BTREE(3) BSD Programmer's Manual BTREE(3)NAMEbtree - btree database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
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 as-
sociated 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 {
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, 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 dupli-
cate keys into the tree will fail.
If the database contains duplicate keys, the order of re-
trieval of key/data pairs is undefined if the get() rou-
tine 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 substan-
tially improves access time. In addition, physical writes are de-
layed 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 argu-
ment is considered to be respectively less than, equal to, or
greater than the second key argument. The same comparison func-
tion 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 rou-
tine 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 it can produce significantly re-
duced 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 re-
claimed, 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 or-
dered 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 ALSOdbopen(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.
BUGS
Only big and little endian byte order is supported.
MirOS BSD #10-current August 18, 1994 2
