summaryrefslogtreecommitdiffstats
path: root/lib/libc/db/man/btree.3
blob: 99b2b1502ee79bcf628e5065e9e94e0294aeeb69 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
.\" Copyright (c) 1990, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"	@(#)btree.3	8.4 (Berkeley) 8/18/94
.\" $FreeBSD$
.\"
.Dd August 18, 1994
.Dt BTREE 3
.Os
.Sh NAME
.Nm btree
.Nd "btree database access method"
.Sh SYNOPSIS
.In sys/types.h
.In db.h
.Sh DESCRIPTION
The routine
.Fn dbopen
is the library interface to database files.
One of the supported file formats is
.Nm
files.
The general description of the database access methods is in
.Xr dbopen 3 ,
this manual page describes only the
.Nm
specific information.
.Pp
The
.Nm
data structure is a sorted, balanced tree structure storing
associated key/data pairs.
.Pp
The
.Nm
access method specific data structure provided to
.Fn dbopen
is defined in the
.Aq Pa db.h
include file as follows:
.Bd -literal
typedef struct {
	u_long flags;
	u_int cachesize;
	int maxkeypage;
	int minkeypage;
	u_int psize;
	int (*compare)(const DBT *key1, const DBT *key2);
	size_t (*prefix)(const DBT *key1, const DBT *key2);
	int lorder;
} BTREEINFO;
.Ed
.Pp
The elements of this structure are as follows:
.Bl -tag -width indent
.It Va flags
The flag value is specified by
.Em or Ns 'ing
any of the following values:
.Bl -tag -width indent
.It Dv R_DUP
Permit duplicate keys in the tree, i.e. permit insertion if the key to be
inserted already exists in the tree.
The default behavior, as described in
.Xr dbopen 3 ,
is to overwrite a matching key when inserting a new key or to fail if
the
.Dv R_NOOVERWRITE
flag is specified.
The
.Dv R_DUP
flag is overridden by the
.Dv R_NOOVERWRITE
flag, and if the
.Dv R_NOOVERWRITE
flag is specified, attempts to insert duplicate keys into
the tree will fail.
.Pp
If the database contains duplicate keys, the order of retrieval of
key/data pairs is undefined if the
.Va get
routine is used, however,
.Va seq
routine calls with the
.Dv R_CURSOR
flag set will always return the logical
.Dq first
of any group of duplicate keys.
.El
.It Va cachesize
A suggested maximum size (in bytes) of the memory cache.
This value is
.Em only
advisory, and the access method will allocate more memory rather than fail.
Since every search examines the root page of the tree, caching the most
recently used pages substantially improves access time.
In addition, physical writes are delayed as long as possible, so a moderate
cache can reduce the number of I/O operations significantly.
Obviously, using a cache increases (but only increases) the likelihood of
corruption or lost data if the system crashes while a tree is being modified.
If
.Va cachesize
is 0 (no size is specified) a default cache is used.
.It Va maxkeypage
The maximum number of keys which will be stored on any single page.
Not currently implemented.
.\" The maximum number of keys which will be stored on any single page.
.\" Because of the way the
.\" .Nm
.\" data structure works,
.\" .Va maxkeypage
.\" must always be greater than or equal to 2.
.\" If
.\" .Va maxkeypage
.\" is 0 (no maximum number of keys is specified) the page fill factor is
.\" made as large as possible (which is almost invariably what is wanted).
.It Va minkeypage
The minimum number of keys which will be stored on any single page.
This value is used to determine which keys will be stored on overflow
pages, i.e. if a key or data item is longer than the pagesize divided
by the minkeypage value, it will be stored on overflow pages instead
of in the page itself.
If
.Va minkeypage
is 0 (no minimum number of keys is specified) a value of 2 is used.
.It Va psize
Page size is the size (in bytes) of the pages used for nodes in the tree.
The minimum page size is 512 bytes and the maximum page size is 64K.
If
.Va psize
is 0 (no page size is specified) a page size is chosen based on the
underlying file system I/O block size.
.It Va compare
Compare is the key comparison function.
It must return an integer less than, equal to, or greater than zero if the
first key argument is considered to be respectively less than, equal to,
or greater than the second key argument.
The same comparison function must be used on a given tree every time it
is opened.
If
.Va compare
is
.Dv NULL
(no comparison function is specified), the keys are compared
lexically, with shorter keys considered less than longer keys.
.It Va prefix
.Va Prefix
is the prefix comparison function.
If specified, this routine must return the number of bytes of the second key
argument which are necessary to determine that it is greater than the first
key argument.
If the keys are equal, the key length should be returned.
Note, the usefulness of this routine is very data dependent, but, in some
data sets can produce significantly reduced tree sizes and search times.
If
.Va prefix
is
.Dv NULL
(no prefix function is specified),
.Em and
no comparison function is specified, a default lexical comparison routine
is used.
If
.Va prefix
is
.Dv NULL
and a comparison routine is specified, no prefix comparison is
done.
.It Va lorder
The byte order for integers in the stored database metadata.
The number should represent the order as an integer; for example,
big endian order would be the number 4,321.
If
.Va lorder
is 0 (no order is specified) the current host order is used.
.El
.Pp
If the file already exists (and the
.Dv O_TRUNC
flag is not specified), the
values specified for the parameters
.Va flags , lorder
and
.Va psize
are ignored
in favor of the values used when the tree was created.
.Pp
Forward sequential scans of a tree are from the least key to the greatest.
.Pp
Space freed up by deleting key/data pairs from the tree is never reclaimed,
although it is normally made available for reuse.
This means that the
.Nm
storage structure is grow-only.
The only solutions are to avoid excessive deletions, or to create a fresh
tree periodically from a scan of an existing one.
.Pp
Searches, insertions, and deletions in a
.Nm
will all complete in
O lg base N where base is the average fill factor.
Often, inserting ordered data into
.Nm Ns s
results in a low fill factor.
This implementation has been modified to make ordered insertion the best
case, resulting in a much better than normal page fill factor.
.Sh ERRORS
The
.Nm
access method routines may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr dbopen 3 .
.Sh SEE ALSO
.Xr dbopen 3 ,
.Xr hash 3 ,
.Xr mpool 3 ,
.Xr recno 3
.Rs
.%T "The Ubiquitous B-tree"
.%A Douglas Comer
.%J "ACM Comput. Surv. 11"
.%N 2
.%D June 1979
.%P 121-138
.Re
.Rs
.%A Bayer
.%A Unterauer
.%T "Prefix B-trees"
.%J "ACM Transactions on Database Systems"
.%N 1
.%V Vol. 2
.%D March 1977
.%P 11-26
.Re
.Rs
.%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
.%A D. E. Knuth
.%D 1968
.%P 471-480
.Re
.Sh BUGS
Only big and little endian byte order is supported.
OpenPOWER on IntegriCloud