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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 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.
COLOPHON
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2020-12-21 BTREE(3)