NAME
btree
—
btree database access method
SYNOPSIS
#include
<sys/types.h>
#include <db.h>
DESCRIPTION
The
dbopen
()
routine 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 { 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, 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. TheR_DUP
flag is overridden by theR_NOOVERWRITE
flag, and if theR_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 theR_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 it 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 isNULL
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.
ERRORS
The btree
access method routines may fail
and set errno for any of the errors specified for the
library routine
dbopen(3).
SEE ALSO
dbopen(3), hash(3), mpool(3), recno(3)
Douglas Comer, The Ubiquitous B-tree, ACM Comput. Surv. 11, pp 121-138, June 1979.
Rudolf Bayer and Karl Unterauer, Prefix B-trees, ACM Transactions on Database Systems, Vol. 2, 1, pp 11-26, March 1977.
D. E. Knuth, The Art of Computer Programming Vol. 3: Sorting and Searching, pp 471-480, 1968.
BUGS
Only big and little endian byte order is supported.