Berkeley DB: Db.join
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Db.join
import com.sleepycat.db.*;
public Dbc join(Dbc curslist, int flags)
throws DbException;
Description
The Db.join method creates a specialized cursor for use in performing
joins on secondary indices. Your data must be organized in the following
manner in order to take advantage of this:
- The actual data should be stored in the database represented by the
primary Db handle.
- Secondary indices should be stored in separate database files, whose keys
are the values of the secondary indices and whose data items are the
primary keys corresponding to the records having the designated secondary
key value. It is acceptable (and expected) that there may be duplicate
entries in the secondary indices.
These duplicate entries should be sorted. For more information see
the Db.DB_DUPSORT flag to the DbInfo.set_flags method.
The primary argument contains the Db handle of the
primary database, which is keyed by the data values found in entries
in the curslist.
The curslist argument contains a null terminated array of
Dbc objects. Each Dbc must have been initialized to
reference the key on which the underlying database should be joined.
Typically, this initialization is done by a Dbc.get call with
the Db.DB_SET flag specified.
The flags parameter is currently unused, and must be set to 0.
The returned cursor has the standard cursor functions, that behave as
follows:
- Dbc.get
- Iterates over the values associated with the keys to which each item in
curslist has been initialized. Any data value which appears in
all items specified by the curslist argument is then used as a
key into the primary, and the key/data pair found in the
primary is returned.
The flags parameter must be set to 0 or the following value:
- Db.DB_JOIN_ITEM
- Do not use the data value found in all of the cursors as a lookup
key for the primary, but simply return it in the key parameter
instead. The data parameter is left unchanged.
In addition, the following value may be set by logically OR'ing it into the
flags parameter:
- 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.
- Dbc.put
- Returns EINVAL.
- Dbc.del
- Returns EINVAL.
- Dbc.close
- Close the cursor and release all resources. (Closing the
cursors in curslist is the responsibility of the caller.)
In a transaction protected environment, all of the cursors listed in
curslist must have been created within the same transaction.
The Db.join
method throws an exception that encapsulates an errno on failure.
Errors
If a fatal error occurs in Berkeley DB, the Db.join method may fail and throw a
DbRunRecoveryException, at which point all subsequent database
calls will also fail in the same way.
The Db.join
method may fail and throw an exception
for any of the errors specified for the following Berkeley DB and C library
functions:
fflush(3),
fprintf(3),
free(3),
malloc(3),
memset(3),
vfprintf(3),
and
vsnprintf(3).
In addition, the Db.join
method may fail and throw an exception
encapsulating errno
for the following conditions:
- EINVAL
- An invalid flag value or parameter was specified.
The c_put or c_del functions were called.
Class
Db
See Also
Db.close,
Db.cursor,
Db.del,
Db.fd,
Db.get,
Db.get_byteswapped,
Db.get_type,
Db.join,
Db.open,
Db.put,
Db.stat
and
Db.sync.
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