|
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>
Note well: This page documents interfaces provided in glibc up until
version 2.1. Since version 2.2, glibc no longer provides these
interfaces. Probably, you are looking for the APIs provided by the
libdb library instead.
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 ORing 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
filesystem 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 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2012-04-23 BTREE(3)
Pages that refer to this page: dbopen(3), hash(3), mpool(3), recno(3)