btree - btree database access method
#include <sys/types.h>
#include <db.h>
The dbopen() routine 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);
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, 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 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 parameters 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.
The btree access method routines may fail and set errno for
any of the
errors specified for the library routine dbopen(3).
dbopen(3), hash(3), mpool(3), recno(3)
Douglas Comer, "The Ubiquitous B-tree", ACM Comput. Surv.
11, pp 121-138,
June 1979.
Rudolf Bayer and Karl Unterauer, "Prefix B-trees", ACM
Transactions on
Database Systems, Vol. 2, 1, pp 11-26, March 1977.
D. E. Knuth, The Art of Computer Programming Vol. 3: Sorting
and
Searching, pp 471-480, 1968.
Only big and little endian byte order is supported.
OpenBSD 3.6 August 18, 1994
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