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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;
       } 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 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 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 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 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)

       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 471-480.

BUGS    [Toc]    [Back]

       Only big and little endian byte order is supported.

				  1994-08-18			      BTREE(3)
[ Back ]
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