create trigger : functions

Hi,
I have written a 'C' function to be called during INSERT trigger on a
table.

Are there any restrictions on the functions that can be called?

I know you can call SPI_* functions. But, can I call PQ* functions ?
e.g PQsetdb.

Is there any document which describes how the functions written for
trigger are executed?
Do they executed within the same process as server process or another
process is started to execute the commands?

regards,

Sandeep