Add some basic helpers to make use of dynahash hashtables easier.

src/backend/distributed/utils/hash_helpers.c

@@ -0,0 +1,34 @@
+/*-------------------------------------------------------------------------
+ *
+ * hash_helpers.c
+ *   Helpers for dynahash.c style hash tables.
+ *
+ * Copyright (c) 2016, Citus Data, Inc.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "distributed/hash_helpers.h"
+#include "utils/hsearch.h"
+
+
+/*
+ * Empty a hash, without destroying the hash table itself.
+ */
+void
+hash_delete_all(HTAB *htab)
+{
+	HASH_SEQ_STATUS status;
+	void *entry = NULL;
+
+	hash_seq_init(&status, htab);
+	while ((entry = hash_seq_search(&status)) != 0)
+	{
+		bool found = false;
+
+		hash_search(htab, entry, HASH_REMOVE, &found);
+		Assert(found);
+	}
+}

src/include/distributed/hash_helpers.h

@@ -0,0 +1,30 @@
+/*-------------------------------------------------------------------------
+ * hash_helpers.h
+ *
+ * Copyright (c) 2016, Citus Data, Inc.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef HASH_HELPERS_H
+#define HASH_HELPERS_H
+
+#include "utils/hsearch.h"
+
+/*
+ * Combine two hash values, resulting in another hash value, with decent bit
+ * mixing.
+ *
+ * Similar to boost's hash_combine().
+ */
+static inline uint32
+hash_combine(uint32 a, uint32 b)
+{
+	a ^= b + 0x9e3779b9 + (a << 6) + (a >> 2);
+	return a;
+}
+
+
+extern void hash_delete_all(HTAB *htab);
+
+#endif