DB_BTREE(3)

NAME
db_btree - btree database access method

DESCRIPTION
specific details of the btree access method.

The btree data structure is a sorted, balanced tree structure storing associated key/data pairs. Searches, insertions, and deletions in the 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.

ACCESS METHOD SPECIFIC INFORMATION
The btree access method specific data structure provided to db_open is typedef'd and named BTREEINFO. A BTREEINFO structure has at least the following fields, which may be initialized before calling db_open:

u_int 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.

int (*compare)(const DBT *, const DBT *);
    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.

u_long 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 db_open(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 function is used, however, seq function calls with the R_CURSOR flag set will always return the logical ``first'' of any group of duplicate keys.

int 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.

int maxkeypage;
    The maximum number of keys which will be stored on any single page. This functionality is not currently implemented.

int 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.

size_t (*prefix)(const DBT *, const DBT *);
    Prefix is the prefix comparison function. If specified, this function 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 function 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 function is used. If prefix is NULL and a comparison function is specified, no prefix comparison is done.

u_int 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.

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.

DB OPERATIONS
The functions returned by db_open for the btree access method are as described in db_open(3), with the following exceptions and additions:

type
    The type is DB_BTREE.

del Space freed up by deleting key/data pairs from the tree is never reclaimed, although it is reused where possible. 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.

put The put function takes the following additional flags:

    R_SETCURSOR
        Store the key/data pair, setting or initializing the position of the cursor to reference it.

seq Forward sequential scans of a tree are from the least key to the greatest.

    The returned key for the seq function is not necessarily an exact match for the specified key in the btree access method. The returned key is the smallest key greater than or equal to the specified key, permitting partial key matches and range searches.

    The seq function takes the following additional flags:

    R_LAST
        The last key/data pair of the database is returned, and the cursor is set or initialized to reference it.

    R_PREV
        Retrieve the key/data pair immediately before the cursor. If the cursor is not yet set, this is the same as the R_LAST flag.

ERRORS
The btree access method functions may fail and set errno for any of the errors specified for the library function db_open(3).

SEE ALSO
db_hash(3), db_lock(3), db_log(3), db_mpool(3), db_open(3), db_recno(3), db_txn(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.