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1 | .\" Copyright (c) 1990, 1993 |
2 | .\" The Regents of the University of California. All rights reserved. | |
3 | .\" | |
47009d5e | 4 | .\" SPDX-License-Identifier: BSD-4-Clause-UC |
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5 | .\" |
6 | .\" @(#)btree.3 8.4 (Berkeley) 8/18/94 | |
7 | .\" | |
1ae6b2c7 | 8 | .TH BTREE 3 2020-12-21 GNU "Linux Programmer's Manual" |
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9 | .\".UC 7 |
10 | .SH NAME | |
11 | btree \- btree database access method | |
b813014f AC |
12 | .SH LIBRARY |
13 | Standard C library | |
14 | .RI ( libc ", " \-lc ) | |
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15 | .SH SYNOPSIS |
16 | .nf | |
17 | .ft B | |
18 | #include <sys/types.h> | |
19 | #include <db.h> | |
20 | .ft R | |
21 | .fi | |
22 | .SH DESCRIPTION | |
df21098d MK |
23 | .IR "Note well" : |
24 | This page documents interfaces provided in glibc up until version 2.1. | |
25 | Since version 2.2, glibc no longer provides these interfaces. | |
26 | Probably, you are looking for the APIs provided by the | |
27 | .I libdb | |
28 | library instead. | |
847e0d88 | 29 | .PP |
fea681da | 30 | The routine |
fb186734 | 31 | .BR dbopen (3) |
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32 | is the library interface to database files. |
33 | One of the supported file formats is btree files. | |
34 | The general description of the database access methods is in | |
31e9a9ec | 35 | .BR dbopen (3), |
76c637e1 | 36 | this manual page describes only the btree-specific information. |
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37 | .PP |
38 | The btree data structure is a sorted, balanced tree structure storing | |
39 | associated key/data pairs. | |
40 | .PP | |
76c637e1 | 41 | The btree access-method-specific data structure provided to |
fb186734 | 42 | .BR dbopen (3) |
e5056894 MK |
43 | is defined in the |
44 | .I <db.h> | |
45 | include file as follows: | |
e646a1ba | 46 | .PP |
088a639b | 47 | .in +4n |
e646a1ba | 48 | .EX |
fea681da | 49 | typedef struct { |
aeb4b1fc MK |
50 | unsigned long flags; |
51 | unsigned int cachesize; | |
52 | int maxkeypage; | |
53 | int minkeypage; | |
54 | unsigned int psize; | |
55 | int (*compare)(const DBT *key1, const DBT *key2); | |
56 | size_t (*prefix)(const DBT *key1, const DBT *key2); | |
57 | int lorder; | |
fea681da | 58 | } BTREEINFO; |
b8302363 | 59 | .EE |
ce4b0e57 | 60 | .in |
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61 | .PP |
62 | The elements of this structure are as follows: | |
63 | .TP | |
e5056894 | 64 | .I flags |
cebca1bd | 65 | The flag value is specified by ORing any of the following values: |
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66 | .RS |
67 | .TP | |
628d8d62 | 68 | .B R_DUP |
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69 | Permit duplicate keys in the tree, that is, |
70 | permit insertion if the key to be | |
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71 | inserted already exists in the tree. |
72 | The default behavior, as described in | |
31e9a9ec | 73 | .BR dbopen (3), |
fea681da | 74 | is to overwrite a matching key when inserting a new key or to fail if |
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75 | the |
76 | .B R_NOOVERWRITE | |
77 | flag is specified. | |
78 | The | |
79 | .B R_DUP | |
80 | flag is overridden by the | |
81 | .B R_NOOVERWRITE | |
82 | flag, and if the | |
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83 | .B R_NOOVERWRITE |
84 | flag is specified, attempts to insert duplicate keys into | |
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85 | the tree will fail. |
86 | .IP | |
87 | If the database contains duplicate keys, the order of retrieval of | |
88 | key/data pairs is undefined if the | |
89 | .I get | |
90 | routine is used, however, | |
91 | .I seq | |
628d8d62 MK |
92 | routine calls with the |
93 | .B R_CURSOR | |
94 | flag set will always return the logical | |
40d6f0f0 | 95 | "first" of any group of duplicate keys. |
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96 | .RE |
97 | .TP | |
e5056894 | 98 | .I cachesize |
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99 | A suggested maximum size (in bytes) of the memory cache. |
100 | This value is | |
836f07c1 | 101 | .I only |
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102 | advisory, and the access method will allocate more memory rather than fail. |
103 | Since every search examines the root page of the tree, caching the most | |
104 | recently used pages substantially improves access time. | |
105 | In addition, physical writes are delayed as long as possible, so a moderate | |
106 | cache can reduce the number of I/O operations significantly. | |
107 | Obviously, using a cache increases (but only increases) the likelihood of | |
108 | corruption or lost data if the system crashes while a tree is being modified. | |
109 | If | |
110 | .I cachesize | |
113839c3 | 111 | is 0 (no size is specified), a default cache is used. |
fea681da | 112 | .TP |
e5056894 | 113 | .I maxkeypage |
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114 | The maximum number of keys which will be stored on any single page. |
115 | Not currently implemented. | |
116 | .\" The maximum number of keys which will be stored on any single page. | |
117 | .\" Because of the way the btree data structure works, | |
118 | .\" .I maxkeypage | |
119 | .\" must always be greater than or equal to 2. | |
120 | .\" If | |
121 | .\" .I maxkeypage | |
113839c3 | 122 | .\" is 0 (no maximum number of keys is specified), the page fill factor is |
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123 | .\" made as large as possible (which is almost invariably what is wanted). |
