btree - btree database access method


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


       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 struc­
       ture storing associated key/data pairs.

       The btree access method specific data  structure  provided
       to  dbopen  is  defined in the <db.h> include file as fol­

       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. per­
                     mit  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

                     If the database contains duplicate keys, the
                     order  of  retrieval  of  key/data  pairs is
                     undefined if the get routine is  used,  how­
                     ever,  seq  routine  calls with the R_CURSOR

              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 delayed as long as possible, so a moder­
              ate 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,  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 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 func­
              tion  is  specified),  the  keys are compared lexi­
              cally,  with  shorter  keys  considered  less  than
              longer keys.

       prefix Prefix is the prefix comparison function.  If spec­
              ified, this routine must return the number of bytes
              of  the  second key argument which are necessary to
              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  rou­
              tine 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 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 modi­
       fied 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), 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
       Only big and little endian byte order is supported.