如何做一个光标指删除的行？ [英] How does a cursor refer to deleted rows?

问题描述

当我得到一个光标从Android中似乎包含的行固定数量并不无论发生了什么DB改变一个SQLite数据库查询回来。如果，例如，某些行被删除，而游标打开我仍然可以参考已经被删除的行。这是很好的，但它是如何工作的？

When I get a Cursor back from a sqlite DB query in Android it seems to contain a fixed number of rows that does not change regardless of what happens to the DB. If, for example, some of the rows are deleted while the Cursor is open I can still reference rows that have been deleted. This is nice, but how does it work?

一个猜测可能是sqlite的都知道我有一个指向该行中的光标等保持旧数据的副本中。但是，如果我有一百万行，删除它们，并与一百万个不同的行替换他们，好像很多数据缓存！

One guess might be that sqlite knows I have a Cursor that is pointing to that row and so keeps a copy of the old data around. But what if I had a million rows, deleted them and replaced them with a million different rows, seems like a lot of data to cache!

更新：

我现在想，这种缓存我们在Android中看到的不是真正的防弹，而不是SQLite的一部分。我创建了一个测试，建立一个数据库，它1,000,000行，然后我问打印出一些成果数据库，并留下了游标打开，接下来我删除了一半的行，最后我访问了我敞开，结果光标是崩溃：

I am now thinking that this caching we see in Android isn't really bullet-proof and isn't part of SQLite. I created a test which builds a database with 1,000,000 rows in it, then I queried the db printed out some results and left the cursor open, next I deleted half the rows, finally I tried to access the cursor that I left open and the result is a crash:

04-05 18:17:16.141 E/AndroidRuntime(19655): java.lang.IllegalStateException: Couldn't read row 0, col 0 from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.
04-05 18:17:16.141 E/AndroidRuntime(19655):     at android.database.CursorWindow.nativeGetLong(Native Method)
04-05 18:17:16.141 E/AndroidRuntime(19655):     at android.database.CursorWindow.getLong(CursorWindow.java:507)
04-05 18:17:16.141 E/AndroidRuntime(19655):     at android.database.AbstractWindowedCursor.getLong(AbstractWindowedCursor.java:75)
04-05 18:17:16.141 E/AndroidRuntime(19655):     at android.database.AbstractCursor.moveToPosition(AbstractCursor.java:220)
04-05 18:17:16.141 E/AndroidRuntime(19655):     at android.database.AbstractCursor.moveToFirst(AbstractCursor.java:237)
04-05 18:17:16.141 E/AndroidRuntime(19655):     at com.jacob.DirectDbActivity$4.run(DirectDbActivity.java:88)

这似乎是在Android中的一个错误，因为很多数据库是小的缓存窗口可以容纳所有的结果，但在大型数据库这会打你。

This seems like a bug in Android, since most databases are small the cache window can hold all the results but in large databases this is going to hit you.

推荐答案

上次我检查，游标是基于创建它的SQL数据库的快照。很像一个数据集。如果更改了数据库中的游标可能不知道，他们的快照是旧的。您可以对光标进行重新查询基础上创建光标原来的SQL来获得新的数据。

Last I checked, a cursor is a snapshot of the database based on the SQL that created it. Much like a Data set. If you change the database the cursors may not know that their snapshot is old. You can perform a re-query on the cursor to get fresh data based on the original SQL that created the cursor.

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