Index performance

Hi,

I have a table with a primarry key made of two columns.
One of these has about 150 distinct values which are unique IDs, and
the other has over 3 million almost unique data values.
This table is added to in real time, at least 10 rows per second.

If I do a select which uses the pkey index, where equal to the ID
column, and greater than one of the values, which should return about
1500 rows, it sometimes takes 1/2 minute to return, and other times
takes only seconds.

Is it the number of rows being added in real time, that is maybe
causing the index to be locked?

I tried creating a partial index where equal to the ID column, and
that sped it up cconsiderably and consistently, but this is a headache
to maintain.

Any ideas?
Thanks
Brian

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