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man:btree

BTREE(3) Linux Programmer's Manual BTREE(3)

NAME

     btree - btree database access method

SYNOPSIS

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

DESCRIPTION

     Note  well:  This  page documents interfaces provided in glibc up until
     version 2.1.  Since version 2.2, glibc no longer provides these  inter-
     faces.   Probably,  you  are looking for the APIs provided by the libdb
     library instead.
     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 val-
            ues:
            R_DUP  Permit duplicate keys in the tree, that is, permit inser-
                   tion  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  rou-
                   tine  is used, however, seq routine calls with the R_CUR-
                   SOR 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  substan-
            tially  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  corrup-
            tion  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 64 KiB.  If psize is 0 (no page size is specified),
            a  page  size  is  chosen based on the underlying filesystem I/O
            block size.
     compare
            Compare is the key comparison function.  It must return an inte-
            ger  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  argu-
            ment  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 signifi-
            cantly  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  exam-
            ple, 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 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 solu-
     tions are to avoid excessive deletions, or to create a fresh tree peri-
     odically 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 implemen-
     tation 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 Sys-
     tems, 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 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.
                                2017-09-15                          BTREE(3)
/data/webs/external/dokuwiki/data/pages/man/btree.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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