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<title>Berkeley DB Reference Guide: Access Methods</title>
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<h3>Berkeley DB Reference Guide: Access Methods</h3>
<p>
<h1 align=center>Logical record numbers</h1>
<p>
Both the Berkeley DB Btree and Recno <a href="intro.html">access methods</a> can be
accessed using logical record numbers.
<p>
In all cases for the Recno access method, and in some cases with the
Btree access method, a record number may be specified to reference a
specific key/data pair. In the case of Btrees supporting duplicate
data items, i.e., a single key may have more than one associated data
item, the logical record refers to a key and all of its data items.
<p>
Record numbers are 32-bit unsigned types, which limits the number of
logical records in a database to 4,294,967,296.
<p>
Logical record numbers are 1-based, not 0-based, i.e., the first record
in the database is record number 1.
<p>
Record numbers in Btree databases are always mutable, i.e., as records
are deleted or inserted, the logical record number for other records in
the database can change. The Recno access method can be configured to
run in either mutable or fixed mode: mutable, where logical record numbers
change as records are deleted or inserted, and fixed, where record numbers
never change regardless of the database operation. (See
<a href="../../ref/am/renumber.html">Logically renumbering records (DB_RENUMBER)</a>
for more information.
<p>
Configuring Btree databases to support record numbers can severely limit
the throughput of applications with multiple concurrent threads writing
the database.
<p>
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