256 lines
8.1 KiB
Groff
256 lines
8.1 KiB
Groff
.\" $NetBSD: btree.3,v 1.12 2010/03/22 19:30:53 joerg Exp $
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.\"
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.\" 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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.\"
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.Dd April 17, 2003
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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 btree 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 btree specific information.
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.Pp
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The btree 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 btree 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 maxkeypagex
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.It Fa flags
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The flag value is specified by or'ing any of the following values:
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.Bl -tag -width R_DUP -offset 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
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be 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 the tree
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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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.Em get
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routine is used, however,
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.Em 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 Fa 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
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fail.
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Since every search examines the root page of the tree, caching the
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most recently used pages substantially improves access time.
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In addition, physical writes are delayed as long as possible, so a
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moderate cache can reduce the number of I/O operations significantly.
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Obviously, using a cache increases (but only increases) the likelihood
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of corruption or lost data if the system crashes while a tree is being
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modified.
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If
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.Fa cachesize
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is 0 (no size is specified) a default cache is used.
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.It Fa 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 btree data structure works,
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.\" .Fa maxkeypage
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.\" must always be greater than or equal to 2.
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.\" If
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.\" .Fa 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 Fa 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
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.Fa minkeypage
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value, it will be stored on overflow pages instead of in the page
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itself.
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If
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.Fa 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 Fa psize
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Page size is the size (in bytes) of the pages used for nodes in the
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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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.Fa 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 Fa 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
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the first key argument is considered to be respectively less than,
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equal to, or greater than the second key argument.
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The same comparison function must be used on a given tree every time
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it is opened.
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If
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.Fa 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 Fa prefix
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Prefix is the prefix comparison function.
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If specified, this routine must return the number of bytes of the
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second key argument which are necessary to determine that it is
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greater than the first 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
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some data sets can produce significantly reduced tree sizes and search
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times.
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If
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.Fa 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
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routine is used.
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If
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.Fa 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 done.
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.It Fa 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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.Fa 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 values specified for the parameters flags,
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lorder and psize are ignored in favor of the values used when the tree
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was created.
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.Pp
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Forward sequential scans of a tree are from the least key to the
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greatest.
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.Pp
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Space freed up by deleting key/data pairs from the tree is never
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reclaimed, although it is normally made available for reuse.
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This means that the btree storage structure is grow-only.
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The only solutions are to avoid excessive deletions, or to create a
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fresh tree periodically from a scan of an existing one.
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.Pp
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Searches, insertions, and deletions in a btree 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 btrees results in a low fill
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factor.
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This implementation has been modified to make ordered insertion the
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best 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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.Pp
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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."
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.%V 2
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.%N 11
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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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.%T "Prefix B-trees"
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.%A "Bayer"
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.%A "Unterauer"
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.%J "ACM Transactions on Database Systems"
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.%V Vol. 2
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.%N 1
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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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