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32.\" @(#)btree.3 8.4 (Berkeley) 8/18/94
33.\" $FreeBSD: src/lib/libc/db/man/btree.3,v 1.3.2.3 2003/03/15 15:11:05 trhodes Exp $
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.Aq Pa 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.