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@(#)btree.3 8.4 (Berkeley) 8/18/94
.Dd April 17, 2003 .Dt BTREE 3 .Os .Sh NAME .Nm btree .Nd btree database access method .Sh SYNOPSIS n sys/types.h n db.h .Sh DESCRIPTION The routine .Fn dbopen is the library interface to database files. One of the supported file formats is btree files. The general description of the database access methods is in .Xr dbopen 3 , this manual page describes only the btree specific information.
p The btree data structure is a sorted, balanced tree structure storing associated key/data pairs.
p The btree access method specific data structure provided to .Fn dbopen is defined in the n db.h include file as follows: d -literal typedef struct { u_long flags; u_int cachesize; int maxkeypage; int minkeypage; u_int psize; int (*compare)(const DBT *key1, const DBT *key2); size_t (*prefix)(const DBT *key1, const DBT *key2); int lorder; } BTREEINFO; .Ed
p The elements of this structure are as follows: l -tag -width maxkeypagex t Fa flags The flag value is specified by or'ing any of the following values: l -tag -width R_DUP -offset indent t Dv R_DUP Permit duplicate keys in the tree, i.e. permit insertion if the key to be inserted already exists in the tree. The default behavior, as described in .Xr dbopen 3 , is to overwrite a matching key when inserting a new key or to fail if the .Dv R_NOOVERWRITE flag is specified. The .Dv R_DUP flag is overridden by the .Dv R_NOOVERWRITE flag, and if the .Dv R_NOOVERWRITE flag is specified, attempts to insert duplicate keys into the tree will fail.
p
If the database contains duplicate keys, the order of retrieval of
key/data pairs is undefined if the
.Em get
routine is used, however,
.Em seq
routine calls with the
.Dv R_CURSOR
flag set will always return the logical
.Dq first
of any group of duplicate keys.
.El
t Fa cachesize A suggested maximum size (in bytes) of the memory cache.
This value is
.Em only
advisory, and the access method will allocate more memory rather than
fail.
Since every search examines the root page of the tree, caching the
most recently used pages substantially improves access time.
In addition, physical writes are delayed as long as possible, so a
moderate cache can reduce the number of I/O operations significantly.
Obviously, using a cache increases (but only increases) the likelihood
of corruption or lost data if the system crashes while a tree is being
modified.
If
.Fa cachesize
is 0 (no size is specified) a default cache is used.
t Fa maxkeypage The maximum number of keys which will be stored on any single page.
Not currently implemented.
The maximum number of keys which will be stored on any single page.
Because of the way the btree data structure works,
.Fa maxkeypage
must always be greater than or equal to 2.
If
.Fa maxkeypage
is 0 (no maximum number of keys is specified) the page fill factor is
made as large as possible (which is almost invariably what is wanted).
t Fa minkeypage The minimum number of keys which will be stored on any single page.
This value is used to determine which keys will be stored on overflow
pages, i.e., if a key or data item is longer than the pagesize divided
by the
.Fa minkeypage
value, it will be stored on overflow pages instead of in the page
itself.
If
.Fa minkeypage
is 0 (no minimum number of keys is specified) a value of 2 is used.
t Fa psize Page size is the size (in bytes) of the pages used for nodes in the
tree.
The minimum page size is 512 bytes and the maximum page size is 64K.
If
.Fa psize
is 0 (no page size is specified) a page size is chosen based on the
underlying file system I/O block size.
t Fa compare Compare is the key comparison function.
It must return an integer less than, equal to, or greater than zero if
the first key argument is considered to be respectively less than,
equal to, or greater than the second key argument.
The same comparison function must be used on a given tree every time
it is opened.
If
.Fa compare
is
.Dv NULL
(no comparison function is specified), the keys are compared
lexically, with shorter keys considered less than longer keys.
t Fa prefix Prefix is the prefix comparison function.
If specified, this routine must return the number of bytes of the
second key argument which are necessary to determine that it is
greater than the first key argument.
If the keys are equal, the key length should be returned.
Note, the usefulness of this routine is very data dependent, but, in
some data sets can produce significantly reduced tree sizes and search
times.
If
.Fa prefix
is
.Dv NULL
(no prefix function is specified),
.Em and
no comparison function is specified, a default lexical comparison
routine is used.
If
.Fa prefix
is
.Dv NULL
and a comparison routine is specified, no prefix comparison is done.
t Fa lorder The byte order for integers in the stored database metadata.
The number should represent the order as an integer; for example,
big endian order would be the number 4,321.
If
.Fa lorder
is 0 (no order is specified) the current host order is used.
.El
p If the file already exists (and the .Dv O_TRUNC flag is not specified), the values specified for the parameters flags, lorder and psize are ignored in favor of the values used when the tree was created.
p Forward sequential scans of a tree are from the least key to the greatest.
p Space freed up by deleting key/data pairs from the tree is never reclaimed, although it is normally made available for reuse. This means that the btree storage structure is grow-only. The only solutions are to avoid excessive deletions, or to create a fresh tree periodically from a scan of an existing one.
p Searches, insertions, and deletions in a btree will all complete in O lg base N where base is the average fill factor. Often, inserting ordered data into btrees results in a low fill factor. This implementation has been modified to make ordered insertion the best case, resulting in a much better than normal page fill factor. .Sh ERRORS The .Nm access method routines may fail and set .Va errno for any of the errors specified for the library routine .Xr dbopen 3 . .Sh SEE ALSO .Xr dbopen 3 , .Xr hash 3 , .Xr mpool 3 , .Xr recno 3 .Rs .%T "The Ubiquitous B-tree" .%A "Douglas Comer" .%J "ACM Comput. Surv." .%V 2 .%N 11 .%D June 1979 .%P 121-138 .Re .Rs .%T "Prefix B-trees" .%A "Bayer" .%A "Unterauer" .%J "ACM Transactions on Database Systems" .%V Vol. 2 .%N 1 .%D March 1977 .%P 11-26 .Re .Rs .%B "The Art of Computer Programming Vol. 3: Sorting and Searching" .%A "D.E. Knuth" .%D 1968 .%P 471-480 .Re .Sh BUGS Only big and little endian byte order is supported.
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