.\" Copyright (c) 1990, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" from: @(#)btree.3 8.1 (Berkeley) 6/4/93 .\" $Id: btree.3,v 1.2 1993/08/01 07:45:41 mycroft Exp $ .\" .TH BTREE 3 .\".UC 7 .SH NAME btree \- btree database access method .SH SYNOPSIS .nf .ft B #include #include .ft R .fi .SH DESCRIPTION The routine .IR dbopen is the library interface to database files. One of the supported file formats is btree files. The general description of the database access methods is in .IR dbopen (3), this manual page describes only the btree specific information. .PP The btree data structure is a sorted, balanced tree structure storing associated key/data pairs. .PP The btree access method specific data structure provided to .I dbopen is defined in the include file as follows: .PP typedef struct { .RS u_long flags; .br u_int cachesize; .br index_t psize; .br int lorder; .\" .br .\" int maxkeypage; .br int minkeypage; .br int (*compare)(const DBT *key1, const DBT *key2); .br int (*prefix)(const DBT *key1, const DBT *key2); .RE } BTREEINFO; .PP The elements of this structure are as follows: .TP flags The flag value is specified by .IR or 'ing any of the following values: .RS .TP R_DUP Permit duplicate keys in the tree, i.e. permit insertion if the key to be inserted already exists in the tree. The default behavior, as described in .IR dbopen (3), is to overwrite a matching key when inserting a new key or to fail if the R_NOOVERWRITE flag is specified. The R_DUP flag is overridden by the R_NOOVERWRITE flag, and if the R_NOOVERWRITE flag is specified, attempts to insert duplicate keys into the tree will fail. .IP If the database contains duplicate keys, the order of retrieval of key/data pairs is undefined if the .I get routine is used, however, .I seq routine calls with the R_CURSOR flag set will always return the logical ``first'' of any group of duplicate keys. .RE .TP cachesize A suggested maximum size (in bytes) of the memory cache. This value is .B only advisory, and the access method will allocate more memory rather than fail. Since every search examines the root page of the tree, caching the most recently used pages substantially improves access time. In addition, physical writes are delayed as long as possible, so a moderate cache can reduce the number of I/O operations significantly. Obviously, using a cache increases (but only increases) the likelihood of corruption or lost data if the system crashes while a tree is being modified. If .I cachesize is 0 (no size is specified) a default cache is used. .TP psize Page size is the size (in bytes) of the pages used for nodes in the tree. The minimum page size is 512 bytes and the maximum page size is 64K. If .I psize is 0 (no page size is specified) a page size is chosen based on the underlying file system I/O block size. .TP lorder The byte order for integers in the stored database metadata. The number should represent the order as an integer; for example, big endian order would be the number 4,321. If .I lorder is 0 (no order is specified) the current host order is used. .\" .TP .\" maxkeypage .\" The maximum number of keys which will be stored on any single page. .\" Because of the way the btree data structure works, .\" .I maxkeypage .\" must always be greater than or equal to 2. .\" If .\" .I maxkeypage .\" is 0 (no maximum number of keys is specified) the page fill factor is .\" made as large as possible (which is almost invariably what is wanted). .TP minkeypage The minimum number of keys which will be stored on any single page. This value is used to determine which keys will be stored on overflow pages, i.e. if a key or data item is longer than the pagesize divided by the minkeypage value, it will be stored on overflow pages instead of in the page itself. If .I minkeypage is 0 (no minimum number of keys is specified) a value of 2 is used. .TP compare Compare is the key comparison function. It must return an integer less than, equal to, or greater than zero if the first key argument is considered to be respectively less than, equal to, or greater than the second key argument. The same comparison function must be used on a given tree every time it is opened. If .I compare is NULL (no comparison function is specified), the keys are compared lexically, with shorter keys considered less than longer keys. .TP prefix Prefix is the prefix comparison function. If specified, this routine must return the number of bytes of the second key argument which are necessary to determine that it is greater than the first key argument. If the keys are equal, the key length should be returned. Note, the usefulness of this routine is very data dependent, but, in some data sets can produce significantly reduced tree sizes and search times. If .I prefix is NULL (no prefix function is specified), .B and no comparison function is specified, a default lexical comparison routine is used. If .I prefix is NULL and a comparison routine is specified, no prefix comparison is done. .PP If the file already exists (and the O_TRUNC flag is not specified), the values specified for the parameters flags, lorder and psize are ignored in favor of the values used when the tree was created. .PP Forward sequential scans of a tree are from the least key to the greatest. .PP Space freed up by deleting key/data pairs from the tree is never reclaimed, although it is normally made available for reuse. This means that the btree storage structure is grow-only. The only solutions are to avoid excessive deletions, or to create a fresh tree periodically from a scan of an existing one. .PP Searches, insertions, and deletions in a btree will all complete in O lg base N where base is the average fill factor. Often, inserting ordered data into btrees results in a low fill factor. This implementation has been modified to make ordered insertion the best case, resulting in a much better than normal page fill factor. .SH "SEE ALSO" .IR dbopen (3), .IR hash (3), .IR mpool (3), .IR recno (3) .sp .IR "The Ubiquitous B-tree" , Douglas Comer, ACM Comput. Surv. 11, 2 (June 1979), 121-138. .sp .IR "Prefix B-trees" , Bayer and Unterauer, ACM Transactions on Database Systems, Vol. 2, 1 (March 1977), 11-26. .sp .IR "The Art of Computer Programming Vol. 3: Sorting and Searching" , D.E. Knuth, 1968, pp 471-480. .SH BUGS Only big and little endian byte order is supported.