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BTREE(3)										 BTREE(3)

       btree - btree database access method

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

       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

       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 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 advi-
	      sory, 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 likeli-
	      hood  of	corruption or lost data if the system crashes while a tree is being modi-
	      fied.  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

	      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 min-
	      imum 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 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 compari-
	      son 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 num-
	      ber 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

       Only big and little endian byte order is supported.

					    1994-08-18					 BTREE(3)
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