Function trim_array

src/backend/utils/adt/arrayfuncs.c:6800–6837

* Trim the last N elements from an array by building an appropriate slice. * Only the first dimension is trimmed. */

 * Only the first dimension is trimmed.
 */
Datum
trim_array(PG_FUNCTION_ARGS)
{
	ArrayType  *v = PG_GETARG_ARRAYTYPE_P(0);
	int			n = PG_GETARG_INT32(1);
	int			array_length = ARR_DIMS(v)[0];
	int16		elmlen;
	bool		elmbyval;
	char		elmalign;
	int			lower[MAXDIM];
	int			upper[MAXDIM];
	bool		lowerProvided[MAXDIM];
	bool		upperProvided[MAXDIM];
	Datum		result;

	/* Per spec, throw an error if out of bounds */
	if (n < 0 || n > array_length)
		ereport(ERROR,
				(errcode(ERRCODE_ARRAY_ELEMENT_ERROR),
				 errmsg("number of elements to trim must be between 0 and %d",
						array_length)));

	/* Set all the bounds as unprovided except the first upper bound */
	memset(lowerProvided, false, sizeof(lowerProvided));
	memset(upperProvided, false, sizeof(upperProvided));
	upper[0] = ARR_LBOUND(v)[0] + array_length - n - 1;
	upperProvided[0] = true;

	/* Fetch the needed information about the element type */
	get_typlenbyvalalign(ARR_ELEMTYPE(v), &elmlen, &elmbyval, &elmalign);

	/* Get the slice */
	result = array_get_slice(PointerGetDatum(v), 1,
							 upper, lower, upperProvided, lowerProvided,
							 -1, elmlen, elmbyval, elmalign);

	PG_RETURN_DATUM(result);
}