Function ExecHashGetHashValue

src/backend/executor/nodeHash.c:2087–2192

* ExecHashGetHashValue * Compute the hash value for a tuple * * The tuple to be tested must be in econtext->ecxt_outertuple (thus Vars in * the hashkeys expressions need to have OUTER_VAR as varno). If outer_tuple * is false (meaning it's the HashJoin's inner node, Hash), econtext, * hashkeys, and slot need to be from Hash, with hashkeys/slot referencing and * being suitable for tuples fro

 * hashkeys_null indicates all the hashkeys are null.
 */
bool
ExecHashGetHashValue(HashState *hashState, HashJoinTable hashtable,
					 ExprContext *econtext,
					 List *hashkeys,
					 bool outer_tuple,
					 bool keep_nulls,
					 uint32 *hashvalue,
					 bool *hashkeys_null)
{
	uint32		hashkey = 0;
	FmgrInfo   *hashfunctions;
	List	   *keyvalues = NIL;
	ListCell   *hk;
	int			i = 0;
	MemoryContext oldContext;
	bool		result = true;
	bool		build_runtime_filter;

	Assert(hashkeys_null);

	(*hashkeys_null) = true;

	/*
	 * We reset the eval context each time to reclaim any memory leaked in the
	 * hashkey expressions.
	 */
	ResetExprContext(econtext);

	oldContext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);

	if (outer_tuple)
		hashfunctions = hashtable->outer_hashfunctions;
	else
		hashfunctions = hashtable->inner_hashfunctions;
	
	build_runtime_filter = !outer_tuple && hashState->rfstate != NULL &&
						   !hashState->rfstate->build_suspend;

	foreach(hk, hashkeys)
	{
		ExprState  *keyexpr = (ExprState *) lfirst(hk);
		Datum		keyval;
		uint32		hkey = 0;
		bool		isNull = false;

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

		/*
		 * Get the join attribute value of the tuple
		 */
		keyval = ExecEvalExpr(keyexpr, econtext, &isNull);

		if (!isNull)
		{
			*hashkeys_null = false;
		}