xref: /dragonfly/lib/libc/db/man/btree.3 (revision c3762235) 
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28.\"	@(#)btree.3	8.4 (Berkeley) 8/18/94
29.\" $FreeBSD: head/lib/libc/db/man/btree.3 165903 2007-01-09 00:28:16Z imp $
30.\"
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 LIBRARY
38.Lb libc
39.Sh SYNOPSIS
40.In sys/types.h
41.In db.h
42.Sh DESCRIPTION
43The routine
44.Fn dbopen
45is the library interface to database files.
46One of the supported file formats is
47.Nm
48files.
49The general description of the database access methods is in
50.Xr dbopen 3 ,
51this manual page describes only the
52.Nm
53specific information.
54.Pp
55The
56.Nm
57data structure is a sorted, balanced tree structure storing
58associated key/data pairs.
59.Pp
60The
61.Nm
62access method specific data structure provided to
63.Fn dbopen
64is defined in the
65.In db.h
66include file as follows:
67.Bd -literal
68typedef struct {
69	unsigned long flags;
70	unsigned int cachesize;
71	int maxkeypage;
72	int minkeypage;
73	unsigned int psize;
74	int (*compare)(const DBT *key1, const DBT *key2);
75	size_t (*prefix)(const DBT *key1, const DBT *key2);
76	int lorder;
77} BTREEINFO;
78.Ed
79.Pp
80The elements of this structure are as follows:
81.Bl -tag -width indent
82.It Va flags
83The flag value is specified by
84.Em or Ns 'ing
85any of the following values:
86.Bl -tag -width indent
87.It Dv R_DUP
88Permit duplicate keys in the tree, i.e., permit insertion if the key to be
89inserted already exists in the tree.
90The default behavior, as described in
91.Xr dbopen 3 ,
92is to overwrite a matching key when inserting a new key or to fail if
93the
94.Dv R_NOOVERWRITE
95flag is specified.
96The
97.Dv R_DUP
98flag is overridden by the
99.Dv R_NOOVERWRITE
100flag, and if the
101.Dv R_NOOVERWRITE
102flag is specified, attempts to insert duplicate keys into
103the tree will fail.
104.Pp
105If the database contains duplicate keys, the order of retrieval of
106key/data pairs is undefined if the
107.Va get
108routine is used, however,
109.Va seq
110routine calls with the
111.Dv R_CURSOR
112flag set will always return the logical
113.Dq first
114of any group of duplicate keys.
115.El
116.It Va cachesize
117A suggested maximum size (in bytes) of the memory cache.
118This value is
119.Em only
120advisory, and the access method will allocate more memory rather than fail.
121Since every search examines the root page of the tree, caching the most
122recently used pages substantially improves access time.
123In addition, physical writes are delayed as long as possible, so a moderate
124cache can reduce the number of I/O operations significantly.
125Obviously, using a cache increases (but only increases) the likelihood of
126corruption or lost data if the system crashes while a tree is being modified.
127If
128.Va cachesize
129is 0 (no size is specified) a default cache is used.
130.It Va maxkeypage
131The maximum number of keys which will be stored on any single page.
132Not currently implemented.
133.\" The maximum number of keys which will be stored on any single page.
134.\" Because of the way the
135.\" .Nm
136.\" data structure works,
137.\" .Va maxkeypage
138.\" must always be greater than or equal to 2.
139.\" If
140.\" .Va maxkeypage
141.\" is 0 (no maximum number of keys is specified) the page fill factor is
142.\" made as large as possible (which is almost invariably what is wanted).
143.It Va minkeypage
144The minimum number of keys which will be stored on any single page.
145This value is used to determine which keys will be stored on overflow
146pages, i.e., if a key or data item is longer than the pagesize divided
147by the minkeypage value, it will be stored on overflow pages instead
148of in the page itself.
149If
150.Va minkeypage
151is 0 (no minimum number of keys is specified) a value of 2 is used.
152.It Va psize
153Page size is the size (in bytes) of the pages used for nodes in the tree.
154The minimum page size is 512 bytes and the maximum page size is 64K.
155If
156.Va psize
157is 0 (no page size is specified) a page size is chosen based on the
158underlying file system I/O block size.
159.It Va compare
160Compare is the key comparison function.
161It must return an integer less than, equal to, or greater than zero if the
162first key argument is considered to be respectively less than, equal to,
163or greater than the second key argument.
164The same comparison function must be used on a given tree every time it
165is opened.
166If
167.Va compare
168is
169.Dv NULL
170(no comparison function is specified), the keys are compared
171lexically, with shorter keys considered less than longer keys.
172.It Va prefix
173The
174.Va prefix
175element
176is the prefix comparison function.
177If specified, this routine must return the number of bytes of the second key
178argument which are necessary to determine that it is greater than the first
179key argument.
180If the keys are equal, the key length should be returned.
181Note, the usefulness of this routine is very data dependent, but, in some
182data sets can produce significantly reduced tree sizes and search times.
183If
184.Va prefix
185is
186.Dv NULL
187(no prefix function is specified),
188.Em and
189no comparison function is specified, a default lexical comparison routine
190is used.
191If
192.Va prefix
193is
194.Dv NULL
195and a comparison routine is specified, no prefix comparison is
196done.
197.It Va lorder
198The byte order for integers in the stored database metadata.
199The number should represent the order as an integer; for example,
200big endian order would be the number 4,321.
201If
202.Va lorder
203is 0 (no order is specified) the current host order is used.
204.El
205.Pp
206If the file already exists (and the
207.Dv O_TRUNC
208flag is not specified), the
209values specified for the
210.Va flags , lorder
211and
212.Va psize
213arguments
214are ignored
215in favor of the values used when the tree was created.
216.Pp
217Forward sequential scans of a tree are from the least key to the greatest.
218.Pp
219Space freed up by deleting key/data pairs from the tree is never reclaimed,
220although it is normally made available for reuse.
221This means that the
222.Nm
223storage structure is grow-only.
224The only solutions are to avoid excessive deletions, or to create a fresh
225tree periodically from a scan of an existing one.
226.Pp
227Searches, insertions, and deletions in a
228.Nm
229will all complete in
230O lg base N where base is the average fill factor.
231Often, inserting ordered data into
232.Nm Ns s
233results in a low fill factor.
234This implementation has been modified to make ordered insertion the best
235case, resulting in a much better than normal page fill factor.
236.Sh ERRORS
237The
238.Nm
239access method routines may fail and set
240.Va errno
241for any of the errors specified for the library routine
242.Xr dbopen 3 .
243.Sh SEE ALSO
244.Xr dbopen 3 ,
245.Xr hash 3 ,
246.Xr mpool 3 ,
247.Xr recno 3
248.Rs
249.%T "The Ubiquitous B-tree"
250.%A Douglas Comer
251.%J "ACM Comput. Surv. 11"
252.%N 2
253.%D June 1979
254.%P 121-138
255.Re
256.Rs
257.%A Bayer
258.%A Unterauer
259.%T "Prefix B-trees"
260.%J "ACM Transactions on Database Systems"
261.%N 1
262.%V Vol. 2
263.%D March 1977
264.%P 11-26
265.Re
266.Rs
267.%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
268.%A D. E. Knuth
269.%D 1968
270.%P 471-480
271.Re
272.Sh BUGS
273Only big and little endian byte order is supported.
274