1 .\" Copyright (c) 1990, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
7 .\" 1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\" 2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\" 3. All advertising materials mentioning features or use of this software
13 .\" must display the following acknowledgement:
14 .\" This product includes software developed by the University of
15 .\" California, Berkeley and its contributors.
16 .\" 4. Neither the name of the University nor the names of its contributors
17 .\" may be used to endorse or promote products derived from this software
18 .\" without specific prior written permission.
20 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
21 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
24 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
32 .\" @(#)btree.3 8.4 (Berkeley) 8/18/94
33 .\" $FreeBSD: src/lib/libc/db/man/btree.3,v 1.3.2.3 2003/03/15 15:11:05 trhodes Exp $
34 .\" $DragonFly: src/lib/libc/db/man/btree.3,v 1.2 2003/06/17 04:26:41 dillon Exp $
41 .Nd "btree database access method"
48 is the library interface to database files.
49 One of the supported file formats is
52 The general description of the database access methods is in
54 this manual page describes only the
60 data structure is a sorted, balanced tree structure storing
61 associated key/data pairs.
65 access method specific data structure provided to
69 include file as follows:
77 int (*compare)(const DBT *key1, const DBT *key2);
78 size_t (*prefix)(const DBT *key1, const DBT *key2);
83 The elements of this structure are as follows:
84 .Bl -tag -width indent
86 The flag value is specified by
88 any of the following values:
89 .Bl -tag -width indent
91 Permit duplicate keys in the tree, i.e. permit insertion if the key to be
92 inserted already exists in the tree.
93 The default behavior, as described in
95 is to overwrite a matching key when inserting a new key or to fail if
101 flag is overridden by the
105 flag is specified, attempts to insert duplicate keys into
108 If the database contains duplicate keys, the order of retrieval of
109 key/data pairs is undefined if the
111 routine is used, however,
113 routine calls with the
115 flag set will always return the logical
117 of any group of duplicate keys.
120 A suggested maximum size (in bytes) of the memory cache.
123 advisory, and the access method will allocate more memory rather than fail.
124 Since every search examines the root page of the tree, caching the most
125 recently used pages substantially improves access time.
126 In addition, physical writes are delayed as long as possible, so a moderate
127 cache can reduce the number of I/O operations significantly.
128 Obviously, using a cache increases (but only increases) the likelihood of
129 corruption or lost data if the system crashes while a tree is being modified.
132 is 0 (no size is specified) a default cache is used.
134 The maximum number of keys which will be stored on any single page.
135 Not currently implemented.
136 .\" The maximum number of keys which will be stored on any single page.
137 .\" Because of the way the
139 .\" data structure works,
141 .\" must always be greater than or equal to 2.
144 .\" is 0 (no maximum number of keys is specified) the page fill factor is
145 .\" made as large as possible (which is almost invariably what is wanted).
147 The minimum number of keys which will be stored on any single page.
148 This value is used to determine which keys will be stored on overflow
149 pages, i.e. if a key or data item is longer than the pagesize divided
150 by the minkeypage value, it will be stored on overflow pages instead
151 of in the page itself.
154 is 0 (no minimum number of keys is specified) a value of 2 is used.
156 Page size is the size (in bytes) of the pages used for nodes in the tree.
157 The minimum page size is 512 bytes and the maximum page size is 64K.
160 is 0 (no page size is specified) a page size is chosen based on the
161 underlying file system I/O block size.
163 Compare is the key comparison function.
164 It must return an integer less than, equal to, or greater than zero if the
165 first key argument is considered to be respectively less than, equal to,
166 or greater than the second key argument.
167 The same comparison function must be used on a given tree every time it
173 (no comparison function is specified), the keys are compared
174 lexically, with shorter keys considered less than longer keys.
179 is the prefix comparison function.
180 If specified, this routine must return the number of bytes of the second key
181 argument which are necessary to determine that it is greater than the first
183 If the keys are equal, the key length should be returned.
184 Note, the usefulness of this routine is very data dependent, but, in some
185 data sets can produce significantly reduced tree sizes and search times.
190 (no prefix function is specified),
192 no comparison function is specified, a default lexical comparison routine
198 and a comparison routine is specified, no prefix comparison is
201 The byte order for integers in the stored database metadata.
202 The number should represent the order as an integer; for example,
203 big endian order would be the number 4,321.
206 is 0 (no order is specified) the current host order is used.
209 If the file already exists (and the
211 flag is not specified), the
212 values specified for the
218 in favor of the values used when the tree was created.
220 Forward sequential scans of a tree are from the least key to the greatest.
222 Space freed up by deleting key/data pairs from the tree is never reclaimed,
223 although it is normally made available for reuse.
226 storage structure is grow-only.
227 The only solutions are to avoid excessive deletions, or to create a fresh
228 tree periodically from a scan of an existing one.
230 Searches, insertions, and deletions in a
233 O lg base N where base is the average fill factor.
234 Often, inserting ordered data into
236 results in a low fill factor.
237 This implementation has been modified to make ordered insertion the best
238 case, resulting in a much better than normal page fill factor.
242 access method routines may fail and set
244 for any of the errors specified for the library routine
252 .%T "The Ubiquitous B-tree"
254 .%J "ACM Comput. Surv. 11"
263 .%J "ACM Transactions on Database Systems"
270 .%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
276 Only big and little endian byte order is supported.