How do I index the following statement with an un-anchored search pattern
SELECT somefield FROM sometable WHERE lower(somefield2) like '%foo%';
Some rows have more than 2k bytes.
The length aside, a btree index would not help that query. You could create a hash index but that would also only help if the query wants an exact match for the whole column. not for a substring pattern. To do what you want first, add the
CREATE EXTENSION pg_trgm;
Then create a trigram index:
CREATE INDEX trgm_idx ON sometable USING GIN (somefield2 gin_trgm_ops); -- can also be GIST
A trigram index can help find matches for SQL's
ILIKE and regex patterns.
For more information see the docs on