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