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NAME
btree - btree database access method
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
Note well: This page documents interfaces provided up until glibc 2.1. Since glibc 2.2,
glibc no longer provides these interfaces. 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 values:
R_DUP Permit duplicate keys in the tree, that is, 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, 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 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 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 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 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 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 Systems, Vol. 2, 1
(March 1977), 11-26.
The Art of Computer Programming Vol. 3: Sorting and Searching, D.E. Knuth, 1968, pp
471-480.