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 struc-
ture storing associated key/data pairs.
The btree access method specific data structure provided
to dbopen is defined in the <db.h> include file as fol-
lows:
typedef struct {
u_long flags;
u_int cachesize;
index_t psize;
int lorder;
int minkeypage;
int (*compare)(const DBT *key1, const DBT *key2);
int (*prefix)(const DBT *key1, const DBT *key2);
} 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. per-
mit 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, how-
ever, seq routine calls with the R_CURSOR
flag set will always return the logical
``first'' of any group of duplicate keys.
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BTREE(3) BTREE(3)
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 delayed as long as possible, so a moder-
ate 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.
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.
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.
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.
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 func-
tion is specified), the keys are compared lexi-
cally, with shorter keys considered less than
longer keys.
prefix Prefix is the prefix comparison function. If spec-
ified, 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
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BTREE(3) BTREE(3)
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 rou-
tine is specified, no prefix comparison is done.
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 modi-
fied to make ordered insertion the best case, resulting in
a much better than normal page fill factor.
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.
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