Function hash_array

src/backend/utils/adt/arrayfuncs.c:4048–4174

 */

Datum
hash_array(PG_FUNCTION_ARGS)
{
	LOCAL_FCINFO(locfcinfo, 1);
	AnyArrayType *array = PG_GETARG_ANY_ARRAY_P(0);
	int			ndims = AARR_NDIM(array);
	int		   *dims = AARR_DIMS(array);
	Oid			element_type = AARR_ELEMTYPE(array);
	uint32		result = 1;
	int			nitems;
	TypeCacheEntry *typentry;
	int			typlen;
	bool		typbyval;
	char		typalign;
	int			i;
	array_iter	iter;

	/*
	 * We arrange to look up the hash function only once per series of calls,
	 * assuming the element type doesn't change underneath us.  The typcache
	 * is used so that we have no memory leakage when being used as an index
	 * support function.
	 */
	typentry = (TypeCacheEntry *) fcinfo->flinfo->fn_extra;
	if (typentry == NULL ||
		typentry->type_id != element_type)
	{
		typentry = lookup_type_cache(element_type,
									 TYPECACHE_HASH_PROC_FINFO);
		if (!OidIsValid(typentry->hash_proc_finfo.fn_oid) && element_type != RECORDOID)
			ereport(ERROR,
					(errcode(ERRCODE_UNDEFINED_FUNCTION),
					 errmsg("could not identify a hash function for type %s",
							format_type_be(element_type))));

		/*
		 * The type cache doesn't believe that record is hashable (see
		 * cache_record_field_properties()), but since we're here, we're
		 * committed to hashing, so we can assume it does.  Worst case, if any
		 * components of the record don't support hashing, we will fail at
		 * execution.
		 */
		if (element_type == RECORDOID)
		{
			MemoryContext oldcontext;
			TypeCacheEntry *record_typentry;

			oldcontext = MemoryContextSwitchTo(fcinfo->flinfo->fn_mcxt);

			/*
			 * Make fake type cache entry structure.  Note that we can't just
			 * modify typentry, since that points directly into the type cache.
			 */
			record_typentry = palloc0(sizeof(*record_typentry));
			record_typentry->type_id = element_type;

			/* fill in what we need below */
			record_typentry->typlen = typentry->typlen;