How check execution plan of a function

Hi there,

I have a function which returns setof record based on a specific query.
I try to check the execution plan of that query, so I write EXPLAIN ANALYZE
before my select, I call the function and I see the result which shows an
actual time about 5 seconds. But when I call my function after I remove
EXPLAIN ANALYZE it takes more than 300 seconds, and I cancel it.

What's happen, or how can I see the function execution plan to optimize it ?

TIA,
Sabin