Nested Cursors over two different Stores - xodus

Question

I have following code:
...
Transaction xodusTransaction = xodusEnvironment.beginReadonlyTransaction();
Store leftStore = xodusEnvironment.openStore(leftName, StoreConfig.USE_EXISTING, xodusTransaction, false);
Store rightStore = xodusEnvironment.openStore(rightName, StoreConfig.USE_EXISTING, xodusTransaction, false);
try(Cursor leftCursor = leftStore.openCursor(xodusTransaction);
Cursor rightCursor = rightStore.openCursor(xodusTransaction)) {
while(leftCursor.getNext()) {
while(rightCursor.getNext()) {
// Do actual work with data from both stores
}
}
}
...
I expect that internal loop will be fired N*M times, where N - cardinality of leftStore and M - cardinality of rightStore.
On practice external loop fires only once and internal loop fires M-times.
If I rewrite the code in following way (flattering nested loops):
...
while(leftCursor.getNext()) {
...
}
while(rightCursor.getNext()) {
...
}
...
Then both loops fires as expected N-times for leftStore and M-times for rightStore.
The question is: is it possible to make nested cursor traveling? If yes, kindly please guide me.
Thank you!
-Taras

Answer 1

Once cursor.getNext() returned false (there is no next key/value pair), it will never return true for this Cursor instance. To traverse a Store again, reopen cursor.
Here is the code traversing two Stores as a matrix, i.e. all pairwise combinations of key/value pairs from both Stores:
try (Cursor leftCursor = leftStore.openCursor(txn)) {
while (leftCursor.getNext()) {
try (Cursor rightCursor = rightStore.openCursor(txn)) {
while (rightCursor.getNext()) {
// Do actual work with data from both stores
}
}
}
}