NAME
btree —
btree database access
method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
The routine
dbopen() 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 *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 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 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, 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 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 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 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).
SEE ALSO
dbopen(3),
hash(3),
mpool(3),
recno(3)
Douglas Comer, The
Ubiquitous B-tree, ACM Comput. Surv.,
11, 2,
121-138, June 1979.
Bayer and
Unterauer, Prefix B-trees,
ACM Transactions on Database Systems,
1, Vol. 2,
11-26, March 1977.
D.E. Knuth, The Art of
Computer Programming Vol. 3: Sorting and Searching,
471-480, 1968.
BUGS
Only big and little endian byte order is supported.