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

# Db.get |

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    import com.sleepycat.db.*;

    public int get(DbTxn txnid, Dbt key, Dbt data, int flags)
    	throws DbException;

# Description

The Db.get method retrieves key/data pairs from the database. The byte array and length of the data associated with the specified key are returned in the structure referenced by data.

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.

If the file is being accessed under transaction protection, the txnid parameter is a transaction ID returned from DbEnv.txn_begin, otherwise, NULL.

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

Db.DB_CONSUME
  Return the record number and data from the available record closest to the head of the queue and delete the record. The cursor will be positioned on the deleted 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.DB_CONSUME to be specified.

Db.DB_CONSUME_WAIT
  The Db.DB_CONSUME_WAIT flag is the same as the Db.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.DB_CONSUME_WAIT to be specified.

Db.DB_GET_BOTH
  Retrieve the key/data pair only if both the key and data match the arguments.

Db.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, not just the data item as is done for all other uses of the Db.get method.

  The data field of the specified key must be a byte array large enough to hold a logical record number (i.e., an int). This record number determines the record to be retrieved.

  For Db.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 flag may be set by bitwise inclusively OR'ing it into the flags parameter:

Db.DB_RMW
  Acquire write locks instead of read locks when doing the retrieval. Setting this flag may decrease the likelihood of deadlock during a read-modify-write cycle by immediately 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.

  As the Db.get interface will not hold locks across Berkeley DB interface calls in non-transactional environments, the Db.DB_RMW flag to the Db.get call is only meaningful in the presence of transactions.

If the database is a Queue or Recno database and the requested key exists, but was never explicitly created by the application or was later deleted, the Db.get method returns Db.DB_KEYEMPTY.

Otherwise, if the requested key is not in the database, the Db.get function returns Db.DB_NOTFOUND.

Otherwise, the Db.get method throws an exception that encapsulates a non-zero error value on failure.

# Errors

The Db.get method may fail and throw an exception encapsulating a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

  A record number of 0 was specified.

  The Db.DB_THREAD flag was specified to the Db.open method and none of the Db.DB_DBT_MALLOC, Db.DB_DBT_REALLOC or Db.DB_DBT_USERMEM flags were set in the Dbt.

If the operation was selected to resolve a deadlock, the Db.get method will fail and throw a DbDeadlockException exception.

If the requested item could not be returned due to insufficient memory, the Db.get method will fail and throw a DbMemoryException exception.

The Db.get method may fail and throw an exception for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the Db.get method may fail and throw a DbRunRecoveryException, in which case all subsequent Berkeley DB calls will fail in the same way.

### Class

Db

# See Also

Db.close, Db.cursor, Db.del, Db.fd, Db.get, Db.get_byteswapped, Db.get_type, Db.join, Db.key_range, Db.open, Db.put, Db.remove, Db.set_bt_minkey, Db.set_cachesize, Db.set_errcall, Db.set_errpfx, Db.set_flags, Db.set_h_ffactor, Db.set_h_nelem, Db.set_lorder, Db.set_pagesize, Db.set_q_extentsize, Db.set_re_delim, Db.set_re_len, Db.set_re_pad, Db.set_re_source, Db.stat, Db.sync, Db.upgrade and Db.verify.`

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