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<! "@(#)select.so 10.10 (Sleepycat) 12/16/98">
<!Copyright 1997, 1998 by Sleepycat Software, Inc. All rights reserved.>
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<title>Berkeley DB Reference Guide: Access Methods</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btr
ee,hash,hashing,transaction,transactions,locking,logging,access method,access me
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<h3>Berkeley DB Reference Guide: Access Methods</h3>
<p>
<h1 align=center>Selecting an access method</h1>
<p>
Keys in the Btree are stored in sorted order, and the relationship between
them is defined by that sort order. The default sort order is lexical,
but applications may provide any sort order they choose. For this reason,
the Btree access method should be used when there is a locality of
reference between keys. Locality of reference means that accessing one
particular key in the Btree means that the application is more likely to
want to access keys that are somehow related to the key being accessed.
For example, if keys are timestamps, and it's likely that a request for an 8AM
timestamp will be followed by a request for a 9AM timestamp, the
Btree access method is generally the right choice. Or, for example, if the
keys are individual surnames, and the application will want to review all
individuals with the same last name, the Btree access method is again
the right choice.
<p>
The Hash access method should be used when there is no locality of
reference between keys.
<p>
The Recno access method should be used when it is desirable to store
and retrieve records based on logical record numbers.
<p>
It is possible to store and retrieve records based on logical record
numbers in the Btree access method.
However, those record numbers 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. Generally, the Recno access method should be
used when the primary method for accessing the database is a logical
record number.)
<p>
In addition, the Recno access method provides support for databases whose
permanent storage is a flat text file and the database is used as a fast,
temporary storage area while the data is being read or modified.
<p>
Because the access method implementation unavoidably interacts with the
application's data set, locking requirements and data access patterns,
using one access method instead of another can sometimes provide superior
performance. For applications whose data can potentially be stored in more
than one type of database, it may be useful to benchmark the application's
performance using the two access methods.
<p>
One of the strengths of Berkeley DB is that it provides multiple access methods
and provides almost identical interfaces to the different access methods.
Because of this, it is a simple task to modify an application to use a
different access method. This allows applications to benchmark the
different Berkeley DB access methods against each other for their particular
data set and access pattern.
<p>
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