Function TupleHashTableHash_internal

src/backend/executor/execGrouping.c:436–496

* If tuple is NULL, use the input slot instead. This convention avoids the * need to materialize virtual input tuples unless they actually need to get * copied into the table. * * Also, the caller must select an appropriate memory context for running * the hash functions. (dynahash.c doesn't change CurrentMemoryContext.) */

 * the hash functions. (dynahash.c doesn't change CurrentMemoryContext.)
 */
static uint32
TupleHashTableHash_internal(struct tuplehash_hash *tb,
							const MinimalTuple tuple)
{
	TupleHashTable hashtable = (TupleHashTable) tb->private_data;
	int			numCols = hashtable->numCols;
	AttrNumber *keyColIdx = hashtable->keyColIdx;
	uint32		hashkey = hashtable->hash_iv;
	TupleTableSlot *slot;
	FmgrInfo   *hashfunctions;
	int			i;

	if (tuple == NULL)
	{
		/* Process the current input tuple for the table */
		slot = hashtable->inputslot;
		hashfunctions = hashtable->in_hash_funcs;
	}
	else
	{
		/*
		 * Process a tuple already stored in the table.
		 *
		 * (this case never actually occurs due to the way simplehash.h is
		 * used, as the hash-value is stored in the entries)
		 */
		slot = hashtable->tableslot;
		ExecStoreMinimalTuple(tuple, slot, false);
		hashfunctions = hashtable->tab_hash_funcs;
	}

	for (i = 0; i < numCols; i++)
	{
		AttrNumber	att = keyColIdx[i];
		Datum		attr;
		bool		isNull;

		/* rotate hashkey left 1 bit at each step */
		hashkey = (hashkey << 1) | ((hashkey & 0x80000000) ? 1 : 0);

		attr = slot_getattr(slot, att, &isNull);

		if (!isNull)			/* treat nulls as having hash key 0 */
		{
			uint32		hkey;

			hkey = DatumGetUInt32(FunctionCall1Coll(&hashfunctions[i],
													hashtable->tab_collations[i],
													attr));
			hashkey ^= hkey;
		}
	}

	/*
	 * The way hashes are combined above, among each other and with the IV,
	 * doesn't lead to good bit perturbation. As the IV's goal is to lead to
	 * achieve that, perform a round of hashing of the combined hash -
	 * resulting in near perfect perturbation.