NAME
btree - btree database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
The routine dbopen(3) 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(3)
is defined in the <db.h> include file as follows:
typedef struct {
unsigned long flags;
unsigned int cachesize;
int maxkeypage;
int minkeypage;
unsigned int psize;
int (*compare)(const DBT *key1, const DBT *key2);
size_t (*prefix)(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, that is, 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, that is, 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 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 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 arguments flags, lorder and psize 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).
BUGS
Only big and little endian byte order is supported.
SEE ALSO
dbopen(3), hash(3), mpool(3), recno(3)
The Ubiquitous B-tree, Douglas Comer, ACM Comput. Surv. 11, 2 (June
1979), 121-138.
Prefix B-trees, Bayer and Unterauer, ACM Transactions on Database
Systems, Vol. 2, 1 (March 1977), 11-26.
The Art of Computer Programming Vol. 3: Sorting and Searching, D.E.
Knuth, 1968, pp 471-480.
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
1994-08-18