Postgres full text search on words, not lexemes
Just use the simple text search configuration, which works with the simple dictionary and does not do any stemming:

CREATE INDEX ON tbl USING GIN (to_tsvector('simple', data));


And query accordingly:

SELECT *
FROM   tbl
WHERE  to_tsvector('simple', data) @@ to_tsquery('simple', 'foo');


Related:


Get partial match from GIN indexed TSVECTOR column
Finding the most commonly used non-stop words in a column
PostgreSQL full text search on many columns