xref: /dragonfly/lib/libc/db/man/btree.3 (revision b40e316c) 
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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.\" $DragonFly: src/lib/libc/db/man/btree.3,v 1.2 2003/06/17 04:26:41 dillon Exp $
35.\"
36.Dd August 18, 1994
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
50.Nm
51files.
52The general description of the database access methods is in
53.Xr dbopen 3 ,
54this manual page describes only the
55.Nm
56specific information.
57.Pp
58The
59.Nm
60data structure is a sorted, balanced tree structure storing
61associated key/data pairs.
62.Pp
63The
64.Nm
65access method specific data structure provided to
66.Fn dbopen
67is defined in the
68.Aq Pa db.h
69include file as follows:
70.Bd -literal
71typedef struct {
72	u_long flags;
73	u_int cachesize;
74	int maxkeypage;
75	int minkeypage;
76	u_int psize;
77	int (*compare)(const DBT *key1, const DBT *key2);
78	size_t (*prefix)(const DBT *key1, const DBT *key2);
79	int lorder;
80} BTREEINFO;
81.Ed
82.Pp
83The elements of this structure are as follows:
84.Bl -tag -width indent
85.It Va flags
86The flag value is specified by
87.Em or Ns 'ing
88any of the following values:
89.Bl -tag -width indent
90.It Dv R_DUP
91Permit duplicate keys in the tree, i.e. permit insertion if the key to be
92inserted already exists in the tree.
93The default behavior, as described in
94.Xr dbopen 3 ,
95is to overwrite a matching key when inserting a new key or to fail if
96the
97.Dv R_NOOVERWRITE
98flag is specified.
99The
100.Dv R_DUP
101flag is overridden by the
102.Dv R_NOOVERWRITE
103flag, and if the
104.Dv R_NOOVERWRITE
105flag is specified, attempts to insert duplicate keys into
106the tree will fail.
107.Pp
108If the database contains duplicate keys, the order of retrieval of
109key/data pairs is undefined if the
110.Va get
111routine is used, however,
112.Va seq
113routine calls with the
114.Dv R_CURSOR
115flag set will always return the logical
116.Dq first
117of any group of duplicate keys.
118.El
119.It Va cachesize
120A suggested maximum size (in bytes) of the memory cache.
121This value is
122.Em only
123advisory, and the access method will allocate more memory rather than fail.
124Since every search examines the root page of the tree, caching the most
125recently used pages substantially improves access time.
126In addition, physical writes are delayed as long as possible, so a moderate
127cache can reduce the number of I/O operations significantly.
128Obviously, using a cache increases (but only increases) the likelihood of
129corruption or lost data if the system crashes while a tree is being modified.
130If
131.Va cachesize
132is 0 (no size is specified) a default cache is used.
133.It Va maxkeypage
134The maximum number of keys which will be stored on any single page.
135Not currently implemented.
136.\" The maximum number of keys which will be stored on any single page.
137.\" Because of the way the
138.\" .Nm
139.\" data structure works,
140.\" .Va maxkeypage
141.\" must always be greater than or equal to 2.
142.\" If
143.\" .Va maxkeypage
144.\" is 0 (no maximum number of keys is specified) the page fill factor is
145.\" made as large as possible (which is almost invariably what is wanted).
146.It Va minkeypage
147The minimum number of keys which will be stored on any single page.
148This value is used to determine which keys will be stored on overflow
149pages, i.e. if a key or data item is longer than the pagesize divided
150by the minkeypage value, it will be stored on overflow pages instead
151of in the page itself.
152If
153.Va minkeypage
154is 0 (no minimum number of keys is specified) a value of 2 is used.
155.It Va psize
156Page size is the size (in bytes) of the pages used for nodes in the tree.
157The minimum page size is 512 bytes and the maximum page size is 64K.
158If
159.Va psize
160is 0 (no page size is specified) a page size is chosen based on the
161underlying file system I/O block size.
162.It Va compare
163Compare is the key comparison function.
164It must return an integer less than, equal to, or greater than zero if the
165first key argument is considered to be respectively less than, equal to,
166or greater than the second key argument.
167The same comparison function must be used on a given tree every time it
168is opened.
169If
170.Va compare
171is
172.Dv NULL
173(no comparison function is specified), the keys are compared
174lexically, with shorter keys considered less than longer keys.
175.It Va prefix
176The
177.Va prefix
178element
179is the prefix comparison function.
180If specified, this routine must return the number of bytes of the second key
181argument which are necessary to determine that it is greater than the first
182key argument.
183If the keys are equal, the key length should be returned.
184Note, the usefulness of this routine is very data dependent, but, in some
185data sets can produce significantly reduced tree sizes and search times.
186If
187.Va prefix
188is
189.Dv NULL
190(no prefix function is specified),
191.Em and
192no comparison function is specified, a default lexical comparison routine
193is used.
194If
195.Va prefix
196is
197.Dv NULL
198and a comparison routine is specified, no prefix comparison is
199done.
200.It Va lorder
201The byte order for integers in the stored database metadata.
202The number should represent the order as an integer; for example,
203big endian order would be the number 4,321.
204If
205.Va lorder
206is 0 (no order is specified) the current host order is used.
207.El
208.Pp
209If the file already exists (and the
210.Dv O_TRUNC
211flag is not specified), the
212values specified for the
213.Va flags , lorder
214and
215.Va psize
216arguments
217are ignored
218in favor of the values used when the tree was created.
219.Pp
220Forward sequential scans of a tree are from the least key to the greatest.
221.Pp
222Space freed up by deleting key/data pairs from the tree is never reclaimed,
223although it is normally made available for reuse.
224This means that the
225.Nm
226storage structure is grow-only.
227The only solutions are to avoid excessive deletions, or to create a fresh
228tree periodically from a scan of an existing one.
229.Pp
230Searches, insertions, and deletions in a
231.Nm
232will all complete in
233O lg base N where base is the average fill factor.
234Often, inserting ordered data into
235.Nm Ns s
236results in a low fill factor.
237This implementation has been modified to make ordered insertion the best
238case, resulting in a much better than normal page fill factor.
239.Sh ERRORS
240The
241.Nm
242access method routines may fail and set
243.Va errno
244for any of the errors specified for the library routine
245.Xr dbopen 3 .
246.Sh SEE ALSO
247.Xr dbopen 3 ,
248.Xr hash 3 ,
249.Xr mpool 3 ,
250.Xr recno 3
251.Rs
252.%T "The Ubiquitous B-tree"
253.%A Douglas Comer
254.%J "ACM Comput. Surv. 11"
255.%N 2
256.%D June 1979
257.%P 121-138
258.Re
259.Rs
260.%A Bayer
261.%A Unterauer
262.%T "Prefix B-trees"
263.%J "ACM Transactions on Database Systems"
264.%N 1
265.%V Vol. 2
266.%D March 1977
267.%P 11-26
268.Re
269.Rs
270.%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
271.%A D. E. Knuth
272.%D 1968
273.%P 471-480
274.Re
275.Sh BUGS
276Only big and little endian byte order is supported.
277