| blob | 1936386966430ee60bf540f67173e7703c127755 |
1 .\" $NetBSD: btree.3,v 1.12 2010/03/22 19:30:53 joerg Exp $
2 .\"
3 .\" Copyright (c) 1990, 1993
4 .\" The Regents of the University of California. All rights reserved.
5 .\"
6 .\" Redistribution and use in source and binary forms, with or without
7 .\" modification, are permitted provided that the following conditions
8 .\" are met:
9 .\" 1. Redistributions of source code must retain the above copyright
10 .\" notice, this list of conditions and the following disclaimer.
11 .\" 2. Redistributions in binary form must reproduce the above copyright
12 .\" notice, this list of conditions and the following disclaimer in the
13 .\" documentation and/or other materials provided with the distribution.
14 .\" 3. Neither the name of the University nor the names of its contributors
15 .\" may be used to endorse or promote products derived from this software
16 .\" without specific prior written permission.
17 .\"
18 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
19 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
20 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
21 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
22 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
23 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
24 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
25 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
26 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
27 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 .\" SUCH DAMAGE.
29 .\"
30 .\" @(#)btree.3 8.4 (Berkeley) 8/18/94
31 .\"
32 .Dd April 17, 2003
33 .Dt BTREE 3
34 .Os
35 .Sh NAME
36 .Nm btree
37 .Nd btree database access method
38 .Sh SYNOPSIS
39 .In sys/types.h
40 .In db.h
41 .Sh DESCRIPTION
42 The routine
43 .Fn dbopen
44 is the library interface to database files.
45 One of the supported file formats is btree files.
46 The general description of the database access methods is in
47 .Xr dbopen 3 ,
48 this manual page describes only the btree specific information.
49 .Pp
50 The btree data structure is a sorted, balanced tree structure storing
51 associated key/data pairs.
52 .Pp
53 The btree access method specific data structure provided to
54 .Fn dbopen
55 is defined in the
56 .In db.h
57 include file as follows:
58 .Bd -literal
59 typedef struct {
60 u_long flags;
61 u_int cachesize;
62 int maxkeypage;
63 int minkeypage;
64 u_int psize;
65 int (*compare)(const DBT *key1, const DBT *key2);
66 size_t (*prefix)(const DBT *key1, const DBT *key2);
67 int lorder;
68 } BTREEINFO;
69 .Ed
70 .Pp
71 The elements of this structure are as follows:
72 .Bl -tag -width maxkeypagex
73 .It Fa flags
74 The flag value is specified by or'ing any of the following values:
75 .Bl -tag -width R_DUP -offset indent
76 .It Dv R_DUP
77 Permit duplicate keys in the tree, i.e. permit insertion if the key to
78 be inserted already exists in the tree.
79 The default behavior, as described in
80 .Xr dbopen 3 ,
81 is to overwrite a matching key when inserting a new key or to fail if
82 the
83 .Dv R_NOOVERWRITE
84 flag is specified.
85 The
86 .Dv R_DUP
87 flag is overridden by the
88 .Dv R_NOOVERWRITE
89 flag, and if the
90 .Dv R_NOOVERWRITE
91 flag is specified, attempts to insert duplicate keys into the tree
92 will fail.
93 .Pp
94 If the database contains duplicate keys, the order of retrieval of
95 key/data pairs is undefined if the
96 .Em get
97 routine is used, however,
98 .Em seq
99 routine calls with the
100 .Dv R_CURSOR
101 flag set will always return the logical
102 .Dq first
103 of any group of duplicate keys.
104 .El
105 .It Fa cachesize
106 A suggested maximum size (in bytes) of the memory cache.
107 This value is
108 .Em only
109 advisory, and the access method will allocate more memory rather than
110 fail.
111 Since every search examines the root page of the tree, caching the
112 most recently used pages substantially improves access time.
113 In addition, physical writes are delayed as long as possible, so a
114 moderate cache can reduce the number of I/O operations significantly.
115 Obviously, using a cache increases (but only increases) the likelihood
116 of corruption or lost data if the system crashes while a tree is being
117 modified.
118 If
119 .Fa cachesize
120 is 0 (no size is specified) a default cache is used.
121 .It Fa maxkeypage
122 The maximum number of keys which will be stored on any single page.
123 Not currently implemented.
124 .\" The maximum number of keys which will be stored on any single page.
125 .\" Because of the way the btree data structure works,
126 .\" .Fa maxkeypage
127 .\" must always be greater than or equal to 2.
