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32 .\" @(#)btree.3 8.4 (Berkeley) 8/18/94
34 .TH BTREE 3 "August 18, 1994"
37 btree \- btree database access method
41 #include <sys/types.h>
48 is the library interface to database files.
49 One of the supported file formats is btree files.
50 The general description of the database access methods is in
52 this manual page describes only the btree specific information.
54 The btree data structure is a sorted, balanced tree structure storing
55 associated key/data pairs.
57 The btree access method specific data structure provided to
59 is defined in the <db.h> include file as follows:
73 int (*compare)(const DBT *key1, const DBT *key2);
75 size_t (*prefix)(const DBT *key1, const DBT *key2);
81 The elements of this structure are as follows:
84 The flag value is specified by
86 any of the following values:
90 Permit duplicate keys in the tree, i.e. permit insertion if the key to be
91 inserted already exists in the tree.
92 The default behavior, as described in
94 is to overwrite a matching key when inserting a new key or to fail if
95 the R_NOOVERWRITE flag is specified.
96 The R_DUP flag is overridden by the R_NOOVERWRITE flag, and if the
97 R_NOOVERWRITE flag is specified, attempts to insert duplicate keys into
100 If the database contains duplicate keys, the order of retrieval of
101 key/data pairs is undefined if the
103 routine is used, however,
105 routine calls with the R_CURSOR flag set will always return the logical
106 ``first'' of any group of duplicate keys.
110 A suggested maximum size (in bytes) of the memory cache.
113 advisory, and the access method will allocate more memory rather than fail.
114 Since every search examines the root page of the tree, caching the most
115 recently used pages substantially improves access time.
116 In addition, physical writes are delayed as long as possible, so a moderate
117 cache can reduce the number of I/O operations significantly.
118 Obviously, using a cache increases (but only increases) the likelihood of
119 corruption or lost data if the system crashes while a tree is being modified.
122 is 0 (no size is specified) a default cache is used.
125 The maximum number of keys which will be stored on any single page.
126 Not currently implemented.
127 .\" The maximum number of keys which will be stored on any single page.
128 .\" Because of the way the btree data structure works,
130 .\" must always be greater than or equal to 2.
133 .\" is 0 (no maximum number of keys is specified) the page fill factor is
134 .\" made as large as possible (which is almost invariably what is wanted).
137 The minimum number of keys which will be stored on any single page.
138 This value is used to determine which keys will be stored on overflow
139 pages, i.e. if a key or data item is longer than the pagesize divided
140 by the minkeypage value, it will be stored on overflow pages instead
141 of in the page itself.
144 is 0 (no minimum number of keys is specified) a value of 2 is used.
147 Page size is the size (in bytes) of the pages used for nodes in the tree.
148 The minimum page size is 512 bytes and the maximum page size is 64K.
151 is 0 (no page size is specified) a page size is chosen based on the
152 underlying file system I/O block size.
155 Compare is the key comparison function.
156 It must return an integer less than, equal to, or greater than zero if the
157 first key argument is considered to be respectively less than, equal to,
158 or greater than the second key argument.
159 The same comparison function must be used on a given tree every time it
163 is NULL (no comparison function is specified), the keys are compared
164 lexically, with shorter keys considered less than longer keys.
167 Prefix is the prefix comparison function.
168 If specified, this routine must return the number of bytes of the second key
169 argument which are necessary to determine that it is greater than the first
171 If the keys are equal, the key length should be returned.
172 Note, the usefulness of this routine is very data dependent, but, in some
173 data sets can produce significantly reduced tree sizes and search times.
176 is NULL (no prefix function is specified),
178 no comparison function is specified, a default lexical comparison routine
182 is NULL and a comparison routine is specified, no prefix comparison is
186 The byte order for integers in the stored database metadata.
187 The number should represent the order as an integer; for example,
188 big endian order would be the number 4,321.
191 is 0 (no order is specified) the current host order is used.
193 If the file already exists (and the O_TRUNC flag is not specified), the
194 values specified for the parameters flags, lorder and psize are ignored
195 in favor of the values used when the tree was created.
197 Forward sequential scans of a tree are from the least key to the greatest.
199 Space freed up by deleting key/data pairs from the tree is never reclaimed,
200 although it is normally made available for reuse.
201 This means that the btree storage structure is grow-only.
202 The only solutions are to avoid excessive deletions, or to create a fresh
203 tree periodically from a scan of an existing one.
205 Searches, insertions, and deletions in a btree will all complete in
206 O lg base N where base is the average fill factor.
207 Often, inserting ordered data into btrees results in a low fill factor.
208 This implementation has been modified to make ordered insertion the best
209 case, resulting in a much better than normal page fill factor.
213 access method routines may fail and set
215 for any of the errors specified for the library routine
223 .IR "The Ubiquitous B-tree" ,
224 Douglas Comer, ACM Comput. Surv. 11, 2 (June 1979), 121-138.
226 .IR "Prefix B-trees" ,
227 Bayer and Unterauer, ACM Transactions on Database Systems, Vol. 2, 1
230 .IR "The Art of Computer Programming Vol. 3: Sorting and Searching" ,
231 D.E. Knuth, 1968, pp 471-480.
233 Only big and little endian byte order is supported.