NAME | SYNOPSIS | DESCRIPTION | ERRORS | BUGS | SEE ALSO | COLOPHON
BTREE(3) Linux Programmer's Manual BTREE(3)
btree - btree database access method
#include <sys/types.h>
#include <db.h>
The routine dbopen(3) 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(3) is
defined in the <db.h> include file as follows:
typedef struct {
unsigned long flags;
unsigned int cachesize;
int maxkeypage;
int minkeypage;
unsigned 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, that is, 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, 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 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 arguments 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.
The btree access method routines may fail and set errno for any of the errors
specified for the library routine dbopen(3).
Only big and little endian byte order is supported.
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.
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
1994-08-18 BTREE(3)