BTREE(3) UNIX System V (August 18, 1994) 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);
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.
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)
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.
BUGS
Only big and little endian byte order is supported.
This version of berkeley db (1.85) is free software which is
not developed nor maintained by SGI. It is known to have
some bugs that are unlikely to get fixed (See NOTES below)
in particular, the following btree operations are known to
have problems, up to corrupting databases, and should be
avoided according to http://www.sleepycat.com/db.185.html:
o Btree cursor (seq) operations.
o Large numbers of btree duplicates (specifically,
migrating duplicate keys into internal pages).
o Large numbers of btree deletes (you should periodically
dump and rebuild the database if you delete large
numbers of records).
NOTES
This version of berkeley db is 1.85. A newer enhanced
version db-2.x requires licensing. Check out
http://www.sleepycat.com/ for details.
Page 4 (printed 4/30/98)