Introduce hash_search_with_hash_value() function

This new function iterates hash entries with given hash values.  This function
is designed to avoid full sequential hash search in the syscache invalidation
callbacks.

Discussion: https://postgr.es/m/5812a6e5-68ae-4d84-9d85-b443176966a1%40sigaev.ru
Author: Teodor Sigaev
Reviewed-by: Aleksander Alekseev, Tom Lane, Michael Paquier, Roman Zharkov
Reviewed-by: Andrei Lepikhov

src/backend/utils/hash/dynahash.c

@@ -1387,10 +1387,30 @@ hash_seq_init(HASH_SEQ_STATUS *status, HTAB *hashp)
 	status->hashp = hashp;
 	status->curBucket = 0;
 	status->curEntry = NULL;
+	status->hasHashvalue = false;
 	if (!hashp->frozen)
 		register_seq_scan(hashp);
 }
 
+/*
+ * Same as above but scan by the given hash value.
+ * See also hash_seq_search().
+ */
+void
+hash_seq_init_with_hash_value(HASH_SEQ_STATUS *status, HTAB *hashp,
+							  uint32 hashvalue)
+{
+	HASHBUCKET *bucketPtr;
+
+	hash_seq_init(status, hashp);
+
+	status->hasHashvalue = true;
+	status->hashvalue = hashvalue;
+
+	status->curBucket = hash_initial_lookup(hashp, hashvalue, &bucketPtr);
+	status->curEntry = *bucketPtr;
+}
+
 void *
 hash_seq_search(HASH_SEQ_STATUS *status)
 {
@@ -1404,6 +1424,24 @@ hash_seq_search(HASH_SEQ_STATUS *status)
 	uint32		curBucket;
 	HASHELEMENT *curElem;
 
+	if (status->hasHashvalue)
+	{
+		/*
+		 * Scan entries only in the current bucket because only this bucket
+		 * can contain entries with the given hash value.
+		 */
+		while ((curElem = status->curEntry) != NULL)
+		{
+			status->curEntry = curElem->link;
+			if (status->hashvalue != curElem->hashvalue)
+				continue;
+			return (void *) ELEMENTKEY(curElem);
+		}
+
+		hash_seq_term(status);
+		return NULL;
+	}
+
 	if ((curElem = status->curEntry) != NULL)
 	{
 		/* Continuing scan of curBucket... */

src/include/utils/hsearch.h

@@ -122,6 +122,8 @@ typedef struct
 	HTAB	   *hashp;
 	uint32		curBucket;		/* index of current bucket */
 	HASHELEMENT *curEntry;		/* current entry in bucket */
+	bool		hasHashvalue;	/* true if hashvalue was provided */
+	uint32		hashvalue;		/* hashvalue to start seqscan over hash */
 } HASH_SEQ_STATUS;
 
 /*
@@ -141,6 +143,9 @@ extern bool hash_update_hash_key(HTAB *hashp, void *existingEntry,
 								 const void *newKeyPtr);
 extern long hash_get_num_entries(HTAB *hashp);
 extern void hash_seq_init(HASH_SEQ_STATUS *status, HTAB *hashp);
+extern void hash_seq_init_with_hash_value(HASH_SEQ_STATUS *status,
+										  HTAB *hashp,
+										  uint32 hashvalue);
 extern void *hash_seq_search(HASH_SEQ_STATUS *status);
 extern void hash_seq_term(HASH_SEQ_STATUS *status);
 extern void hash_freeze(HTAB *hashp);