Originální popis anglicky:
btree - btree database access method
Návod, kniha: Library Functions Manual
#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 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 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 flag value is specified by or'ing any of the
following values:
- R_DUP
- Permit duplicate keys in the tree, i.e. 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, i.e. 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 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.
The
btree access method routines may fail and set
errno for any of
the errors specified for the library routine
dbopen(3).
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.
Only big and little endian byte order is supported.
