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NAME    [Toc]    [Back]

     btree - btree database access method

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     The dbopen() routine 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
                   size_t (*prefix)(const DBT  *key1,  const  DBT
                   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
                    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, 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

             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

     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
     has been modified to make ordered insertion the  best  case,
resulting in a
     much better than normal page fill factor.

ERRORS    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

     dbopen(3), hash(3), mpool(3), recno(3)

     Douglas Comer, "The Ubiquitous B-tree",  ACM  Comput.  Surv.
11, pp 121-138,
     June 1979.

     Rudolf  Bayer  and  Karl  Unterauer,  "Prefix  B-trees", ACM
Transactions on
     Database Systems, Vol. 2, 1, pp 11-26, March 1977.

     D. E. Knuth, The Art of Computer Programming Vol. 3: Sorting
     Searching, pp 471-480, 1968.

BUGS    [Toc]    [Back]

     Only big and little endian byte order is supported.

OpenBSD      3.6                          August     18,     1994
[ Back ]
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