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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 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, 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  The prefix element 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 flags,
     lorder and psize arguments 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 solu-tions solutions
     tions are to avoid excessive deletions, or to create a fresh tree periodically from a scan of an exist-ing existing
     ing one.

     Searches, insertions, and deletions in a btree will all complete in O lg base N where base is the aver-age average
     age fill factor.  Often, inserting ordered data into btrees results in a low fill factor.  This imple-mentation implementation
     mentation 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                             August 18, 1994                            BSD