-text indexing with dictionary based search, including support for prefix-search, eg to_tsvector(mycol) @@ to_tsquery('search:*')

• text_pattern_ops indexes to support prefix string matches eg LIKE 'abc%' but not infix searches like %blah%;. A reverse()d index may be used for suffix searching.

• pg_tgrm trigram indexes on newer versions as demonstrated in this recent dba.stackexchange.com post.

• An external search and indexing tool like Apache Solr.

From the minimal information given above, I'd say that only a trigram index will be able to help you, since you're doing infix searches on a string and not looking for dictionary words. Unfortunately, trigram indexes are huge and rather inefficient; don't expect some kind of magical performance boost, and keep in mind that they take a lot of work for the database engine to build and keep up to date.

For the like operator use one of the operator classes varchar_pattern_ops or text_pattern_ops

create index test_index on test_table (col varchar_pattern_ops);

That will only work if the pattern does not start with a % in which case another strategy is required.

If you need just to, for instance, get unique substrings in an entire table, you can create a substring index:

CREATE INDEX  i_test_sbstr ON tablename (substring(columname, 5, 3)); 
-- start at position 5, go for 3 characters

It is important that the substring() parameters in the index definition are
the same as you use in your query.

ref: http://www.postgresql.org/message-id/[email protected]