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Berkeley DB Reference Guide:
  Access Methods

### Retrieving records in bulk

When retrieving large numbers of records from the database, the number of method calls can often dominate performance. Berkeley DB offers bulk get interfaces which can significantly increase performance for some applications. To retrieve records in bulk, an application buffer must be specified to the DB->get or DBcursor->get methods. This is done in the C API by setting the data and ulen fields of the data DBT to reference an application buffer, and the flags field of that structure to DB_DBT_USERMEM. In the Berkeley DB C++ and Java APIs, the actions are similar, although there are API-specific methods to set the DBT values. Then, the DB_MULTIPLE or DB_MULTIPLE_KEY flags are specified to the DB->get or DBcursor->get methods, which cause multiple records to be returned in the specified buffer.

The difference between DB_MULTIPLE and DB_MULTIPLE_KEY is as follows: DB_MULTIPLE returns multiple data items for a single key. For example, the DB_MULTIPLE flag would be used to retrieve all of the duplicate data items for a single key in a single call. The DB_MULTIPLE_KEY flag is used to retrieve multiple key/data pairs, where each returned key may or may not have duplicate data items.

Once the DB->get or DBcursor->get method has returned, the application will walk through the buffer handling the returned records. This is implemented for the C and C++ APIs using four macros: DB_MULTIPLE_INIT, DB_MULTIPLE_NEXT, DB_MULTIPLE_KEY_NEXT, and DB_MULTIPLE_RECNO_NEXT. For the Java API, this is implemented as three iterator classes: MultipleDataEntry, MultipleKeyDataEntry, and MultipleRecnoDataEntry.

The DB_MULTIPLE_INIT macro is always called first. It initializes a local application variable and the data DBT for stepping through the set of returned records. Then, the application calls one of the remaining three macros: DB_MULTIPLE_NEXT, DB_MULTIPLE_KEY_NEXT, and DB_MULTIPLE_RECNO_NEXT.

If the DB_MULTIPLE flag was specified to the DB->get or DBcursor->get method, the application will always call the DB_MULTIPLE_NEXT macro. If the DB_MULTIPLE_KEY flag was specified to the DB->get or DBcursor->get method, and, the underlying database is a Btree or Hash database, the application will always call the DB_MULTIPLE_KEY_NEXT macro. If the DB_MULTIPLE_KEY flag was specified to the DB->get or DBcursor->get method, and, the underlying database is a Queue or Recno database, the application will always call the DB_MULTIPLE_RECNO_NEXT macro. The DB_MULTIPLE_NEXT, DB_MULTIPLE_KEY_NEXT, and DB_MULTIPLE_RECNO_NEXT macros are called repeatedly, until the end of the returned records is reached. The end of the returned records is detected by the application's local pointer variable being set to NULL.

The following is an example of a routine that displays the contents of a Btree database using the bulk return interfaces.

>     int
>     rec_display(dbp)
>     	DB *dbp;
>     {
>     	DBC *dbcp;
>     	DBT key, data;
>     	size_t retklen, retdlen;
>     	char *retkey, *retdata;
>     	int ret, t_ret;
>     	void *p;

>     	memset(&key, 0, sizeof(key));
>     	memset(&data, 0, sizeof(data));

>     	/* Review the database in 5MB chunks. */
>     #define	BUFFER_LENGTH	(5 * 1024 * 1024)
>     	if ((data.data = malloc(BUFFER_LENGTH)) == NULL)
>     		return (errno);
>     	data.ulen = BUFFER_LENGTH;
>     	data.flags = DB_DBT_USERMEM;

>     	/* Acquire a cursor for the database. */
>     	if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) {
>     		dbp->err(dbp, ret, "DB->cursor");
>     		free(data.data);
>     		return (ret);
>     	}

>     	for (;;) {
>     		/*
>     		 * Acquire the next set of key/data pairs.  This code does
>     		 * not handle single key/data pairs that won't fit in a
>     		 * BUFFER_LENGTH size buffer, instead returning DB_BUFFER_SMALL
>     		 * to our caller.
>     		 */
>     		if ((ret = dbcp->c_get(dbcp,
>     		    &key, &data, DB_MULTIPLE_KEY | DB_NEXT)) != 0) {
>     			if (ret != DB_NOTFOUND)
>     				dbp->err(dbp, ret, "DBcursor->c_get");
>     			break;
>     		}

>     		for (DB_MULTIPLE_INIT(p, &data);;) {
>     			DB_MULTIPLE_KEY_NEXT(p,
>     			    &data, retkey, retklen, retdata, retdlen);
>     			if (p == NULL)
>     				break;
>     			printf("key: %.*s, data: %.*s\n",
>     			    (int)retklen, retkey, (int)retdlen, retdata);
>     		}
>     	}

>     	if ((t_ret = dbcp->c_close(dbcp)) != 0) {
>     		dbp->err(dbp, ret, "DBcursor->close");
>     		if (ret == 0)
>     			ret = t_ret;
>     	}

>     	free(data.data);

>     	return (ret);
>     }

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