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<title>Berkeley DB Reference Guide: Opening multiple databases in a single file</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
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<h1 align=center>Opening multiple databases in a single file</h1>
<p>Applications may create multiple databases within a single physical
file. This is useful when the databases are both numerous and
reasonably small, in order to avoid creating a large number of
underlying files, or when it is desirable to include secondary index
databases in the same file as the primary index database. Multiple
databases are an administrative convenience and using them is unlikely
to effect database performance. To open or create a file that will
include more than a single database, specify a database name when
calling the <a href="../../api_c/db_open.html">DB->open</a> method.
<p>Physical files do not need to be comprised of a single type of database,
and databases in a file may be of any type (e.g., Btree, Hash or Recno),
except for Queue databases. Queue databases must be created one per file
and cannot share a file with any other database type. There is no limit
on the number of databases that may be created in a single file other than
the standard Berkeley DB file size and disk space limitations.
<p>It is an error to attempt to open a second database in a file that was
not initially created using a database name, that is, the file must
initially be specified as capable of containing multiple databases for a
second database to be created in it.
<p>It is not an error to open a file that contains multiple databases without
specifying a database name, however the database type should be specified
as DB_UNKNOWN and the database must be opened read-only. The handle that
is returned from such a call is a handle on a database whose key values
are the names of the databases stored in the database file and whose data
values are opaque objects. No keys or data values may be modified or
stored using this database handle.
<p>Storing multiple databases in a single file is almost identical to
storing each database in its own separate file. The one crucial
difference is how locking and the underlying memory pool services must
to be configured. As an example, consider two databases instantiated
in two different physical files. If access to each separate database
is single-threaded, there is no reason to perform any locking of any
kind, and the two databases may be read and written simultaneously.
Further, there would be no requirement to create a shared database
environment in which to open the databases. Because multiple databases
in a file exist in a single physical file, opening two databases in the
same file requires that locking be enabled, unless access to the
databases is known to be single-threaded, that is, only one of the
databases is ever accessed at a time. (As the locks for the two
databases can only conflict during page allocation, this additional
locking is unlikely to effect performance.) Further, the databases must
share an underlying memory pool so that per-physical-file information
is updated correctly.
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