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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)btree.3 8.4 (Berkeley) 8/18/94 33.\" $FreeBSD$ 34.\" 35.Dd August 18, 1994 36.Dt BTREE 3 37.Os 38.Sh NAME 39.Nm btree 40.Nd "btree database access method" 41.Sh SYNOPSIS 42.In sys/types.h 43.In db.h 44.Sh DESCRIPTION 45The routine 46.Fn dbopen 47is the library interface to database files. 48One of the supported file formats is 49.Nm 50files. 51The general description of the database access methods is in 52.Xr dbopen 3 , 53this manual page describes only the 54.Nm 55specific information. 56.Pp 57The 58.Nm 59data structure is a sorted, balanced tree structure storing 60associated key/data pairs. 61.Pp 62The 63.Nm 64access method specific data structure provided to 65.Fn dbopen 66is defined in the 67.In db.h 68include file as follows: 69.Bd -literal 70typedef struct { 71 u_long flags; 72 u_int cachesize; 73 int maxkeypage; 74 int minkeypage; 75 u_int psize; 76 int (*compare)(const DBT *key1, const DBT *key2); 77 size_t (*prefix)(const DBT *key1, const DBT *key2); 78 int lorder; 79} BTREEINFO; 80.Ed 81.Pp 82The elements of this structure are as follows: 83.Bl -tag -width indent 84.It Va flags 85The flag value is specified by 86.Em or Ns 'ing 87any of the following values: 88.Bl -tag -width indent 89.It Dv R_DUP 90Permit duplicate keys in the tree, i.e., permit insertion if the key to be 91inserted already exists in the tree. 92The default behavior, as described in 93.Xr dbopen 3 , 94is to overwrite a matching key when inserting a new key or to fail if 95the 96.Dv R_NOOVERWRITE 97flag is specified. 98The 99.Dv R_DUP 100flag is overridden by the 101.Dv R_NOOVERWRITE 102flag, and if the 103.Dv R_NOOVERWRITE 104flag is specified, attempts to insert duplicate keys into 105the tree will fail. 106.Pp 107If the database contains duplicate keys, the order of retrieval of 108key/data pairs is undefined if the 109.Va get 110routine is used, however, 111.Va seq 112routine calls with the 113.Dv R_CURSOR 114flag set will always return the logical 115.Dq first 116of any group of duplicate keys. 117.El 118.It Va cachesize 119A suggested maximum size (in bytes) of the memory cache. 120This value is 121.Em only 122advisory, and the access method will allocate more memory rather than fail. 123Since every search examines the root page of the tree, caching the most 124recently used pages substantially improves access time. 125In addition, physical writes are delayed as long as possible, so a moderate 126cache can reduce the number of I/O operations significantly. 127Obviously, using a cache increases (but only increases) the likelihood of 128corruption or lost data if the system crashes while a tree is being modified. 129If 130.Va cachesize 131is 0 (no size is specified) a default cache is used. 132.It Va maxkeypage 133The maximum number of keys which will be stored on any single page. 134Not currently implemented. 135.\" The maximum number of keys which will be stored on any single page. 136.\" Because of the way the 137.\" .Nm 138.\" data structure works, 139.\" .Va maxkeypage 140.\" must always be greater than or equal to 2. 141.\" If 142.\" .Va maxkeypage 143.\" is 0 (no maximum number of keys is specified) the page fill factor is 144.\" made as large as possible (which is almost invariably what is wanted). 145.It Va minkeypage 146The minimum number of keys which will be stored on any single page. 147This value is used to determine which keys will be stored on overflow 148pages, i.e., if a key or data item is longer than the pagesize divided 149by the minkeypage value, it will be stored on overflow pages instead 150of in the page itself. 151If 152.Va minkeypage 153is 0 (no minimum number of keys is specified) a value of 2 is used. 154.It Va psize 155Page size is the size (in bytes) of the pages used for nodes in the tree. 156The minimum page size is 512 bytes and the maximum page size is 64K. 157If 158.Va psize 159is 0 (no page size is specified) a page size is chosen based on the 160underlying file system I/O block size. 161.It Va compare 162Compare is the key comparison function. 163It must return an integer less than, equal to, or greater than zero if the 164first key argument is considered to be respectively less than, equal to, 165or greater than the second key argument. 166The same comparison function must be used on a given tree every time it 167is opened. 168If 169.Va compare 170is 171.Dv NULL 172(no comparison function is specified), the keys are compared 173lexically, with shorter keys considered less than longer keys. 174.It Va prefix 175The 176.Va prefix 177element 178is the prefix comparison function. 179If specified, this routine must return the number of bytes of the second key 180argument which are necessary to determine that it is greater than the first 181key argument. 182If the keys are equal, the key length should be returned. 183Note, the usefulness of this routine is very data dependent, but, in some 184data sets can produce significantly reduced tree sizes and search times. 185If 186.Va prefix 187is 188.Dv NULL 189(no prefix function is specified), 190.Em and 191no comparison function is specified, a default lexical comparison routine 192is used. 193If 194.Va prefix 195is 196.Dv NULL 197and a comparison routine is specified, no prefix comparison is 198done. 199.It Va lorder 200The byte order for integers in the stored database metadata. 201The number should represent the order as an integer; for example, 202big endian order would be the number 4,321. 203If 204.Va lorder 205is 0 (no order is specified) the current host order is used. 206.El 207.Pp 208If the file already exists (and the 209.Dv O_TRUNC 210flag is not specified), the 211values specified for the 212.Va flags , lorder 213and 214.Va psize 215arguments 216are ignored 217in favor of the values used when the tree was created. 218.Pp 219Forward sequential scans of a tree are from the least key to the greatest. 220.Pp 221Space freed up by deleting key/data pairs from the tree is never reclaimed, 222although it is normally made available for reuse. 223This means that the 224.Nm 225storage structure is grow-only. 226The only solutions are to avoid excessive deletions, or to create a fresh 227tree periodically from a scan of an existing one. 228.Pp 229Searches, insertions, and deletions in a 230.Nm 231will all complete in 232O lg base N where base is the average fill factor. 233Often, inserting ordered data into 234.Nm Ns s 235results in a low fill factor. 236This implementation has been modified to make ordered insertion the best 237case, resulting in a much better than normal page fill factor. 238.Sh ERRORS 239The 240.Nm 241access method routines may fail and set 242.Va errno 243for any of the errors specified for the library routine 244.Xr dbopen 3 . 245.Sh SEE ALSO 246.Xr dbopen 3 , 247.Xr hash 3 , 248.Xr mpool 3 , 249.Xr recno 3 250.Rs 251.%T "The Ubiquitous B-tree" 252.%A Douglas Comer 253.%J "ACM Comput. Surv. 11" 254.%N 2 255.%D June 1979 256.%P 121-138 257.Re 258.Rs 259.%A Bayer 260.%A Unterauer 261.%T "Prefix B-trees" 262.%J "ACM Transactions on Database Systems" 263.%N 1 264.%V Vol. 2 265.%D March 1977 266.%P 11-26 267.Re 268.Rs 269.%B "The Art of Computer Programming Vol. 3: Sorting and Searching" 270.%A D. E. Knuth 271.%D 1968 272.%P 471-480 273.Re 274.Sh BUGS 275Only big and little endian byte order is supported. 276