btree - btree database access method


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


   Note  well:  This  page documents interfaces provided in glibc up until
   version 2.1.   Since  version  2.2,  glibc  no  longer  provides  these
   interfaces.   Probably,  you  are  looking for the APIs provided by the
   libdb library instead.

   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  ORing  any of the following

          R_DUP  Permit duplicate  keys  in  the  tree,  that  is,  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, 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 filesystem I/O block

          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 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

   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).


   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
   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.


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

                              2012-04-23                          BTREE(3)


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