Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
BTREE(3)	       FreeBSD Library Functions Manual		      BTREE(3)

NAME
     btree -- btree database access method

SYNOPSIS
     #include <sys/types.h>
     #include <db.h>

DESCRIPTION
     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	val-
	     ues:

	     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 rou-
		     tine 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 substan-
	     tially 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	inte-
	     ger 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	func-
	     tion 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  The prefix	element	is the prefix comparison function.  If speci-
	     fied, 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	sig-
	     nificantly	reduced	tree sizes and search times.  If prefix	is
	     NULL (no prefix function is specified), and no comparison func-
	     tion is specified,	a default lexical comparison routine is	used.
	     If	prefix is NULL and a comparison	routine	is specified, no pre-
	     fix 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 flags, lorder and	psize arguments	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 great-
     est.

     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, 2, 121-138,
     June 1979.

     Bayer and Unterauer, "Prefix B-trees", ACM	Transactions on	Database
     Systems, 1, Vol. 2, 11-26,	March 1977.

     D.	E. Knuth, The Art of Computer Programming Vol. 3: Sorting and
     Searching,	471-480, 1968.

BUGS
     Only big and little endian	byte order is supported.

FreeBSD	9.2			August 18, 1994			   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | BUGS

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=btree&sektion=3&manpath=FreeBSD+7.2-RELEASE>

home | help