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BTREE(3)		  OpenBSD Programmer's Manual		      BTREE(3)

     btree - btree database access method

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

     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;

     The elements of this structure are as follows:

     flags   The flag value is specified by OR'ing any of the following

	     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  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 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

     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)

     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.

     Only big and little endian byte order is supported.

OpenBSD 4.9			 May 31, 2007			   OpenBSD 4.9

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