Extract constants from EXECUTE queries

Hi hackers, I wanted to extract constants hard coded within prepared statements.
ex:-

PREPARE stmt(text, int) as SELECT * FROM test_table WHERE a = $1 AND b = 99 AND c = $2;
EXECUTE stmt('abc', 1);

I can easily get the parameter values ('abc' and 1) from queryDesc->params, but I need to also extract the constant value (99) from within the queryDesc structure during EXECUTE.

I've tried traversing the plan tree like this:

List *constants = NIL;
extract_constants_from_plan(queryDesc->plannedstmt->planTree, &constants);

ListCell *lc;
foreach(lc, constants)
{
Const *c = (Const *) lfirst(lc);

if (!c->constisnull)
{
char *valueStr = NULL;

switch (c->consttype)
{
case BOOLOID:
valueStr = DatumGetBool(c->constvalue) ? "true" : "false";
break;

case INT2OID:
valueStr = psprintf("%d", DatumGetInt16(c->constvalue));
break;

case INT4OID:
valueStr = psprintf("%d", DatumGetInt32(c->constvalue));
break;

case INT8OID:
valueStr = psprintf("%ld", DatumGetInt64(c->constvalue));
break;

case FLOAT4OID:
valueStr = psprintf("%f", DatumGetFloat4(c->constvalue));
break;

case FLOAT8OID:
valueStr = psprintf("%f", DatumGetFloat8(c->constvalue));
break;

case TEXTOID:
case VARCHAROID:
case BPCHAROID:
valueStr = TextDatumGetCString(c->constvalue);
break;

default:
/* For unknown types, try to convert to string using output function */
valueStr = OidOutputFunctionCall(c->consttype, c->constvalue);
break;
}

FILE* fptr = fopen("/Users/abc/test.txt","a");
fprintf(fptr, "Constant value: %s\n", valueStr);
fclose(fptr);
}
}

But this does not seems to work for select statements, though this works for fine for prepared insert statements. Is there a general/ simpler way to do this?

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