fbbd9655e5
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
272 lines
8.1 KiB
Groff
272 lines
8.1 KiB
Groff
.\" Copyright (c) 1990, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)btree.3 8.4 (Berkeley) 8/18/94
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.\" $FreeBSD$
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.\"
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.Dd August 18, 1994
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.Dt BTREE 3
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.Os
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.Sh NAME
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.Nm btree
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.Nd "btree database access method"
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.Sh SYNOPSIS
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.In sys/types.h
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.In db.h
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.Sh DESCRIPTION
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The routine
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.Fn dbopen
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is the library interface to database files.
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One of the supported file formats is
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.Nm
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files.
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The general description of the database access methods is in
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.Xr dbopen 3 ,
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this manual page describes only the
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.Nm
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specific information.
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.Pp
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The
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.Nm
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data structure is a sorted, balanced tree structure storing
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associated key/data pairs.
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.Pp
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The
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.Nm
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access method specific data structure provided to
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.Fn dbopen
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is defined in the
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.In db.h
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include file as follows:
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.Bd -literal
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typedef struct {
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u_long flags;
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u_int cachesize;
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int maxkeypage;
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int minkeypage;
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u_int psize;
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int (*compare)(const DBT *key1, const DBT *key2);
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size_t (*prefix)(const DBT *key1, const DBT *key2);
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int lorder;
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} BTREEINFO;
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.Ed
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.Pp
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The elements of this structure are as follows:
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.Bl -tag -width indent
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.It Va flags
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The flag value is specified by
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.Em or Ns 'ing
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any of the following values:
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.Bl -tag -width indent
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.It Dv R_DUP
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Permit duplicate keys in the tree, i.e., permit insertion if the key to be
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inserted already exists in the tree.
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The default behavior, as described in
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.Xr dbopen 3 ,
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is to overwrite a matching key when inserting a new key or to fail if
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the
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.Dv R_NOOVERWRITE
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flag is specified.
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The
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.Dv R_DUP
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flag is overridden by the
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.Dv R_NOOVERWRITE
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flag, and if the
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.Dv R_NOOVERWRITE
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flag is specified, attempts to insert duplicate keys into
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the tree will fail.
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.Pp
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If the database contains duplicate keys, the order of retrieval of
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key/data pairs is undefined if the
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.Va get
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routine is used, however,
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.Va seq
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routine calls with the
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.Dv R_CURSOR
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flag set will always return the logical
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.Dq first
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of any group of duplicate keys.
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.El
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.It Va cachesize
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A suggested maximum size (in bytes) of the memory cache.
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This value is
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.Em only
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advisory, and the access method will allocate more memory rather than fail.
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Since every search examines the root page of the tree, caching the most
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recently used pages substantially improves access time.
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In addition, physical writes are delayed as long as possible, so a moderate
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cache can reduce the number of I/O operations significantly.
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Obviously, using a cache increases (but only increases) the likelihood of
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corruption or lost data if the system crashes while a tree is being modified.
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If
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.Va cachesize
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is 0 (no size is specified) a default cache is used.
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.It Va maxkeypage
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The maximum number of keys which will be stored on any single page.
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Not currently implemented.
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.\" The maximum number of keys which will be stored on any single page.
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.\" Because of the way the
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.\" .Nm
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.\" data structure works,
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.\" .Va maxkeypage
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.\" must always be greater than or equal to 2.
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.\" If
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.\" .Va maxkeypage
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.\" is 0 (no maximum number of keys is specified) the page fill factor is
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.\" made as large as possible (which is almost invariably what is wanted).
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.It Va minkeypage
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The minimum number of keys which will be stored on any single page.
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This value is used to determine which keys will be stored on overflow
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pages, i.e., if a key or data item is longer than the pagesize divided
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by the minkeypage value, it will be stored on overflow pages instead
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of in the page itself.
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If
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.Va minkeypage
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is 0 (no minimum number of keys is specified) a value of 2 is used.
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.It Va psize
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Page size is the size (in bytes) of the pages used for nodes in the tree.
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The minimum page size is 512 bytes and the maximum page size is 64K.
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If
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.Va psize
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is 0 (no page size is specified) a page size is chosen based on the
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underlying file system I/O block size.
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.It Va compare
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Compare is the key comparison function.
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It must return an integer less than, equal to, or greater than zero if the
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first key argument is considered to be respectively less than, equal to,
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or greater than the second key argument.
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The same comparison function must be used on a given tree every time it
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is opened.
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If
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.Va compare
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is
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.Dv NULL
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(no comparison function is specified), the keys are compared
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lexically, with shorter keys considered less than longer keys.
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.It Va prefix
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The
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.Va prefix
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element
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is the prefix comparison function.
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If specified, this routine must return the number of bytes of the second key
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argument which are necessary to determine that it is greater than the first
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key argument.
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If the keys are equal, the key length should be returned.
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Note, the usefulness of this routine is very data dependent, but, in some
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data sets can produce significantly reduced tree sizes and search times.
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If
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.Va prefix
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is
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.Dv NULL
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(no prefix function is specified),
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.Em and
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no comparison function is specified, a default lexical comparison routine
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is used.
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If
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.Va prefix
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is
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.Dv NULL
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and a comparison routine is specified, no prefix comparison is
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done.
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.It Va lorder
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The byte order for integers in the stored database metadata.
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The number should represent the order as an integer; for example,
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big endian order would be the number 4,321.
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If
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.Va lorder
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is 0 (no order is specified) the current host order is used.
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.El
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.Pp
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If the file already exists (and the
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.Dv O_TRUNC
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flag is not specified), the
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values specified for the
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.Va flags , lorder
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and
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.Va psize
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arguments
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are ignored
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in favor of the values used when the tree was created.
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.Pp
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Forward sequential scans of a tree are from the least key to the greatest.
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.Pp
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Space freed up by deleting key/data pairs from the tree is never reclaimed,
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although it is normally made available for reuse.
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This means that the
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.Nm
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storage structure is grow-only.
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The only solutions are to avoid excessive deletions, or to create a fresh
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tree periodically from a scan of an existing one.
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.Pp
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Searches, insertions, and deletions in a
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.Nm
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will all complete in
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O lg base N where base is the average fill factor.
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Often, inserting ordered data into
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.Nm Ns s
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results in a low fill factor.
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This implementation has been modified to make ordered insertion the best
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case, resulting in a much better than normal page fill factor.
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.Sh ERRORS
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The
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.Nm
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access method routines may fail and set
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.Va errno
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for any of the errors specified for the library routine
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.Xr dbopen 3 .
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.Sh SEE ALSO
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.Xr dbopen 3 ,
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.Xr hash 3 ,
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.Xr mpool 3 ,
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.Xr recno 3
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.Rs
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.%T "The Ubiquitous B-tree"
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.%A Douglas Comer
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.%J "ACM Comput. Surv. 11"
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.%N 2
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.%D June 1979
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.%P 121-138
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.Re
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.Rs
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.%A Bayer
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.%A Unterauer
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.%T "Prefix B-trees"
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.%J "ACM Transactions on Database Systems"
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.%N 1
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.%V Vol. 2
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.%D March 1977
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.%P 11-26
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.Re
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.Rs
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.%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
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.%A D. E. Knuth
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.%D 1968
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.%P 471-480
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.Re
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.Sh BUGS
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Only big and little endian byte order is supported.
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