Library Version 18.1.40

Db::get()
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Db::get() 

#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:

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.

Parameters

txnid

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.

key

The key Dbt operated on.

If DB_DBT_PARTIAL is set for the Dbt used for this parameter, and if the flags parameter is not set to DB_CONSUME DB_CONSUME_WAIT, or DB_SET_RECNO, then this method will fail and return EINVAL.

pkey

The pkey parameter is the return key from the primary database. If DB_DBT_PARTIAL is set for the Dbt used for this parameter, then this method will fail and return EINVAL.

data

The data Dbt operated on.

flags

The flags parameter must be set to 0 or one of the following values:

  • DB_CONSUME

    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.

  • DB_CONSUME_WAIT

    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.

  • DB_GET_BOTH

    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.

  • DB_SET_RECNO

    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:

  • DB_IGNORE_LEASE

    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.

  • DB_MULTIPLE

    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, Recno or Heap 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.

  • DB_READ_COMMITTED

    Configure a transactional get operation to have degree 2 isolation (the read is not repeatable).

  • DB_READ_UNCOMMITTED

    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.

  • DB_RMW

    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.

Errors

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:

DbMemoryException or DB_BUFFER_SMALL

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.

DbDeadlockException or DB_LOCK_DEADLOCK

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.

DbLockNotGrantedException or DB_LOCK_NOTGRANTED

A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time.

You attempted to open a database handle that is configured for no waiting exclusive locking, but the exclusive lock could not be immediately obtained. See Db::set_lk_exclusive() for more information.

DbLockNotGrantedException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, DB_LOCK_NOTGRANTED is returned.

DbLockNotGrantedException or DB_LOCK_NOTGRANTED

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.

DbRepHandleDeadException or DB_REP_HANDLE_DEAD

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.

DB_REP_LEASE_EXPIRED

The operation failed because the site's replication master lease has expired.

DbDeadlockException or DB_REP_LOCKOUT

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.

DB_SECONDARY_BAD

A secondary index references a nonexistent primary key.

EINVAL

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.

Class

Db

See Also

Database and Related Methods

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