What kind of index should I use ?

Hi,

I have a table with a lot of fields and I would want to obtain a quicker
query response.

The main characteristics are:

The table is NOT often updated, I mean INSERT and UPDATE are sporadic
sentences, but SELECT sentences are often executed.

The most used query is something like

select distinct field1 from my_table where field1 is not null and field2
like '...'

Should I use a BTREE index or would be better to use a HASH or RTREE ?

Thanks,

Gabi :-)