MemoryContextSwitchTo() confusion

Hi,

I've written a custom C-language function that takes a SQL select
statement as it's input parameter, runs the query using the SPI
interface, iterates over all the results of the select using a cursor
and collates the data using a tsearch type binary tree, then walks the
tree to create a flat vector of ordered nodes all in the first call
(i.e. SRF_IS_FIRSTCALL()). It then contructs tuples, turns them into
Datums and returns all it's records with subsequent calls.

I've used the example code in the documentation, and used the doxygen
docs for tablefunc.c as guidelines.

Everything seems to work fine, except on the final call it has no more
data to return so cleans up all it's internal data structures and
returns with SRF_RETURN_DONE(funcctx).

I had to fiddle about with switching memory contexts rather a lot to
make it work this far, but I'm only guessing as to when it's appropriate
to call MemoryContextSwitchTo(), and to which context to switch to. I
had a few SEGV's and had to add a few MemoryContextSwitchTo() calls in
various places, however, can someone please explain the need for
MemoryContextSwitchTo() and when it's needed, as I'm basically guessing
and I think it's the reason for the SEGV after the final call to
SRF_RETURN_DONE() after it's returned all it's tuples.

I have verified that all the code seems to work, and that it is after
the final call to SRF_RETURN_DONE() that the SEGV happens by writing
debug strings to a file handle.

I can provide a copy of the sourece code, but I thought it best to ask
first.

Regards, Dan...

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