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<title>Berkeley DB Reference Guide: Retrieving records in bulk</title>
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<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></b></td>
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<p align=center><b>Retrieving records in bulk</b></p>
<p>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 <a href="../../api_c/db_get.html">DB->get</a> or <a href="../../api_c/dbc_get.html">DBcursor->get</a> methods. This is done
in the C API by setting the <b>data</b> and <b>ulen</b> fields of the
<b>data</b> <a href="../../api_c/dbt_class.html">DBT</a> to reference an application buffer, and the
<b>flags</b> field of that structure to <a href="../../api_c/dbt_class.html#DB_DBT_USERMEM">DB_DBT_USERMEM</a>. In
the Berkeley DB C++ and Java APIs, the actions are similar, although there
are API-specific methods to set the <a href="../../api_c/dbt_class.html">DBT</a> values. Then, the
<a href="../../api_c/dbc_get.html#DB_MULTIPLE">DB_MULTIPLE</a> or <a href="../../api_c/dbc_get.html#DB_MULTIPLE_KEY">DB_MULTIPLE_KEY</a> flags are specified to
the <a href="../../api_c/db_get.html">DB->get</a> or <a href="../../api_c/dbc_get.html">DBcursor->get</a> methods, which cause multiple records
to be returned in the specified buffer.</p>
<p>The difference between <a href="../../api_c/dbc_get.html#DB_MULTIPLE">DB_MULTIPLE</a> and <a href="../../api_c/dbc_get.html#DB_MULTIPLE_KEY">DB_MULTIPLE_KEY</a>
is as follows: <a href="../../api_c/dbc_get.html#DB_MULTIPLE">DB_MULTIPLE</a> returns multiple data items for a
single key. For example, the <a href="../../api_c/dbc_get.html#DB_MULTIPLE">DB_MULTIPLE</a> flag would be used to
retrieve all of the duplicate data items for a single key in a single
call. The <a href="../../api_c/dbc_get.html#DB_MULTIPLE_KEY">DB_MULTIPLE_KEY</a> flag is used to retrieve multiple
key/data pairs, where each returned key may or may not have duplicate
data items.</p>
<p>Once the <a href="../../api_c/db_get.html">DB->get</a> or <a href="../../api_c/dbc_get.html">DBcursor->get</a> 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:
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_INIT">DB_MULTIPLE_INIT</a>, <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_NEXT">DB_MULTIPLE_NEXT</a>,
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_KEY_NEXT">DB_MULTIPLE_KEY_NEXT</a>, and <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_RECNO_NEXT">DB_MULTIPLE_RECNO_NEXT</a>. For
the Java API, this is implemented as three iterator classes:
<a href="../../java/com/sleepycat/db/MultipleDataEntry.html">MultipleDataEntry</a>,
<a href="../../java/com/sleepycat/db/MultipleKeyDataEntry.html">MultipleKeyDataEntry</a>, and
<a href="../../java/com/sleepycat/db/MultipleRecnoDataEntry.html">MultipleRecnoDataEntry</a>.</p>
<p>The <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_INIT">DB_MULTIPLE_INIT</a> macro is always called first. It
initializes a local application variable and the <b>data</b>
<a href="../../api_c/dbt_class.html">DBT</a> for stepping through the set of returned records. Then,
the application calls one of the remaining three macros:
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_NEXT">DB_MULTIPLE_NEXT</a>, <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_KEY_NEXT">DB_MULTIPLE_KEY_NEXT</a>, and
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_RECNO_NEXT">DB_MULTIPLE_RECNO_NEXT</a>.</p>
<p>If the <a href="../../api_c/dbc_get.html#DB_MULTIPLE">DB_MULTIPLE</a> flag was specified to the <a href="../../api_c/db_get.html">DB->get</a> or
<a href="../../api_c/dbc_get.html">DBcursor->get</a> method, the application will always call the
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_NEXT">DB_MULTIPLE_NEXT</a> macro. If the <a href="../../api_c/dbc_get.html#DB_MULTIPLE_KEY">DB_MULTIPLE_KEY</a> flag
was specified to the <a href="../../api_c/db_get.html">DB->get</a> or <a href="../../api_c/dbc_get.html">DBcursor->get</a> method, and, the
underlying database is a Btree or Hash database, the application will
always call the <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_KEY_NEXT">DB_MULTIPLE_KEY_NEXT</a> macro. If the
<a href="../../api_c/dbc_get.html#DB_MULTIPLE_KEY">DB_MULTIPLE_KEY</a> flag was specified to the <a href="../../api_c/db_get.html">DB->get</a> or
<a href="../../api_c/dbc_get.html">DBcursor->get</a> method, and, the underlying database is a Queue or Recno
database, the application will always call the
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_RECNO_NEXT">DB_MULTIPLE_RECNO_NEXT</a> macro. The <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_NEXT">DB_MULTIPLE_NEXT</a>,
<a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_KEY_NEXT">DB_MULTIPLE_KEY_NEXT</a>, and <a href="../../api_c/dbt_bulk.html#DB_MULTIPLE_RECNO_NEXT">DB_MULTIPLE_RECNO_NEXT</a> 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.</p>
<p>The following is an example of a routine that displays the contents of
a Btree database using the bulk return interfaces.</p>
<blockquote><pre>int
rec_display(dbp)
DB *dbp;
{
DBC *dbcp;
DBT key, data;
size_t retklen, retdlen;
char *retkey, *retdata;
int ret, t_ret;
void *p;
<p>
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
<p>
/* 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;
<p>
/* 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);
}
<p>
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;
}
<p>
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);
}
}
<p>
if ((t_ret = dbcp->c_close(dbcp)) != 0) {
dbp->err(dbp, ret, "DBcursor->close");
if (ret == 0)
ret = t_ret;
}
<p>
free(data.data);
<p>
return (ret);
}</pre></blockquote>
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