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NAME    [Toc]    [Back]

     btree -- btree database access method

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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;

     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.

	     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, 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

	     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  The prefix element 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 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 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    [Toc]    [Back]

     The btree access method routines may fail and set errno for any of the
     errors specified for the library routine dbopen(3).

SEE ALSO    [Toc]    [Back]

     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    [Toc]    [Back]

     Only big and little endian byte order is supported.

FreeBSD 5.2.1			August 18, 1994 		 FreeBSD 5.2.1
[ Back ]
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