SELECT from record-returning function causes function code to be executed multiple times

Hello,

I have a simple C-language function , SimpleSRF(), that returns RECORDs with
two integer columns. When I call the function using SELECT (SIMPLESRF()).*,
the function code is being executed twice. I'm wondering if this is
expected behavior?

The function is defined as:

CREATE OR REPLACE FUNCTION SIMPLESRF(OUT id integer, OUT type integer)
RETURNS SETOF RECORD
AS '$libdir/pg1.dll', 'simplesrf'
LANGUAGE C IMMUTABLE STRICT;

The function follows the basic structure for an SRF in the docs. I included
some output statements in the function like this so that I could keep track
of the repeated calls.

elog(INFO, "Call %i", funcctx->call_cntr);

Calling the function like this: SELECT (SIMPLESRF()).* gives me the
following messages:

INFO: Call 0
INFO: Call 0
INFO: Call 1
INFO: Call 1
INFO: Call 2
INFO: Call 2
INFO: Call 3
INFO: Call 3
INFO: Call 4
INFO: Call 4
INFO: Call 5
INFO: Call 5

Total query runtime: 18 ms.
5 rows retrieved.

Apparently the function code is being executed twice, once for each column
in the return tuple.

Alternatively, I can call the function like this: SELECT * FROM SIMPLESRF()
This gives me the messages I'd expect:

INFO: Call 0
INFO: Call 1
INFO: Call 2
INFO: Call 3
INFO: Call 4
INFO: Call 5

Total query runtime: 12 ms.
5 rows retrieved.

Is this expected behavior? I would think that the function would only need
to be executed once with either calling pattern, especially since it's been
declared as IMMUTABLE.

I do get different plans with the two calling patterns, though I don't know
enough to understand the differences.

EXPLAIN SELECT (SIMPLESRF()).*
"Result (cost=0.00..5.01 rows=1000 width=0)"

EXPLAIN SELECT * FROM SIMPLESRF()
"Function Scan on simplesrf (cost=0.00..10.00 rows=1000 width=8)"

Function code, though I don't think it's relevant:

PG_FUNCTION_INFO_V1(simplesrf);
__declspec(dllexport) Datum simplesrf(PG_FUNCTION_ARGS) {
FuncCallContext *funcctx;
TupleDesc resultTupleDesc;
HeapTuple returnTuple;
Datum returnValues[2];
bool returnNulls[2] = {false};

if (SRF_IS_FIRSTCALL()) {
struct vertex** a;
MemoryContext oldContext;
funcctx = SRF_FIRSTCALL_INIT();
oldContext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

a = (struct vertex**) palloc(4 * sizeof(struct vertex*));

resultTupleDesc = CreateTemplateTupleDesc(2, false);
TupleDescInitEntry(resultTupleDesc, (AttrNumber) 1, "call", INT4OID, -1,
0);
TupleDescInitEntry(resultTupleDesc, (AttrNumber) 2, "call2", INT4OID, -1,
0);
resultTupleDesc = BlessTupleDesc(resultTupleDesc);
funcctx->tuple_desc = resultTupleDesc;
funcctx->max_calls = 5;

MemoryContextSwitchTo(oldContext);
}

funcctx = SRF_PERCALL_SETUP();
resultTupleDesc = funcctx->tuple_desc;
elog(INFO, "Call %i", funcctx->call_cntr);

if (funcctx->call_cntr < funcctx->max_calls) {
returnValues[0] = Int32GetDatum(1);
returnValues[1] = Int32GetDatum(2);
returnTuple = heap_form_tuple(resultTupleDesc, returnValues, returnNulls);

SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(returnTuple));
} else {
SRF_RETURN_DONE(funcctx);
}
}

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