#include <db_cxx.h>

int
Db::key_range(DbTxn *txnid
    Dbt *key, DB_KEY_RANGE *key_range, u_int32_t flags);

The Db::key_range method returns an estimate of the proportion of keys that are less than, equal to, and greater than the specified key. The underlying database must be of type Btree.

The Db::key_range method fills in a structure of type DB_KEY_RANGE. The following data fields are available from the DB_KEY_RANGE structure:

• A value between 0 and 1, the proportion of keys less than the specified key.

• A value between 0 and 1, the proportion of keys equal to the specified key.

• A value between 0 and 1, the proportion of keys greater than the specified key.

Values are in the range of 0 to 1; for example, if the field less is 0.05, 5% of the keys in the database are less than the key parameter. The value for equal will be zero if there is no matching key, and will be non-zero otherwise.

The Db::key_range method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

• The key Dbt operated on.

• The estimates are returned in the key_range parameter, which contains three elements of type double: less, equal, and greater. Values are in the range of 0 to 1; for example, if the field less is 0.05, 5% of the keys in the database are less than the key parameter. The value for equal will be zero if there is no matching key, and will be non-zero otherwise.

• If the operation is part of an application-specified transaction, the txnid parameter is a transaction handle returned from DbEnv::txn_begin; if the operation is part of a Berkeley DB Concurrent Data Store group, the txnid parameter is a handle returned from DbEnv::cdsgroup_begin; otherwise NULL. If no transaction handle is specified, but the operation occurs in a transactional database, the operation will be implicitly transaction protected. The Db::key_range method does not retain the locks it acquires for the life of the transaction, so estimates may not be repeatable.

• The flags parameter is currently unused, and must be set to 0.

The Db::key_range method may fail and throw DbException, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

• The database handle has been invalidated because a replication election unrolled a committed transaction.

• The operation was blocked by client/master synchronization.

• If the underlying database was not of type Btree; or if an invalid flag value or parameter was specified.

If a transactional database environment operation was selected to resolve a deadlock, the Db::key_range method will fail and either return DB_LOCK_DEADLOCK or throw a DbDeadlockException exception.

If a Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time, the Db::key_range method will fail and either return DB_LOCK_NOTGRANTED or throw a DbLockNotGrantedException exception.