Jeff wrote:
I think it was on this list - someone posted a message about SETOF being slower. Tom replied saying it was because it needed to create an on-disk tuplestore.

I was just looking for some clarification - a SETOF function will always write the reslting tuples to disk (Not buffering in say a sort_mem sized buffer)?

I think at least part of what you're seeing is normal function call overhead. As far as tuplestores writing to disk, here's what the source says:

In src/backend/utils/sort/tuplestore.c
 * maxKBytes: how much data to store in memory (any data beyond this
 * amount is paged to disk).  When in doubt, use work_mem.
Tuplestorestate *
tuplestore_begin_heap(bool randomAccess, bool interXact, int maxKBytes)

In src/backend/executor/execQual.c:ExecMakeTableFunctionResult():
tupstore = tuplestore_begin_heap(true, false, work_mem);

So up to work_mem (sort_mem in 7.4 and earlier) should be stored in memory.


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