在两个不同商店中的嵌套游标 [英] Nested Cursors over two different Stores

问题描述

我有以下代码:

...

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
    }
  }
}
... 

我希望内部循环将触发N * M次，其中N-leftStore的基数，M-rightStore的基数.

I expect that internal loop will be fired N*M times, where N - cardinality of leftStore and M - cardinality of rightStore.

在实践中，外部循环仅触发一次，内部循环触发M次.

On practice external loop fires only once and internal loop fires M-times.

如果我按照以下方式(更平滑嵌套循环)重写代码:

If I rewrite the code in following way (flattering nested loops):

...
while(leftCursor.getNext()) {
 ...
}

while(rightCursor.getNext()) {
 ...
}

...

然后，这两个循环都会按预期的方式为leftStore触发N次，为rightStore触发M次.

Then both loops fires as expected N-times for leftStore and M-times for rightStore.

问题是:是否可以使嵌套游标移动?如果是，请指导我.

The question is: is it possible to make nested cursor traveling? If yes, kindly please guide me.

谢谢！

-塔拉斯

推荐答案

一旦cursor.getNext()返回false(不存在 next 键/值对)，它将永远不会返回true此Cursor实例.要再次遍历商店，请重新打开光标.

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.

这是代码以矩阵形式遍历两个商店，即两个商店中键/值对的所有成对组合:

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
            }
        }
    }
}

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