124 | .TP | |
e5056894 | 125 | .I minkeypage |
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126 | The minimum number of keys which will be stored on any single page. |
127 | This value is used to determine which keys will be stored on overflow | |
75b94dc3 | 128 | pages, that is, if a key or data item is longer than the pagesize divided |
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129 | by the minkeypage value, it will be stored on overflow pages instead |
130 | of in the page itself. | |
131 | If | |
132 | .I minkeypage | |
113839c3 | 133 | is 0 (no minimum number of keys is specified), a value of 2 is used. |
fea681da | 134 | .TP |
e5056894 | 135 | .I psize |
fea681da | 136 | Page size is the size (in bytes) of the pages used for nodes in the tree. |
ab5736e9 | 137 | The minimum page size is 512 bytes and the maximum page size is 64\ KiB. |
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138 | If |
139 | .I psize | |
113839c3 | 140 | is 0 (no page size is specified), a page size is chosen based on the |
9ee4a2b6 | 141 | underlying filesystem I/O block size. |
fea681da | 142 | .TP |
e5056894 | 143 | .I compare |
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144 | Compare is the key comparison function. |
145 | It must return an integer less than, equal to, or greater than zero if the | |
146 | first key argument is considered to be respectively less than, equal to, | |
147 | or greater than the second key argument. | |
148 | The same comparison function must be used on a given tree every time it | |
149 | is opened. | |
150 | If | |
151 | .I compare | |
152 | is NULL (no comparison function is specified), the keys are compared | |
153 | lexically, with shorter keys considered less than longer keys. | |
154 | .TP | |
e5056894 | 155 | .I prefix |
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156 | Prefix is the prefix comparison function. |
157 | If specified, this routine must return the number of bytes of the second key | |
158 | argument which are necessary to determine that it is greater than the first | |
159 | key argument. | |
160 | If the keys are equal, the key length should be returned. | |
a43eed0c | 161 | Note, the usefulness of this routine is very data-dependent, but, in some |
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162 | data sets can produce significantly reduced tree sizes and search times. |
163 | If | |
164 | .I prefix | |
165 | is NULL (no prefix function is specified), | |
836f07c1 | 166 | .I and |
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167 | no comparison function is specified, a default lexical comparison routine |
168 | is used. | |
169 | If | |
170 | .I prefix | |
171 | is NULL and a comparison routine is specified, no prefix comparison is | |
172 | done. | |
173 | .TP | |
e5056894 | 174 | .I lorder |
fea681da | 175 | The byte order for integers in the stored database metadata. |
c13182ef | 176 | The number should represent the order as an integer; for example, |
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177 | big endian order would be the number 4,321. |
178 | If | |
179 | .I lorder | |
113839c3 | 180 | is 0 (no order is specified), the current host order is used. |
fea681da | 181 | .PP |
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182 | If the file already exists (and the |
183 | .B O_TRUNC | |
184 | flag is not specified), the | |
c4bb193f | 185 | values specified for the arguments |
e5056894 | 186 | .IR flags , |
d556548b | 187 | .IR lorder , |
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188 | and |
189 | .I psize | |
190 | are ignored | |
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191 | in favor of the values used when the tree was created. |
192 | .PP | |
193 | Forward sequential scans of a tree are from the least key to the greatest. | |
194 | .PP | |
195 | Space freed up by deleting key/data pairs from the tree is never reclaimed, | |
196 | although it is normally made available for reuse. | |
197 | This means that the btree storage structure is grow-only. | |
198 | The only solutions are to avoid excessive deletions, or to create a fresh | |
199 | tree periodically from a scan of an existing one. | |
200 | .PP | |
201 | Searches, insertions, and deletions in a btree will all complete in | |
202 | O lg base N where base is the average fill factor. | |
203 | Often, inserting ordered data into btrees results in a low fill factor. | |
204 | This implementation has been modified to make ordered insertion the best | |
205 | case, resulting in a much better than normal page fill factor. | |
206 | .SH ERRORS | |
207 | The | |
208 | .I btree | |
209 | access method routines may fail and set | |
210 | .I errno | |
211 | for any of the errors specified for the library routine | |
31e9a9ec | 212 | .BR dbopen (3). |
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213 | .SH BUGS |
214 | Only big and little endian byte order is supported. | |
47297adb | 215 | .SH SEE ALSO |
31e9a9ec MK |
216 | .BR dbopen (3), |
217 | .BR hash (3), | |
218 | .BR mpool (3), | |
219 | .BR recno (3) | |
847e0d88 | 220 | .PP |
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221 | .IR "The Ubiquitous B-tree" , |
222 | Douglas Comer, ACM Comput. Surv. 11, 2 (June 1979), 121-138. | |
847e0d88 | 223 | .PP |
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224 | .IR "Prefix B-trees" , |
225 | Bayer and Unterauer, ACM Transactions on Database Systems, Vol. 2, 1 | |
226 | (March 1977), 11-26. | |
847e0d88 | 227 | .PP |
c13182ef | 228 | .IR "The Art of Computer Programming Vol. 3: Sorting and Searching" , |
fea681da | 229 | D.E. Knuth, 1968, pp 471-480. |