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 { 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:
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.
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.
The maximum number of keys which will be stored on any single page. Not currently implemented.
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.
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 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 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.
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.
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