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BTREE(3)		   Linux Programmers Manual		     BTREE(3)

       btree - btree database access method


       The  routine dbopen(3) 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(3)
       is defined in the  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 oring any of the following  val

	      R_DUP  Permit duplicate keys in the tree, that is, permit inser
		     tion 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_CUR
		     SOR  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 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 corrup
	      tion 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, that is, 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

       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 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
	      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 argu
	      ment 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 signifi
	      cantly 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 exam
	      ple, 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 arguments 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 great

       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  solu
       tions are to avoid excessive deletions, or to create a fresh tree peri
       odically 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  implemen
       tation  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).

       Only big and little endian byte order is supported.

       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  Sys
       tems, Vol. 2, 1 (March 1977), 11-26.

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

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

				  1994-08-18			      BTREE(3)

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