Optimising inside transactions - Mailing list pgsql-novice

Hi,

I'm running a transaction with about 1600 INSERTs.
Each INSERT involves a subselect.

I've noticed that if one of the INSERTs fails, the remaining INSERTs run in about
1/2 the time expected.

Is postgresql optimising the inserts, knowing that it will rollback at the end ?

If not, why do the queries run faster after the failure ?

Thanks
JohnT