PostgreSQL, Second Edition

The PostgreSQL SRF Interface

First off, you should know that the new SRF interface is simply a wrapper around the old method. Set-returning functions are still invoked multiple times. The first time through, an SRF initializes its own context structure and stores that structure away so that each subsequent invocation can find it. In the old approach, an SRF examined fmgr_info->fn_extra to determine whether it was being invoked for the first time (if fmgr_info->fn_extra is NULL, this is the first call). In the new approach, you call the SRF_IS_FIRSTCALL() macro instead. You can probably guess what this macro does: It returns TRUE if fmgr_info->fn_extra is NULL (implying that this is the first call). In fact, here's the definition of the SRF_IS_FIRSTCALL() ...