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28 .\" @(#)btree.3 8.4 (Berkeley) 8/18/94
29 .\" $FreeBSD: src/lib/libc/db/man/btree.3,v 1.3.2.3 2003/03/15 15:11:05 trhodes Exp $
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37 .Nd "btree database access method"
44 is the library interface to database files.
45 One of the supported file formats is
48 The general description of the database access methods is in
50 this manual page describes only the
56 data structure is a sorted, balanced tree structure storing
57 associated key/data pairs.
61 access method specific data structure provided to
65 include file as follows:
73 int (*compare)(const DBT *key1, const DBT *key2);
74 size_t (*prefix)(const DBT *key1, const DBT *key2);
79 The elements of this structure are as follows:
80 .Bl -tag -width indent
82 The flag value is specified by
84 any of the following values:
85 .Bl -tag -width indent
87 Permit duplicate keys in the tree, i.e. permit insertion if the key to be
88 inserted already exists in the tree.
89 The default behavior, as described in
91 is to overwrite a matching key when inserting a new key or to fail if
97 flag is overridden by the
101 flag is specified, attempts to insert duplicate keys into
104 If the database contains duplicate keys, the order of retrieval of
105 key/data pairs is undefined if the
107 routine is used, however,
109 routine calls with the
111 flag set will always return the logical
113 of any group of duplicate keys.
116 A suggested maximum size (in bytes) of the memory cache.
119 advisory, and the access method will allocate more memory rather than fail.
120 Since every search examines the root page of the tree, caching the most
121 recently used pages substantially improves access time.
122 In addition, physical writes are delayed as long as possible, so a moderate
123 cache can reduce the number of I/O operations significantly.
124 Obviously, using a cache increases (but only increases) the likelihood of
125 corruption or lost data if the system crashes while a tree is being modified.
128 is 0 (no size is specified) a default cache is used.
130 The maximum number of keys which will be stored on any single page.
131 Not currently implemented.
132 .\" The maximum number of keys which will be stored on any single page.
133 .\" Because of the way the
135 .\" data structure works,
137 .\" must always be greater than or equal to 2.
140 .\" is 0 (no maximum number of keys is specified) the page fill factor is
141 .\" made as large as possible (which is almost invariably what is wanted).
143 The minimum number of keys which will be stored on any single page.
144 This value is used to determine which keys will be stored on overflow
145 pages, i.e. if a key or data item is longer than the pagesize divided
146 by the minkeypage value, it will be stored on overflow pages instead
147 of in the page itself.
150 is 0 (no minimum number of keys is specified) a value of 2 is used.
152 Page size is the size (in bytes) of the pages used for nodes in the tree.
153 The minimum page size is 512 bytes and the maximum page size is 64K.
156 is 0 (no page size is specified) a page size is chosen based on the
157 underlying file system I/O block size.
159 Compare is the key comparison function.
160 It must return an integer less than, equal to, or greater than zero if the
161 first key argument is considered to be respectively less than, equal to,
162 or greater than the second key argument.
163 The same comparison function must be used on a given tree every time it
169 (no comparison function is specified), the keys are compared
170 lexically, with shorter keys considered less than longer keys.
175 is the prefix comparison function.
176 If specified, this routine must return the number of bytes of the second key
177 argument which are necessary to determine that it is greater than the first
179 If the keys are equal, the key length should be returned.
180 Note, the usefulness of this routine is very data dependent, but, in some
181 data sets can produce significantly reduced tree sizes and search times.
186 (no prefix function is specified),
188 no comparison function is specified, a default lexical comparison routine
194 and a comparison routine is specified, no prefix comparison is
197 The byte order for integers in the stored database metadata.
198 The number should represent the order as an integer; for example,
199 big endian order would be the number 4,321.
202 is 0 (no order is specified) the current host order is used.
205 If the file already exists (and the
207 flag is not specified), the
208 values specified for the
214 in favor of the values used when the tree was created.
216 Forward sequential scans of a tree are from the least key to the greatest.
218 Space freed up by deleting key/data pairs from the tree is never reclaimed,
219 although it is normally made available for reuse.
222 storage structure is grow-only.
223 The only solutions are to avoid excessive deletions, or to create a fresh
224 tree periodically from a scan of an existing one.
226 Searches, insertions, and deletions in a
229 O lg base N where base is the average fill factor.
230 Often, inserting ordered data into
232 results in a low fill factor.
233 This implementation has been modified to make ordered insertion the best
234 case, resulting in a much better than normal page fill factor.
238 access method routines may fail and set
240 for any of the errors specified for the library routine
248 .%T "The Ubiquitous B-tree"
250 .%J "ACM Comput. Surv. 11"
259 .%J "ACM Transactions on Database Systems"
266 .%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
272 Only big and little endian byte order is supported.