btree
BTREE(3) 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 val-
ues:
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 rou-
tine is used, however, seq routine calls with the R_CUR-
SOR 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 compari-
son function must be used on a given tree every time it is
opened. If compare is NULL (no comparison function is speci-
fied), the keys are compared lexically, with shorter keys con-
sidered 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 argu-
ment 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 signif-
icantly 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 exam-
ple, 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 solu-
tions 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 implemen-
tation 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 Sys-
tems, 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.
1994-08-18 BTREE(3)
