Will Index improve the speed?

Hi all,

I got a table with many columns of data which got an index on one of the
fields (Tsearch2 Gist).

I thought that maybe if I'll create a new table with 2 fields (primary key
reference to the previous table & the index field from the previous table)
and made the index on the index field, maybe then it will be faster to query
the new smaller table and then left join the results to the first table in
order to get the results.

Is that true or as I suspect the index is "in a way" a smaller table already
which is handling it even better then my idea?

Thanks a lot in advance,
Yonatan