public int get(DbTxn txnid, Dbt key, Dbt data, int flags)
public int pget(DbTxn txnid, Dbt key, Dbt pkey, Dbt data, int flags)
The Db.get method retrieves key/data pairs from the database. The
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 interface will always fail and
If the operation is to be transaction-protected, the txnid
parameter is a transaction handle returned from DbEnv.txn_begin;
The flags value 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 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.
- 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.
- Retrieve the key/data pair only if both the key and data match the
When used with the Db.pget version of this interface
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.DB_GET_BOTH flag with the Db.get
version of this interface and a secondary index handle.
- Retrieve the specified numbered key/data pair from a database. Upon
return, both the key and data items will have been
The data field of the specified key
must be a byte array large enough to hold a logical record number (that
is, 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 flags may be set by
bitwise inclusively OR'ing them into the flags parameter:
- Read modified but not yet committed data. Silently ignored if the
Db.DB_DIRTY_READ flag was not specified when the underlying
database was opened.
- Return multiple data items. The buffer to which the data
argument refers is filled with the specified key's data items. If all
of the data items associated with the key cannot fit into the buffer,
the size field of the data argument is set to the length needed
for the specified items, and a DbMemoryException is thrown. The buffer
to which the data argument refers should be large relative to
the page size of the underlying database, aligned for unsigned integer
access, and be a multiple of 1024 bytes in size.
The Db.DB_MULTIPLE flag may only be used alone, or with the
Db.DB_GET_BOTH and Db.DB_SET_RECNO options. The
Db.DB_MULTIPLE flag may not be used when accessing databases made
into secondary indices using the Db.associate method.
See DbMultipleDataIterator for more information.
- Acquire write locks instead of read locks when doing the retrieval.
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 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 meaningful only in
the presence of transactions.
If the specified key is not in the database, the Db.get method will return Db.DB_NOTFOUND.
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, the Db.get method will return Db.DB_KEYEMPTY.
Otherwise, the Db.get method throws an exception that encapsulates a non-zero error value on
The Db.get method may fail and throw an exception encapsulating a non-zero error for the following conditions:
- A secondary index references a nonexistent primary key.
- 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.
The Db.pget interface was called with a Db handle that
does not refer to a secondary index.
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.
Databases and Related Methods
Copyright Sleepycat Software