If I have something like:

CREATE INDEX   idx_myTable_field_x
ON             myTable
USING          btree (field_x);

SELECT COUNT(field_x), field_x FROM myTable GROUP BY field_x ORDER BY field_x;

Imagine myTable with around 500,000 rows and most of field_x values being unique.

Since I don't use any WHERE clause, will the created index have any effect at all in my query?

Edit: I'm asking this question because I don't get any relevant difference between query-times before and after creating the index; They always take about 8 seconds (which, of course is too much time!). Is this behaviour expected?