postgresql sticking “enable_seqscan=False” in a UDF sensible?

Question

I have a query which is taking 40 seconds to execute over the 80ms that it should if postgres where to filter off the primary key rather than doing a sequential scan. I have to force postgres to filter off the index by setting the above flag.

Having to stick this into the UDF seems like quite a hack to me, I mean it's unreasonable IMO that the query planner is chosing a seq scan over a indexed filter. Most of my SQL experience is off SQL Server, and I haven't experienced it missplaning such basic optimizations.

Edit: To anyone interested, the query exeuction time droped from 80ms to 27 ms with 9.2, holly molley.