SQLiteDatabases和光标 [英] SQLiteDatabases and Cursors
问题描述
我在想，如果有人可以给我的Android游标的简要概述。一对夫妇的具体问题：
I was wondering if someone could give me a brief overview of Android Cursors. A couple of specific questions:
1 - 我有一个数据库查询后返回游标的方法：
1 - I have a method which returns a cursor after a database query:
public static Cursor getVehicles()
{
SQLiteDatabase db = vehicleData.getReadableDatabase();
Cursor cursor = db.query(TABLE_NAME, GET_VEHICLES_FROM_CLAUSE, null, null, null, null, ORDER_BY);
return cursor;
}
为了做家政，我想db.close（）刚刚返回语句之前。然而，这引起了返回指针到包含任何行。这是为什么？
In order to do housekeeping, I tried db.close() just before the return statement. However, this caused the returned cursor to contain no rows. Why is this?
2 - 什么是关闭游标并关闭数据库的区别
2 - What's the difference between closing a cursor and closing a database?
3 - 我是否需要关闭游标上调用，如果它是一个局部变量，或者我可以离开它的垃圾收集清理
3 - Do I need to call close on a Cursor if it is a local variable, or can I leave it to the garbage collector to clean up?
4 - 我的数据库很小，只有我的应用程序使用 - 可我只是把它打开
4 - My database is small and only used by my application - can I just keep it open?
推荐答案
1）光标是只是一个指针由您查询返回的数据，它不包含从查询的所有数据。这是为了提高性能/效率（大的结果集不是一次读取 - >使用更少的内存）。因此，如果您关闭数据库，光标无法检索数据 - >它是空的。
1) The cursor is just a pointer to the data returned by your query, it doesn't contain all the data from your query. This is to increase performance/efficiency (large resultsets aren't read at once -> less memory used). Therefore, if you close the database, the cursor can't retrieve the data -> it's empty.
2）当你关闭游标，所有相关的资源被释放 - >你不能访问这个光标（因为它已被释放）相关的数据，但你可以使用这个或其他游标新的查询。当您关闭数据库，则无法查询它了（直到你重新打开它）。
2) When you close a cursor, all associated resources are released -> you can't access the data associated with this cursor (since it has been released), but you can make new queries using this or other cursors. When you close a database, you can't query it anymore (until you re-open it).
3）始终关闭游标。否则，你会遇到的问题 - 如果游标不关闭，新的查询被封锁的GC会抱怨。
3) Always close cursors. Otherwise you will run into problems - the GC will complain if the cursor isn't closed and new queries are blocked.
4）如果您关闭它，当你的应用程序完成的，是的。
4) If you close it when your app finishes, yes.
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