functions and plans

I am trying to figure out some issues with my functions and I was hoping
someone could help me.
My understanding is that the plan is made when the function is run the
first time. That means that running analyze on a table after a function
is run once won't change the plan.

I have a table that I populate, select and delete the data each time the
function is run. Each time I run the function it takes longer to get the
data. In these tests, the data is the exact same every time.

I tried putting vacuum inside the function, but I got an error, cannot
run vacuum inside of function.

Any thoughts?