db/man/FreeBSD/btree.3

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  28 .\"     @(#)btree.3     8.4 (Berkeley) 8/18/94
  29 .\" $FreeBSD: src/lib/libc/db/man/btree.3,v 1.9 2007/01/09 00:27:51 imp Exp $
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  31 .Dd August 18, 1994
  32 .Dt BTREE 3
  33 .Os
  34 .Sh NAME
  35 .Nm btree
  36 .Nd "btree database access method"
  37 .Sh SYNOPSIS
  38 .In sys/types.h
  39 .In db.h
  40 .Sh DESCRIPTION
  41 The routine
  42 .Fn dbopen
  43 is the library interface to database files.
  44 One of the supported file formats is
  45 .Nm
  46 files.
  47 The general description of the database access methods is in
  48 .Xr dbopen 3 ,
  49 this manual page describes only the
  50 .Nm
  51 specific information.
  52 .Pp
  53 The
  54 .Nm
  55 data structure is a sorted, balanced tree structure storing
  56 associated key/data pairs.
  57 .Pp
  58 The
  59 .Nm
  60 access method specific data structure provided to
  61 .Fn dbopen
  62 is defined in the
  63 .In db.h
  64 include file as follows:
  65 .Bd -literal
  66 typedef struct {
  67         u_long flags;
  68         u_int cachesize;
  69         int maxkeypage;
  70         int minkeypage;
  71         u_int psize;
  72         int (*compare)(const DBT *key1, const DBT *key2);
  73         size_t (*prefix)(const DBT *key1, const DBT *key2);
  74         int lorder;
  75 } BTREEINFO;
  76 .Ed
  77 .Pp
  78 The elements of this structure are as follows:
  79 .Bl -tag -width indent
  80 .It Va flags
  81 The flag value is specified by
  82 .Em or Ns 'ing
  83 any of the following values:
  84 .Bl -tag -width indent
  85 .It Dv R_DUP
  86 Permit duplicate keys in the tree, i.e., permit insertion if the key to be
  87 inserted already exists in the tree.
  88 The default behavior, as described in
  89 .Xr dbopen 3 ,
  90 is to overwrite a matching key when inserting a new key or to fail if
  91 the
  92 .Dv R_NOOVERWRITE
  93 flag is specified.
  94 The
  95 .Dv R_DUP
  96 flag is overridden by the
  97 .Dv R_NOOVERWRITE
  98 flag, and if the
  99 .Dv R_NOOVERWRITE
 100 flag is specified, attempts to insert duplicate keys into
 101 the tree will fail.
 102 .Pp
 103 If the database contains duplicate keys, the order of retrieval of
 104 key/data pairs is undefined if the
 105 .Va get
 106 routine is used, however,
 107 .Va seq
 108 routine calls with the
 109 .Dv R_CURSOR
 110 flag set will always return the logical
 111 .Dq first
 112 of any group of duplicate keys.
 113 .El
 114 .It Va cachesize
 115 A suggested maximum size (in bytes) of the memory cache.
 116 This value is
 117 .Em only
 118 advisory, and the access method will allocate more memory rather than fail.
 119 Since every search examines the root page of the tree, caching the most
 120 recently used pages substantially improves access time.
 121 In addition, physical writes are delayed as long as possible, so a moderate
 122 cache can reduce the number of I/O operations significantly.
 123 Obviously, using a cache increases (but only increases) the likelihood of
 124 corruption or lost data if the system crashes while a tree is being modified.
 125 If
 126 .Va cachesize
 127 is 0 (no size is specified) a default cache is used.
 128 .It Va maxkeypage
 129 The maximum number of keys which will be stored on any single page.
 130 Not currently implemented.
 131 .\" The maximum number of keys which will be stored on any single page.
 132 .\" Because of the way the
 133 .\" .Nm
 134 .\" data structure works,
 135 .\" .Va maxkeypage
 136 .\" must always be greater than or equal to 2.
 137 .\" If
 138 .\" .Va maxkeypage
 139 .\" is 0 (no maximum number of keys is specified) the page fill factor is
 140 .\" made as large as possible (which is almost invariably what is wanted).
 141 .It Va minkeypage
 142 The minimum number of keys which will be stored on any single page.
 143 This value is used to determine which keys will be stored on overflow
 144 pages, i.e., if a key or data item is longer than the pagesize divided
 145 by the minkeypage value, it will be stored on overflow pages instead
 146 of in the page itself.
