btree


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

DESCRIPTION
       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 <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;
           } BTREEINFO;

       The elements of this structure are as follows:

       flags  The flag value is specified by or'ing any of the following  val-
              ues:

              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.

       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 substan-
              tially improves access time.  In addition, physical  writes  are
              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
              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 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-
       est.

       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.


SEE ALSO
       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.

COLOPHON
       This page is part of release 3.23 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)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2014 Hurricane Electric. All Rights Reserved.