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BTREE(3)			   BSD Library Functions Manual 			 BTREE(3)

NAME
     btree -- btree database access method

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

DESCRIPTION
     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 behav-
			       ior, 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.

     cachesize	  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 num-
		  ber 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.

     maxkeypage   The maximum number of keys which will be stored on any single page.  Not cur-
		  rently implemented.

     minkeypage   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	  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 compari-
		  son 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 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 implementation has been modified to make ordered insertion the best case,
     resulting in a much better than normal page fill factor.

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

SEE ALSO
     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
     Only big and little endian byte order is supported.

BSD					  April 17, 2003				      BSD
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