128 .\" If
129 .\" .Fa maxkeypage
130 .\" is 0 (no maximum number of keys is specified) the page fill factor is
131 .\" made as large as possible (which is almost invariably what is wanted).
132 .It Fa minkeypage
133 The minimum number of keys which will be stored on any single page.
134 This value is used to determine which keys will be stored on overflow
135 pages, i.e., if a key or data item is longer than the pagesize divided
136 by the
137 .Fa minkeypage
138 value, it will be stored on overflow pages instead of in the page
139 itself.
140 If
141 .Fa minkeypage
142 is 0 (no minimum number of keys is specified) a value of 2 is used.
143 .It Fa psize
144 Page size is the size (in bytes) of the pages used for nodes in the
145 tree.
146 The minimum page size is 512 bytes and the maximum page size is 64K.
147 If
148 .Fa psize
149 is 0 (no page size is specified) a page size is chosen based on the
150 underlying file system I/O block size.
151 .It Fa compare
152 Compare is the key comparison function.
153 It must return an integer less than, equal to, or greater than zero if
154 the first key argument is considered to be respectively less than,
155 equal to, or greater than the second key argument.
156 The same comparison function must be used on a given tree every time
157 it is opened.
158 If
159 .Fa compare
160 is
161 .Dv NULL
162 (no comparison function is specified), the keys are compared
163 lexically, with shorter keys considered less than longer keys.
164 .It Fa prefix
165 Prefix is the prefix comparison function.
166 If specified, this routine must return the number of bytes of the
167 second key argument which are necessary to determine that it is
168 greater than the first key argument.
169 If the keys are equal, the key length should be returned.
170 Note, the usefulness of this routine is very data dependent, but, in
171 some data sets can produce significantly reduced tree sizes and search
172 times.
173 If
174 .Fa prefix
175 is
176 .Dv NULL
177 (no prefix function is specified),
178 .Em and
179 no comparison function is specified, a default lexical comparison
180 routine is used.
181 If
182 .Fa prefix
183 is
184 .Dv NULL
185 and a comparison routine is specified, no prefix comparison is done.
186 .It Fa lorder
187 The byte order for integers in the stored database metadata.
188 The number should represent the order as an integer; for example,
189 big endian order would be the number 4,321.
190 If
191 .Fa lorder
192 is 0 (no order is specified) the current host order is used.
193 .El
194 .Pp
195 If the file already exists (and the
196 .Dv O_TRUNC
197 flag is not specified), the values specified for the parameters flags,
198 lorder and psize are ignored in favor of the values used when the tree
199 was created.
200 .Pp
201 Forward sequential scans of a tree are from the least key to the
202 greatest.
203 .Pp
204 Space freed up by deleting key/data pairs from the tree is never
205 reclaimed, although it is normally made available for reuse.
206 This means that the btree storage structure is grow-only.
207 The only solutions are to avoid excessive deletions, or to create a
208 fresh tree periodically from a scan of an existing one.
209 .Pp
210 Searches, insertions, and deletions in a btree will all complete in
211 O lg base N where base is the average fill factor.
212 Often, inserting ordered data into btrees results in a low fill
213 factor.
214 This implementation has been modified to make ordered insertion the
215 best case, resulting in a much better than normal page fill factor.
216 .Sh ERRORS
217 The
218 .Nm
219 access method routines may fail and set
220 .Va errno
221 for any of the errors specified for the library routine
222 .Xr dbopen 3 .
223 .Sh SEE ALSO
224 .Xr dbopen 3 ,
225 .Xr hash 3 ,
226 .Xr mpool 3 ,
227 .Xr recno 3
228 .Pp
229 .Rs
230 .%T "The Ubiquitous B-tree"
231 .%A "Douglas Comer"
232 .%J "ACM Comput. Surv."
233 .%V 2
234 .%N 11
235 .%D June 1979
236 .%P 121-138
237 .Re
238 .Rs
239 .%T "Prefix B-trees"
240 .%A "Bayer"
241 .%A "Unterauer"
242 .%J "ACM Transactions on Database Systems"
243 .%V Vol. 2
244 .%N 1
245 .%D March 1977
246 .%P 11-26
247 .Re
248 .Rs
249 .%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
250 .%A "D.E. Knuth"
251 .%D 1968
252 .%P 471-480
253 .Re
254 .Sh BUGS
255 Only big and little endian byte order is supported.