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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 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:
flags
The flags value is specified by ORing any of the following values:
R_DUP
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.
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. 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.
maxkeypage
The maximum number of keys which will be stored on any single page. Not
currently implemented.
minkeypage
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.
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 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.
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.
RESTRICTIONS
Only big and little endian byte order is supported.
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
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
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