1.\" $NetBSD: btree.3,v 1.10 2003/04/17 19:17:48 wiz Exp $ 2.\" 3.\" Copyright (c) 1990, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)btree.3 8.4 (Berkeley) 8/18/94 35.\" 36.Dd April 17, 2003 37.Dt BTREE 3 38.Os 39.Sh NAME 40.Nm btree 41.Nd btree database access method 42.Sh SYNOPSIS 43.In sys/types.h 44.In db.h 45.Sh DESCRIPTION 46The routine 47.Fn dbopen 48is the library interface to database files. 49One of the supported file formats is btree files. 50The general description of the database access methods is in 51.Xr dbopen 3 , 52this manual page describes only the btree specific information. 53.Pp 54The btree data structure is a sorted, balanced tree structure storing 55associated key/data pairs. 56.Pp 57The btree access method specific data structure provided to 58.Fn dbopen 59is defined in the 60.Aq Pa db.h 61include file as follows: 62.Bd -literal 63typedef struct { 64 u_long flags; 65 u_int cachesize; 66 int maxkeypage; 67 int minkeypage; 68 u_int psize; 69 int (*compare)(const DBT *key1, const DBT *key2); 70 size_t (*prefix)(const DBT *key1, const DBT *key2); 71 int lorder; 72} BTREEINFO; 73.Ed 74.Pp 75The elements of this structure are as follows: 76.Bl -tag -width maxkeypagex 77.It Fa flags 78The flag value is specified by or'ing any of the following values: 79.Bl -tag -width R_DUP -offset indent 80.It Dv R_DUP 81Permit duplicate keys in the tree, i.e. permit insertion if the key to 82be inserted already exists in the tree. 83The default behavior, as described in 84.Xr dbopen 3 , 85is to overwrite a matching key when inserting a new key or to fail if 86the 87.Dv R_NOOVERWRITE 88flag is specified. 89The 90.Dv R_DUP 91flag is overridden by the 92.Dv R_NOOVERWRITE 93flag, and if the 94.Dv R_NOOVERWRITE 95flag is specified, attempts to insert duplicate keys into the tree 96will fail. 97.Pp 98If the database contains duplicate keys, the order of retrieval of 99key/data pairs is undefined if the 100.Em get 101routine is used, however, 102.Em seq 103routine calls with the 104.Dv R_CURSOR 105flag set will always return the logical 106.Dq first 107of any group of duplicate keys. 108.El 109.It Fa cachesize 110A suggested maximum size (in bytes) of the memory cache. 111This value is 112.Em only 113advisory, and the access method will allocate more memory rather than 114fail. 115Since every search examines the root page of the tree, caching the 116most recently used pages substantially improves access time. 117In addition, physical writes are delayed as long as possible, so a 118moderate cache can reduce the number of I/O operations significantly. 119Obviously, using a cache increases (but only increases) the likelihood 120of corruption or lost data if the system crashes while a tree is being 121modified. 122If 123.Fa cachesize 124is 0 (no size is specified) a default cache is used. 125.It Fa maxkeypage 126The maximum number of keys which will be stored on any single page. 127Not currently implemented. 128.\" The maximum number of keys which will be stored on any single page. 129.\" Because of the way the btree data structure works, 130.\" .Fa maxkeypage 131.\" must always be greater than or equal to 2. 132.\" If 133.\" .Fa maxkeypage 134.\" is 0 (no maximum number of keys is specified) the page fill factor is 135.\" made as large as possible (which is almost invariably what is wanted). 136.It Fa minkeypage 137The minimum number of keys which will be stored on any single page. 138This value is used to determine which keys will be stored on overflow 139pages, i.e., if a key or data item is longer than the pagesize divided 140by the 141.Fa minkeypage 142value, it will be stored on overflow pages instead of in the page 143itself. 144If 145.Fa minkeypage 146is 0 (no minimum number of keys is specified) a value of 2 is used. 147.It Fa psize 148Page size is the size (in bytes) of the pages used for nodes in the 149tree. 150The minimum page size is 512 bytes and the maximum page size is 64K. 151If 152.Fa psize 153is 0 (no page size is specified) a page size is chosen based on the 154underlying file system I/O block size. 155.It Fa compare 156Compare is the key comparison function. 157It must return an integer less than, equal to, or greater than zero if 158the first key argument is considered to be respectively less than, 159equal to, or greater than the second key argument. 160The same comparison function must be used on a given tree every time 161it is opened. 162If 163.Fa compare 164is 165.Dv NULL 166(no comparison function is specified), the keys are compared 167lexically, with shorter keys considered less than longer keys. 168.It Fa prefix 169Prefix is the prefix comparison function. 170If specified, this routine must return the number of bytes of the 171second key argument which are necessary to determine that it is 172greater than the first key argument. 173If the keys are equal, the key length should be returned. 174Note, the usefulness of this routine is very data dependent, but, in 175some data sets can produce significantly reduced tree sizes and search 176times. 177If 178.Fa prefix 179is 180.Dv NULL 181(no prefix function is specified), 182.Em and 183no comparison function is specified, a default lexical comparison 184routine is used. 185If 186.Fa prefix 187is 188.Dv NULL 189and a comparison routine is specified, no prefix comparison is done. 190.It Fa lorder 191The byte order for integers in the stored database metadata. 192The number should represent the order as an integer; for example, 193big endian order would be the number 4,321. 194If 195.Fa lorder 196is 0 (no order is specified) the current host order is used. 197.El 198.Pp 199If the file already exists (and the 200.Dv O_TRUNC 201flag is not specified), the values specified for the parameters flags, 202lorder and psize are ignored in favor of the values used when the tree 203was created. 204.Pp 205Forward sequential scans of a tree are from the least key to the 206greatest. 207.Pp 208Space freed up by deleting key/data pairs from the tree is never 209reclaimed, although it is normally made available for reuse. 210This means that the btree storage structure is grow-only. 211The only solutions are to avoid excessive deletions, or to create a 212fresh tree periodically from a scan of an existing one. 213.Pp 214Searches, insertions, and deletions in a btree will all complete in 215O lg base N where base is the average fill factor. 216Often, inserting ordered data into btrees results in a low fill 217factor. 218This implementation has been modified to make ordered insertion the 219best case, resulting in a much better than normal page fill factor. 220.Sh ERRORS 221The 222.Nm 223access method routines may fail and set 224.Va errno 225for any of the errors specified for the library routine 226.Xr dbopen 3 . 227.Sh SEE ALSO 228.Xr dbopen 3 , 229.Xr hash 3 , 230.Xr mpool 3 , 231.Xr recno 3 232.Pp 233.Rs 234.%T "The Ubiquitous B-tree" 235.%A "Douglas Comer" 236.%J "ACM Comput. Surv." 237.%V 2 238.%N 11 239.%D June 1979 240.%P 121-138 241.Re 242.Rs 243.%T "Prefix B-trees" 244.%A "Bayer" 245.%A "Unterauer" 246.%J "ACM Transactions on Database Systems" 247.%V Vol. 2 248.%N 1 249.%D March 1977 250.%P 11-26 251.Re 252.Rs 253.%B "The Art of Computer Programming Vol. 3: Sorting and Searching" 254.%A "D.E. Knuth" 255.%D 1968 256.%P 471-480 257.Re 258.Sh BUGS 259Only big and little endian byte order is supported. 260