btree - btree database access method
#include <sys/types.h> #include <db.h>
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 reference 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: The flags
value is specified by ORing any of the following values:
Permits duplicate keys in the tree; that is, it permits
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 get() routine is used and the database contains
duplicate keys, the order of retrieval of
key/data pairs is undefined; however, seq() routine
calls with the R_CURSOR flag set will always return
the logical ``first'' of any group of duplicate
keys. 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. Caching the most recently used pages substantially
improves access time because every search
examines the root page of the tree. In addition, a
moderate cache can reduce the number of I/O operations
significantly because physical writes are
delayed as long as possible. 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.
The maximum number of keys which will be stored on
any single page. Not currently implemented. The
minimum number of keys that will be stored on any
single page. This value is used to determine which
keys will be stored on overflow pages, that is, 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. 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
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 is the prefix comparison function. If specified,
this routine must return the number of bytes
of the second key argument that are necessary to
determine whether it is greater than the first key
argument. If the keys are equal, the key length
should be returned. Note that the usefulness of
this routine is very data dependent, but, in some
data sets, it 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.
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.
Only big and little endian byte order is supported.
The btree access method routines may fail and set errno
for any of the errors specified for the library routine
dbopen(3).
Functions: 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
btree(3)
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