BTREE(3) FreeBSD Library Functions Manual BTREE(3)
NAME
btree - btree database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
The routine 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 dbopen(3), this manual page describes
only the btree specific information.
The btree data structure is a sorted, balanced tree structure storing
associated key/data pairs.
The btree access method specific data structure provided to dbopen() is
defined in the <db.h> include file as follows:
typedef struct {
u_long flags;
u_int cachesize;
int maxkeypage;
int minkeypage;
u_int psize;
int (*compare)(const DBT *key1, const DBT *key2);)(const DBT *key1, const DBT *key2);
size_t (*prefix)(const DBT *key1, const DBT *key2);)(const DBT *key1, const DBT *key2);
int lorder;
} BTREEINFO;
The elements of this structure are as follows:
flags The flag value is specified by or'ing any of the following
values:
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 dbopen(3), is to
overwrite a matching key when inserting a new key or to
fail if the R_NOOVERWRITE flag is specified. The R_DUP
flag is overridden by the R_NOOVERWRITE flag, and if the
R_NOOVERWRITE flag is specified, attempts to insert
duplicate keys into the tree will fail.
If the database contains duplicate keys, the order of
retrieval of key/data pairs is undefined if the get
routine is used, however, seq routine calls with the
R_CURSOR flag set will always return the logical "first"
of any group of duplicate keys.
cachesize
A suggested maximum size (in bytes) of the memory cache. This
value is 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 cachesize is 0 (no size is specified) a default
cache is used.
maxkeypage
The maximum number of keys which will be stored on any single
page. Not currently implemented.
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 minkeypage value, it will be stored on
overflow pages instead of in the page itself. If minkeypage is 0
(no minimum number of keys is specified) a value of 2 is used.
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 psize is 0 (no page size is specified) a
page size is chosen based on the underlying file system I/O block
size.
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 compare is NULL (no comparison function is specified), the
keys are compared lexically, with shorter keys considered less
than longer keys.
prefix The prefix element 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
prefix is NULL (no prefix function is specified), and no
comparison function is specified, a default lexical comparison
routine is used. If prefix is NULL and a comparison routine is
specified, no prefix comparison is done.
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 lorder is 0 (no order
is specified) the current host order is used.
If the file already exists (and the O_TRUNC flag is not specified), the
values specified for the flags, lorder and psize arguments are ignored in
favor of the values used when the tree was created.
Forward sequential scans of a tree are from the least key to the
greatest.
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.
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.
ERRORS
The btree access method routines may fail and set errno for any of the
errors specified for the library routine dbopen(3).
SEE ALSO
dbopen(3), hash(3), mpool(3), recno(3)
Douglas Comer, "The Ubiquitous B-tree", ACM Comput. Surv. 11, 2, 121-138,
June 1979.
Bayer and Unterauer, "Prefix B-trees", ACM Transactions on Database
Systems, 1, Vol. 2, 11-26, March 1977.
D. E. Knuth, The Art of Computer Programming Vol. 3: Sorting and
Searching, 471-480, 1968.
BUGS
Only big and little endian byte order is supported.
FreeBSD 13.1-RELEASE-p6 August 18, 1994 FreeBSD 13.1-RELEASE-p6
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