BTREE(3) manual page
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btree - btree database access method
#include <sys/types.h>
#include <db.h>
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 64K. 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.
The btree access method routines may fail and set
errno for any of the errors specified for the library routine dbopen(3)
.
Only big and little endian byte order is supported.
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.
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