Postgresql multi-column GIN index
The answer is same as the answer for this question: Inner join using an array column

The main point is installing the btree_gin extension (this may require additional package, like postgresql-contrib)

Then, the extension can be enabled easily with: (requires postgresql >= 9.1)

CREATE EXTENSION btree_gin;

And index can be created with:

CREATE INDEX index_name ON items USING GIN (account_id, keywords);