метод доступа к базе данных btree (btree database access method)
Имя (Name)
btree - btree database access method
Синопсис (Synopsis)
#include <sys/types.h>
#include <db.h>
Описание (Description)
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 64 KiB. 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.
Ошибки (Error)
The btree access method routines may fail and set errno for any
of the errors specified for the library routine dbopen(3).
Ошибки (баги) (Bugs)
Only big and little endian byte order is supported.
Смотри также (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.
