btree database access method
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
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
The btree data
structure is a sorted, balanced tree structure storing
associated key/data pairs.
access-method-specific data structure provided to
dbopen(3) is defined in the <db.h>
include file as follows:
unsigned long flags;
unsigned int cachesize;
unsigned int psize;
int (*compare)(const DBT *key1, const DBT *key2);
size_t (*prefix)(const DBT *key1, const DBT *key2);
The elements of
this structure are as follows:
The flag value is specified by ORing any of the
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.
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.
The maximum number of keys
which will be stored on any single page. Not currently
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.
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 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 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.
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.
sequential scans of a tree are from the least key to the
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.
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.
btree access method routines may fail and set
errno for any of the errors specified for the library
Only big and
little endian byte order is supported.
hash(3), mpool(3), recno(3)
Ubiquitous B-tree, Douglas Comer, ACM Comput. Surv. 11,
2 (June 1979), 121-138.
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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