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<b>Db::compact</b>
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<tt>
<b><pre>
#include <db_cxx.h>
<p>
int
Db::compact(DbTxn *txnid,
Dbt *start, Dbt *stop, DB_COMPACT *c_data, u_int32_t flags, Dbt *end);
</pre></b>
<hr size=1 noshade>
<b>Description: Db::compact</b>
<p>The Db::compact method compacts Btree and Recno access method
databases, and optionally returns unused Btree, Hash or Recno database
pages to the underlying filesystem.</p>
<p>The Db::compact method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</p>
<b>Parameters</b> <br>
<b>txnid</b><ul compact><li>If the operation is part of an application-specified transaction, the
<b>txnid</b> parameter is a transaction handle returned from
<a href="../../db4-devel-4.7.25/api_cxx/txn_begin.html">DbEnv::txn_begin</a>; otherwise NULL. If no transaction handle is
specified, but the operation occurs in a transactional database, the
operation will be implicitly transaction protected using multiple
transactions. These transactions will be periodically committed to
avoid locking large sections of the tree. Any deadlocks encountered
cause the compaction operation to retried from the point of the last
transaction commit.</ul>
<b>start</b><ul compact><li>If non-NULL, the <b>start</b> parameter is the starting point for
compaction in a Btree or Recno database. Compaction will start at the
smallest key greater than or equal to the specified key. If NULL,
compaction will start at the beginning of the database.</ul>
<b>stop</b><ul compact><li>If non-NULL, the <b>stop</b> parameter is the stopping point for
compaction in a Btree or Recno database. Compaction will stop at the
page with the smallest key greater than the specified key. If NULL,
compaction will stop at the end of the database.</ul>
<b>c_data</b><ul compact><li><p>If non-NULL, the <b>c_data</b> parameter contains additional compaction
configuration parameters, and returns compaction operation statistics,
in a structure of type DB_COMPACT.</p>
<p>The following input configuration fields are available from the
DB_COMPACT structure:</p>
<br>
<b>int compact_fillpercent;</b><ul compact><li>If non-zero, the goal for filling pages, specified as a percentage
between 1 and 100. Any page in a Btree or Recno databases not at or
above this percentage full will be considered for compaction. The
default behavior is to consider every page for compaction, regardless
of its page fill percentage.</ul>
<b>int compact_pages;</b><ul compact><li>If non-zero, the call will return after that number of pages have been
freed.</ul>
<b>db_timeout_t compact_timeout;</b><ul compact><li>If non-zero, and no <b>txnid</b> parameter was specified, the lock
timeout set for implicit transactions, in microseconds.</ul>
<br>
<p>The following output statistics fields are available from the
DB_COMPACT structure:</p>
<br>
<b>u_int32_t compact_deadlock;</b><ul compact><li>An output statistics parameter: if no <b>txnid</b> parameter was
specified, the number of deadlocks which occurred.</ul>
<b>u_int32_t compact_pages_examine;</b><ul compact><li>An output statistics parameter: the number of database pages reviewed
during the compaction phase.</ul>
<b>u_int32_t compact_pages_free;</b><ul compact><li>An output statistics parameter: the number of database pages freed during
the compaction phase.</ul>
<b>u_int32_t compact_levels;</b><ul compact><li>An output statistics parameter: the number of levels removed from the
Btree or Recno database during the compaction phase.</ul>
<b>u_int32_t compact_pages_truncated;</b><ul compact><li>An output statistics parameter: the number of database pages returned
to the filesystem.</ul>
<br></ul>
<b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or
one of the following values:
<br>
<b><a name="DB_FREELIST_ONLY">DB_FREELIST_ONLY</a></b><ul compact><li>Do no page compaction, only returning pages to the filesystem that are
already free and at the end of the file. This flag must be set if the
database is a Hash access method database.</ul>
<b><a name="DB_FREE_SPACE">DB_FREE_SPACE</a></b><ul compact><li>Return pages to the filesystem when possible.
If this flag is not specified, pages emptied as a result of compaction
will be placed on the free list for re-use, but never returned to the
filesystem.
<p>Note that only pages at the end of a file can be returned to the
filesystem. Because of the one-pass nature of the compaction algorithm,
any not emptied page near the end of the file inhibits returning pages to
the file system. A repeated call to the Db::compact method with a low
<b>compact_fillpercent</b> may be used to return pages in this case.</p></ul>
<br></ul>
<b>end</b><ul compact><li>If non-NULL, the <b>end</b> parameter will be filled in with the
database key marking the end of the compaction operation in a Btree or
Recno database. This is generally the first key of the page where the
operation stopped.</ul>
<br>
<br><b>Errors</b>
<p>The Db::compact method
may fail and throw
<a href="../../db4-devel-4.7.25/api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:</p>
<br>
<b>DB_REP_HANDLE_DEAD</b><ul compact><li>The database handle has been invalidated because a replication election
unrolled a committed transaction.</ul>
<br>
<br>
<b>DB_REP_LOCKOUT</b><ul compact><li>The operation was blocked by client/master synchronization.</ul>
<br>
<br>
<b>EACCES</b><ul compact><li>An attempt was made to modify a read-only database.</ul>
<br>
<br>
<b>EINVAL</b><ul compact><li>An
invalid flag value or parameter was specified.</ul>
<br>
<p>If a transactional database environment operation was selected to
resolve a deadlock, the Db::compact method will fail and
either return <a href="../../db4-devel-4.7.25/ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> or
throw a <a href="../../db4-devel-4.7.25/api_cxx/deadlock_class.html">DbDeadlockException</a> exception.</p>
<p>If a Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable
to grant a lock in the allowed time, the Db::compact method will fail and
either return <a href="../../db4-devel-4.7.25/ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> or
throw a <a href="../../db4-devel-4.7.25/api_cxx/lockng_class.html">DbLockNotGrantedException</a> exception.</p>
<hr size=1 noshade>
<br><b>Class</b>
<a href="../../db4-devel-4.7.25/api_cxx/db_class.html">Db</a>
<br><b>See Also</b>
<a href="../../db4-devel-4.7.25/api_cxx/db_list.html">Databases and Related Methods</a>
</tt>
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