#include <db_cxx.h> int Db::get(DbTxn *txnid, Dbt *key, Dbt *data, u_int32_t flags); int Db::pget(DbTxn *txnid, Dbt *key, Dbt *pkey, Dbt *data, u_int32_t flags);
The Db::get() method retrieves key/data pairs from the database. The
address and length of the data associated with the specified key are returned in the structure to which
data refers.
In the presence of duplicate key values,
Db::get() will return the first data
item for the designated key. Duplicates are sorted by:
Their sort order, if a duplicate sort function was specified.
Any explicit cursor designated insertion.
By insert order. This is the default behavior.
Retrieval of duplicates requires the use of cursor operations. See Dbc::get() for details.
When called on a database that has been made into a secondary index
using the Db::associate()
method, the Db::get() and Db::pget()
methods return the key from the secondary
index and the data item from the primary database. In addition, the
Db::pget() method returns the key from the primary database. In
databases that are not secondary indices, the Db::pget() method will
always fail.
The Db::get() method will return DB_NOTFOUND if
the specified key is not in the database. The Db::get() method will
return DB_KEYEMPTY if
the database is a Queue or Recno database and the specified key
exists, but was never explicitly created by the application or was
later deleted. Unless otherwise specified, the
Db::get()
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 data Dbt operated on.
The flags parameter must be set to 0 or one of the following values:
Return the record number and data from the available record closest to the head of the queue, and delete the record. The record number will be returned in key, as described in Dbt. The data will be returned in the data parameter. A record is available if it is not deleted and is not currently locked. The underlying database must be of type Queue for DB_CONSUME to be specified.
The DB_CONSUME_WAIT flag is the same as the DB_CONSUME flag, except that if the Queue database is empty, the thread of control will wait until there is data in the queue before returning. The underlying database must be of type Queue for DB_CONSUME_WAIT to be specified.
If lock or transaction timeouts have been specified, the Db::get()
method with the DB_CONSUME_WAIT flag may return DB_LOCK_NOTGRANTED.
This failure, by itself, does not require the enclosing transaction be
aborted.
Retrieve the key/data pair only if both the key and data match the arguments.
When using a secondary index handle, the DB_GET_BOTH:
flag causes:
the Db::pget() version of
this method to retun the secondary key/primary
key/data tuple only if both the primary and
secondary keys match the arguments.
the Db::get() version of this method
to result in an error.
Retrieve the specified numbered key/data pair from a database. Upon return, both the key and data items will have been filled in.
The data field of the specified key must be a pointer to a logical record number (that is, a db_recno_t). This record number determines the record to be retrieved.
For DB_SET_RECNO to be specified, the underlying database must be of type Btree, and it must have been created with the DB_RECNUM flag.
In addition, the following flags may be set by bitwise inclusively OR'ing them into the flags parameter:
Return the data item irrespective of the state of master leases. The item will be returned under all conditions: if master leases are not configured, if the request is made to a client, if the request is made to a master with a valid lease, or if the request is made to a master without a valid lease.
Return multiple data items in the buffer to which the data parameter refers.
In the case of Btree or Hash databases, all of the data items associated with the specified key are entered into the buffer. In the case of Queue or Recno databases, all of the data items in the database, starting at, and subsequent to, the specified key, are entered into the buffer.
The buffer to which the data parameter refers must be provided from user memory (see DB_DBT_USERMEM). The buffer must be at least as large as the page size of the underlying database, aligned for unsigned integer access, and be a multiple of 1024 bytes in size. If the buffer size is insufficient, then upon return from the call the size field of the data parameter will have been set to an estimated buffer size, and the error DB_BUFFER_SMALL is returned. (The size is an estimate as the exact size needed may not be known until all entries are read. It is best to initially provide a relatively large buffer, but applications should be prepared to resize the buffer as necessary and repeatedly call the method.)
The DB_MULTIPLE flag may only be used alone, or with the DB_GET_BOTH and DB_SET_RECNO options. The DB_MULTIPLE flag may not be used when accessing databases made into secondary indices using the Db::associate() method.
See the DBT and Bulk Operations for more information on working with bulk get.
Configure a transactional get operation to have degree 2 isolation (the read is not repeatable).
Configure a transactional get operation to have degree 1 isolation, reading modified but not yet committed data. Silently ignored if the DB_READ_UNCOMMITTED flag was not specified when the underlying database was opened.
Acquire write locks instead of read locks when doing the read, if locking is configured. Setting this flag can eliminate deadlock during a read-modify-write cycle by acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock.
Because the Db::get() method will not hold locks across Berkeley DB
calls in non-transactional operations, the
DB_RMW flag to the
Db::get() call is meaningful only in the presence of transactions.
The key Dbt operated on.
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::get()
method may fail and throw a DbException
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
The requested item could not be returned due to undersized buffer.
DbMemoryException is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, DB_BUFFER_SMALL is returned.
A transactional database environment operation was selected to resolve a deadlock.
DbDeadlockException is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, DB_LOCK_DEADLOCK is returned.
A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time.
DbLockNotGrantedException is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, DB_LOCK_NOTGRANTED is returned.
The DB_CONSUME_WAIT flag was specified, lock or transaction timers
were configured and the lock could not be granted before the wait-time
expired.
DbLockNotGrantedException is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, DB_LOCK_NOTGRANTED is returned.
When a client synchronizes with the master, it is possible for committed
transactions to be rolled back. This invalidates all the database and cursor
handles opened in the replication environment. Once this occurs, an attempt to use
such a handle will
throw a DbRepHandleDeadException (if
your application is configured to throw exceptions), or
return DB_REP_HANDLE_DEAD.
The application will need to discard the handle and open a new one in order to
continue processing.
The operation was blocked by client/master synchronization.
DbDeadlockException is thrown if
your Berkeley DB API is configured to throw exceptions.
Otherwise, DB_REP_LOCKOUT is returned.
If a record number of 0 was specified; the
DB_THREAD flag was
specified to the Db::open() method and none of the
DB_DBT_MALLOC,
DB_DBT_REALLOC or
DB_DBT_USERMEM
flags were set in the Dbt;
the Db::pget() method was called with a
Db handle
that does not refer to a secondary index; or if an invalid flag value
or parameter was specified.