If you have two or more secondary databases associated with a primary
database, then you can retrieve primary records based on the intersection of
multiple secondary entries. You do this using a
JoinCursor.
Throughout this document we have presented a class that stores inventory information on grocery That class is fairly simple with a limited number of data members, few of which would be interesting from a query perspective. But suppose, instead, that we were storing information on something with many more characteristics that can be queried, such as an automobile. In that case, you may be storing information such as color, number of doors, fuel mileage, automobile type, number of passengers, make, model, and year, to name just a few.
In this case, you would still likely be using some unique value to key your
primary entries (in the United States, the automobile's VIN would be
ideal for this purpose). You would then create a
class
that identifies
all the characteristics of the automobiles in your inventory.
You would
also have to create some mechanism by which you would move instances of
this class in and out of Java byte arrays. We
described the concepts and mechanisms by which you can perform these
activities in Database Records.
To query this data, you might then create multiple secondary databases, one for each of the characteristics that you want to query. For example, you might create a secondary for color, another for number of doors, another for number of passengers, and so forth. Of course, you will need a unique key creator for each such secondary database. You do all of this using the concepts and techniques described throughout this chapter.
Once you have created this primary database and all interesting secondaries, what you have is the ability to retrieve automobile records based on a single characteristic. You can, for example, find all the automobiles that are red. Or you can find all the automobiles that have four doors. Or all the automobiles that are minivans.
The next most natural step, then, is to form compound queries, or joins.
For example, you might want to find all the automobiles that are red,
and that were built by Toyota, and that are minivans. You can do this
using a
JoinCursor class instance.
To use a join cursor:
Open two or more secondary cursors. These cursors for secondary databases that are associated with the same primary database.
Position each such cursor to the secondary key
value in which you are interested. For example, to build on
the previous description, the cursor for the color
database is positioned to the red records
while the cursor for the model database is positioned to the
minivan records, and the cursor for the
make database is positioned to Toyota.
Create an array of secondary cursors, and place in it each of the cursors that are participating in your join query.
Obtain a join cursor. You do this using the
Database.join()
method. You must pass this method the array of secondary cursors that you
opened and positioned in the previous steps.
Iterate over the set of matching records
using JoinCursor.getNext()
until
OperationStatus is not SUCCESS.
Close your join cursor.
If you are done with them, close all your secondary cursors.
For example:
package db.GettingStarted;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.JoinCursor;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
import com.sleepycat.db.SecondaryCursor;
import com.sleepycat.db.SecondaryDatabase;
...
// Database and secondary database opens omitted for brevity.
// Assume a primary database handle:
// automotiveDB
// Assume 3 secondary database handles:
// automotiveColorDB -- index based on automobile color
// automotiveTypeDB -- index based on automobile type
// automotiveMakeDB -- index based on the manufacturer
Database automotiveDB = null;
SecondaryDatabase automotiveColorDB = null;
SecondaryDatabase automotiveTypeDB = null;
SecondaryDatabase automotiveMakeDB = null;
// Query strings:
String theColor = "red";
String theType = "minivan";
String theMake = "Toyota";
// Secondary cursors used for the query:
SecondaryCursor colorSecCursor = null;
SecondaryCursor typeSecCursor = null;
SecondaryCursor makeSecCursor = null;
// The join cursor
JoinCursor joinCursor = null;
// These are needed for our queries
DatabaseEntry foundKey = new DatabaseEntry();
DatabaseEntry foundData = new DatabaseEntry();
// All cursor operations are enclosed in a try block to ensure that they
// get closed in the event of an exception.
try {
// Database entries used for the query:
DatabaseEntry color = new DatabaseEntry(theColor.getBytes("UTF-8"));
DatabaseEntry type = new DatabaseEntry(theType.getBytes("UTF-8"));
DatabaseEntry make = new DatabaseEntry(theMake.getBytes("UTF-8"));
colorSecCursor = automotiveColorDB.openSecondaryCursor(null, null);
typeSecCursor = automotiveTypeDB.openSecondaryCursor(null, null);
makeSecCursor = automotiveMakeDB.openSecondaryCursor(null, null);
// Position all our secondary cursors to our query values.
OperationStatus colorRet =
colorSecCursor.getSearchKey(color, foundData, LockMode.DEFAULT);
OperationStatus typeRet =
typeSecCursor.getSearchKey(type, foundData, LockMode.DEFAULT);
OperationStatus makeRet =
makeSecCursor.getSearchKey(make, foundData, LockMode.DEFAULT);
// If all our searches returned successfully, we can proceed
if (colorRet == OperationStatus.SUCCESS &&
typeRet == OperationStatus.SUCCESS &&
makeRet == OperationStatus.SUCCESS) {
// Get a secondary cursor array and populate it with our
// positioned cursors
SecondaryCursor[] cursorArray = {colorSecCursor,
typeSecCursor,
makeSecCursor};
// Create the join cursor
joinCursor = automotiveDB.join(cursorArray, null);
// Now iterate over the results, handling each in turn
while (joinCursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {
// Do something with the key and data retrieved in
// foundKey and foundData
}
}
} catch (DatabaseException dbe) {
// Error reporting goes here
} catch (Exception e) {
// Error reporting goes here
} finally {
try {
// Make sure to close out all our cursors
if (colorSecCursor != null) {
colorSecCursor.close();
}
if (typeSecCursor != null) {
typeSecCursor.close();
}
if (makeSecCursor != null) {
makeSecCursor.close();
}
if (joinCursor != null) {
joinCursor.close();
}
} catch (DatabaseException dbe) {
// Error reporting goes here
}
}
You can set JoinCursor properties using the
JoinConfig class. Currently there is just one property that you can
set:
JoinConfig.setNoSort()
Specifies whether automatic sorting of input cursors is disabled. The cursors are sorted from the one that refers to the least number of data items to the one that refers to the most.
If the data is structured so that cursors with many data items also share many common elements, higher performance will result from listing those cursors before cursors with fewer data items. Turning off sorting permits applications to specify cursors in the proper order given this scenario.
The default value is false (automatic cursor sorting is performed).
For example:
// All database and environments omitted JoinConfig config = new JoinConfig(); config.setNoSort(true); JoinCursor joinCursor = myDb.join(cursorArray, config);