 147 If
 148 .Va minkeypage
 149 is 0 (no minimum number of keys is specified) a value of 2 is used.
 150 .It Va psize
 151 Page size is the size (in bytes) of the pages used for nodes in the tree.
 152 The minimum page size is 512 bytes and the maximum page size is 64K.
 153 If
 154 .Va psize
 155 is 0 (no page size is specified) a page size is chosen based on the
 156 underlying file system I/O block size.
 157 .It Va compare
 158 Compare is the key comparison function.
 159 It must return an integer less than, equal to, or greater than zero if the
 160 first key argument is considered to be respectively less than, equal to,
 161 or greater than the second key argument.
 162 The same comparison function must be used on a given tree every time it
 163 is opened.
 164 If
 165 .Va compare
 166 is
 167 .Dv NULL
 168 (no comparison function is specified), the keys are compared
 169 lexically, with shorter keys considered less than longer keys.
 170 .It Va prefix
 171 The
 172 .Va prefix
 173 element
 174 is the prefix comparison function.
 175 If specified, this routine must return the number of bytes of the second key
 176 argument which are necessary to determine that it is greater than the first
 177 key argument.
 178 If the keys are equal, the key length should be returned.
 179 Note, the usefulness of this routine is very data dependent, but, in some
 180 data sets can produce significantly reduced tree sizes and search times.
 181 If
 182 .Va prefix
 183 is
 184 .Dv NULL
 185 (no prefix function is specified),
 186 .Em and
 187 no comparison function is specified, a default lexical comparison routine
 188 is used.
 189 If
 190 .Va prefix
 191 is
 192 .Dv NULL
 193 and a comparison routine is specified, no prefix comparison is
 194 done.
 195 .It Va lorder
 196 The byte order for integers in the stored database metadata.
 197 The number should represent the order as an integer; for example,
 198 big endian order would be the number 4,321.
 199 If
 200 .Va lorder
 201 is 0 (no order is specified) the current host order is used.
 202 .El
 203 .Pp
 204 If the file already exists (and the
 205 .Dv O_TRUNC
 206 flag is not specified), the
 207 values specified for the
 208 .Va flags , lorder
 209 and
 210 .Va psize
 211 arguments
 212 are ignored
 213 in favor of the values used when the tree was created.
 214 .Pp
 215 Forward sequential scans of a tree are from the least key to the greatest.
 216 .Pp
 217 Space freed up by deleting key/data pairs from the tree is never reclaimed,
 218 although it is normally made available for reuse.
 219 This means that the
 220 .Nm
 221 storage structure is grow-only.
 222 The only solutions are to avoid excessive deletions, or to create a fresh
 223 tree periodically from a scan of an existing one.
 224 .Pp
 225 Searches, insertions, and deletions in a
 226 .Nm
 227 will all complete in
 228 O lg base N where base is the average fill factor.
 229 Often, inserting ordered data into
 230 .Nm Ns s
 231 results in a low fill factor.
 232 This implementation has been modified to make ordered insertion the best
 233 case, resulting in a much better than normal page fill factor.
 234 .Sh ERRORS
 235 The
 236 .Nm
 237 access method routines may fail and set
 238 .Va errno
 239 for any of the errors specified for the library routine
 240 .Xr dbopen 3 .
 241 .Sh SEE ALSO
 242 .Xr dbopen 3 ,
 243 .Xr hash 3 ,
 244 .Xr mpool 3 ,
 245 .Xr recno 3
 246 .Rs
 247 .%T "The Ubiquitous B-tree"
 248 .%A Douglas Comer
 249 .%J "ACM Comput. Surv. 11"
 250 .%N 2
 251 .%D June 1979
 252 .%P 121-138
 253 .Re
 254 .Rs
 255 .%A Bayer
 256 .%A Unterauer
 257 .%T "Prefix B-trees"
 258 .%J "ACM Transactions on Database Systems"
 259 .%N 1
 260 .%V Vol. 2
 261 .%D March 1977
 262 .%P 11-26
 263 .Re
 264 .Rs
 265 .%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
 266 .%A D. E. Knuth
 267 .%D 1968
 268 .%P 471-480
 269 .Re
 270 .Sh BUGS
 271 Only big and little endian byte order is supported.