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NAME

       btree - btree database access method

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 ORing any of the following values:

              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.

       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 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.

       maxkeypage
              The maximum number of keys which will be stored on any single page.  Not  currently
              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, 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 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 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

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

BUGS

       Only big and little endian byte order is supported.

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  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.

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.

                                            1994-08-18                                   BTREE(3)