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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 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 $ 30.\" $DragonFly: src/lib/libc/db/man/btree.3,v 1.4 2006/05/26 19:39:36 swildner Exp $ 31.\" 32.Dd August 18, 1994 33.Dt BTREE 3 34.Os 35.Sh NAME 36.Nm btree 37.Nd "btree database access method" 38.Sh SYNOPSIS 39.In sys/types.h 40.In db.h 41.Sh DESCRIPTION 42The routine 43.Fn dbopen 44is the library interface to database files. 45One of the supported file formats is 46.Nm 47files. 48The general description of the database access methods is in 49.Xr dbopen 3 , 50this manual page describes only the 51.Nm 52specific information. 53.Pp 54The 55.Nm 56data structure is a sorted, balanced tree structure storing 57associated key/data pairs. 58.Pp 59The 60.Nm 61access method specific data structure provided to 62.Fn dbopen 63is defined in the 64.In db.h 65include file as follows: 66.Bd -literal 67typedef struct { 68 u_long flags; 69 u_int cachesize; 70 int maxkeypage; 71 int minkeypage; 72 u_int psize; 73 int (*compare)(const DBT *key1, const DBT *key2); 74 size_t (*prefix)(const DBT *key1, const DBT *key2); 75 int lorder; 76} BTREEINFO; 77.Ed 78.Pp 79The elements of this structure are as follows: 80.Bl -tag -width indent 81.It Va flags 82The flag value is specified by 83.Em or Ns 'ing 84any of the following values: 85.Bl -tag -width indent 86.It Dv R_DUP 87Permit duplicate keys in the tree, i.e. permit insertion if the key to be 88inserted already exists in the tree. 89The default behavior, as described in 90.Xr dbopen 3 , 91is to overwrite a matching key when inserting a new key or to fail if 92the 93.Dv R_NOOVERWRITE 94flag is specified. 95The 96.Dv R_DUP 97flag is overridden by the 98.Dv R_NOOVERWRITE 99flag, and if the 100.Dv R_NOOVERWRITE 101flag is specified, attempts to insert duplicate keys into 102the tree will fail. 103.Pp 104If the database contains duplicate keys, the order of retrieval of 105key/data pairs is undefined if the 106.Va get 107routine is used, however, 108.Va seq 109routine calls with the 110.Dv R_CURSOR 111flag set will always return the logical 112.Dq first 113of any group of duplicate keys. 114.El 115.It Va cachesize 116A suggested maximum size (in bytes) of the memory cache. 117This value is 118.Em only 119advisory, and the access method will allocate more memory rather than fail. 120Since every search examines the root page of the tree, caching the most 121recently used pages substantially improves access time. 122In addition, physical writes are delayed as long as possible, so a moderate 123cache can reduce the number of I/O operations significantly. 124Obviously, using a cache increases (but only increases) the likelihood of 125corruption or lost data if the system crashes while a tree is being modified. 126If 127.Va cachesize 128is 0 (no size is specified) a default cache is used. 129.It Va maxkeypage 130The maximum number of keys which will be stored on any single page. 131Not currently implemented. 132.\" The maximum number of keys which will be stored on any single page. 133.\" Because of the way the 134.\" .Nm 135.\" data structure works, 136.\" .Va maxkeypage 137.\" must always be greater than or equal to 2. 138.\" If 139.\" .Va maxkeypage 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). 142.It Va minkeypage 143The minimum number of keys which will be stored on any single page. 144This value is used to determine which keys will be stored on overflow 145pages, i.e. if a key or data item is longer than the pagesize divided 146by the minkeypage value, it will be stored on overflow pages instead 147of in the page itself. 148If 149.Va minkeypage 150is 0 (no minimum number of keys is specified) a value of 2 is used. 151.It Va psize 152Page size is the size (in bytes) of the pages used for nodes in the tree. 153The minimum page size is 512 bytes and the maximum page size is 64K. 154If 155.Va psize 156is 0 (no page size is specified) a page size is chosen based on the 157underlying file system I/O block size. 158.It Va compare 159Compare is the key comparison function. 160It must return an integer less than, equal to, or greater than zero if the 161first key argument is considered to be respectively less than, equal to, 162or greater than the second key argument. 163The same comparison function must be used on a given tree every time it 164is opened. 165If 166.Va compare 167is 168.Dv NULL 169(no comparison function is specified), the keys are compared 170lexically, with shorter keys considered less than longer keys. 171.It Va prefix 172The 173.Va prefix 174element 175is the prefix comparison function. 176If specified, this routine must return the number of bytes of the second key 177argument which are necessary to determine that it is greater than the first 178key argument. 179If the keys are equal, the key length should be returned. 180Note, the usefulness of this routine is very data dependent, but, in some 181data sets can produce significantly reduced tree sizes and search times. 182If 183.Va prefix 184is 185.Dv NULL 186(no prefix function is specified), 187.Em and 188no comparison function is specified, a default lexical comparison routine 189is used. 190If 191.Va prefix 192is 193.Dv NULL 194and a comparison routine is specified, no prefix comparison is 195done. 196.It Va lorder 197The byte order for integers in the stored database metadata. 198The number should represent the order as an integer; for example, 199big endian order would be the number 4,321. 200If 201.Va lorder 202is 0 (no order is specified) the current host order is used. 203.El 204.Pp 205If the file already exists (and the 206.Dv O_TRUNC 207flag is not specified), the 208values specified for the 209.Va flags , lorder 210and 211.Va psize 212arguments 213are ignored 214in favor of the values used when the tree was created. 215.Pp 216Forward sequential scans of a tree are from the least key to the greatest. 217.Pp 218Space freed up by deleting key/data pairs from the tree is never reclaimed, 219although it is normally made available for reuse. 220This means that the 221.Nm 222storage structure is grow-only. 223The only solutions are to avoid excessive deletions, or to create a fresh 224tree periodically from a scan of an existing one. 225.Pp 226Searches, insertions, and deletions in a 227.Nm 228will all complete in 229O lg base N where base is the average fill factor. 230Often, inserting ordered data into 231.Nm Ns s 232results in a low fill factor. 233This implementation has been modified to make ordered insertion the best 234case, resulting in a much better than normal page fill factor. 235.Sh ERRORS 236The 237.Nm 238access method routines may fail and set 239.Va errno 240for any of the errors specified for the library routine 241.Xr dbopen 3 . 242.Sh SEE ALSO 243.Xr dbopen 3 , 244.Xr hash 3 , 245.Xr mpool 3 , 246.Xr recno 3 247.Rs 248.%T "The Ubiquitous B-tree" 249.%A Douglas Comer 250.%J "ACM Comput. Surv. 11" 251.%N 2 252.%D June 1979 253.%P 121-138 254.Re 255.Rs 256.%A Bayer 257.%A Unterauer 258.%T "Prefix B-trees" 259.%J "ACM Transactions on Database Systems" 260.%N 1 261.%V Vol. 2 262.%D March 1977 263.%P 11-26 264.Re 265.Rs 266.%B "The Art of Computer Programming Vol. 3: Sorting and Searching" 267.%A D. E. Knuth 268.%D 1968 269.%P 471-480 270.Re 271.Sh BUGS 272Only big and little endian byte order is supported. 273