BTREE(3)                NetBSD 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 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 unde-
                               fined 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 sin-
                  gle page.  Not currently implemented.

     minkeypage   The minimum number of keys which will be stored on any sin-
                  gle 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
                  comparison routine is used.  If prefix is NULL and a compar-
                  ison 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 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 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).

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.

NetBSD 5.1                      April 17, 2003                      NetBSD 5.1

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