Berkeley DB: Db::get
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++

Db::get |

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    #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);

Description: Db::get

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 insert order, except where this order has been overridden by cursor operations. 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
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 or throw a DbLockNotGrantedException exception. 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 used with the Db::pget method version of this method on a secondary index handle, return the secondary key/primary key/data tuple only if both the primary and secondary keys match the arguments. It is an error to use the DB_GET_BOTH flag with the Db::get version of this method and a secondary index handle.

  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 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 DbMultipleDataIterator for more information.

  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.

key

- The key Dbt operated on.

pkey

- The pkey parameter is the return key from the primary database.

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.

Errors

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

DB_REP_HANDLE_DEAD

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

DB_REP_LEASE_EXPIRED

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

DB_REP_LOCKOUT

- The operation was blocked by client/master synchronization.

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.

If the requested item could not be returned due to undersized buffer, the Db::get method will fail and either return DB_BUFFER_SMALL or throw a DbMemoryException exception.

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

If 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, the Db::get method will fail and either return DB_LOCK_NOTGRANTED or throw a DbLockNotGrantedException exception.

Class Db
See Also Databases and Related Methods`

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