SQLite游标在升级时返回旧列

时间：2019-12-02 08:47:43

标签： android android-sqlite

当我升级数据库时，数据库的版本号会更新，但是当查询被触发时，前两次会返回较旧的游标列。

对于前 迁移前的列数：16 迁移后预期的列数：21

但是，在运行迁移之后 数据库版本号已更新，但第一个查询的列数为16 然后在下一次查询或应用重启时更新为21。

有人知道为什么会这样吗？

以下是相同的模拟代码

public class MainActivity extends AppCompatActivity {
    DbHelper helper;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Stetho.initializeWithDefaults(this);
        helper = new DbHelper(this);
        SQLiteDatabase database = helper.getWritableDatabase();
        database.execSQL("CREATE TABLE mytable (names TEXT, co TEXT)");
        Cursor c1 = database.rawQuery("SELECT * FROM mytable", null);
        Log.d("rajas", "Count - " + c1.getColumnCount());
        c1.close();
        database.execSQL("ALTER TABLE mytable ADD COLUMN pass TEXT");
        Cursor c2 = database.rawQuery("SELECT * FROM mytable", null);
        Log.d("rajas", "Count  - " + c2.getColumnCount());
    }
    class DbHelper extends SQLiteOpenHelper {
        public DbHelper(Context context) {
            super(context, "dbdbb", null, 1);
        }
        @Override
        public void onCreate(SQLiteDatabase db) {
        }
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        }
    }
}

在上面的代码中，迁移前后的计数将保持为2。

1 个答案:

答案 0 :(得分：1)

这似乎是一个错误。

有很多可以解决的问题：-

    通过使用table_info（或 table_xinfo 包括隐藏的列）PRAGMA，
  • 获取总（未隐藏）的列数。
    • 请注意，此方法不是在游标中查看表，因此应谨慎使用它。
  • 或关闭并重新打开数据库。
    • 这种方法的缺点是，打开数据库相对而言是昂贵的资源。
  • 选择特定列

例如，请考虑：-

    helper = new DbHelper(this);

    SQLiteDatabase database = helper.getWritableDatabase();
    database.execSQL("CREATE TABLE mytable (names TEXT, co TEXT)");
    Cursor c1 = database.rawQuery("SELECT * FROM mytable", null);
    Log.d("rajas", "Count - " + c1.getColumnCount() + " Alt count = " + (c1.getColumnNames()).length);
    c1.close();
    Cursor ti = database.rawQuery("PRAGMA table_info(mytable);",null);
    DatabaseUtils.dumpCursor(ti);

    database.execSQL("ALTER TABLE mytable ADD COLUMN pass TEXT");

    Cursor c2 = database.rawQuery("SELECT * FROM mytable", null);
    Log.d("rajas", "Count  - " + c2.getColumnCount() + " Alt count = " + (c2.getColumnNames()).length);
    ti = database.rawQuery("PRAGMA table_info(mytable);",null);
    DatabaseUtils.dumpCursor(ti);
    ti.close();

    Cursor c3 = database.rawQuery("SELECT names,co,pass FROM mytable", null);
    Log.d("rajas", "Count  - " + c3.getColumnCount() + " Alt count = " + (c3.getColumnNames()).length);
    ti = database.rawQuery("PRAGMA table_info(mytable);",null);
    DatabaseUtils.dumpCursor(ti);
    c3.close();
    ti.close();

    database.close();
    database = helper.getWritableDatabase();
    Cursor c4 = database.rawQuery("SELECT * FROM mytable", null);
    Log.d("rajas", "Count  - " + c4.getColumnCount() + " Alt count = " + (c4.getColumnNames()).length);
    c4.close();

结果为：-

2019-12-03 07:48:23.484 D/rajas: Count - 2 Alt count = 2
2019-12-03 07:48:23.485 I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@5df21a1
2019-12-03 07:48:23.486 I/System.out: 0 {
2019-12-03 07:48:23.486 I/System.out:    cid=0
2019-12-03 07:48:23.486 I/System.out:    name=names
2019-12-03 07:48:23.486 I/System.out:    type=TEXT
2019-12-03 07:48:23.486 I/System.out:    notnull=0
2019-12-03 07:48:23.486 I/System.out:    dflt_value=null
2019-12-03 07:48:23.486 I/System.out:    pk=0
2019-12-03 07:48:23.486 I/System.out: }
2019-12-03 07:48:23.486 I/System.out: 1 {
2019-12-03 07:48:23.486 I/System.out:    cid=1
2019-12-03 07:48:23.486 I/System.out:    name=co
2019-12-03 07:48:23.486 I/System.out:    type=TEXT
2019-12-03 07:48:23.486 I/System.out:    notnull=0
2019-12-03 07:48:23.486 I/System.out:    dflt_value=null
2019-12-03 07:48:23.486 I/System.out:    pk=0
2019-12-03 07:48:23.486 I/System.out: }
2019-12-03 07:48:23.486 I/System.out: <<<<<
2019-12-03 07:48:23.491 D/rajas: Count  - 2 Alt count = 2
2019-12-03 07:48:23.491 I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@fc68fc6
2019-12-03 07:48:23.491 I/System.out: 0 {
2019-12-03 07:48:23.491 I/System.out:    cid=0
2019-12-03 07:48:23.491 I/System.out:    name=names
2019-12-03 07:48:23.492 I/System.out:    type=TEXT
2019-12-03 07:48:23.492 I/System.out:    notnull=0
2019-12-03 07:48:23.492 I/System.out:    dflt_value=null
2019-12-03 07:48:23.492 I/System.out:    pk=0
2019-12-03 07:48:23.492 I/System.out: }
2019-12-03 07:48:23.492 I/System.out: 1 {
2019-12-03 07:48:23.492 I/System.out:    cid=1
2019-12-03 07:48:23.492 I/System.out:    name=co
2019-12-03 07:48:23.492 I/System.out:    type=TEXT
2019-12-03 07:48:23.492 I/System.out:    notnull=0
2019-12-03 07:48:23.492 I/System.out:    dflt_value=null
2019-12-03 07:48:23.492 I/System.out:    pk=0
2019-12-03 07:48:23.492 I/System.out: }
2019-12-03 07:48:23.492 I/System.out: 2 {
2019-12-03 07:48:23.492 I/System.out:    cid=2
2019-12-03 07:48:23.492 I/System.out:    name=pass
2019-12-03 07:48:23.492 I/System.out:    type=TEXT
2019-12-03 07:48:23.492 I/System.out:    notnull=0
2019-12-03 07:48:23.492 I/System.out:    dflt_value=null
2019-12-03 07:48:23.492 I/System.out:    pk=0
2019-12-03 07:48:23.492 I/System.out: }
2019-12-03 07:48:23.493 I/System.out: <<<<<
2019-12-03 07:48:23.493 D/rajas: Count  - 3 Alt count = 3
2019-12-03 07:48:23.493 I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@7dbb87
2019-12-03 07:48:23.494 I/System.out: 0 {
2019-12-03 07:48:23.494 I/System.out:    cid=0
2019-12-03 07:48:23.494 I/System.out:    name=names
2019-12-03 07:48:23.494 I/System.out:    type=TEXT
2019-12-03 07:48:23.494 I/System.out:    notnull=0
2019-12-03 07:48:23.494 I/System.out:    dflt_value=null
2019-12-03 07:48:23.494 I/System.out:    pk=0
2019-12-03 07:48:23.494 I/System.out: }
2019-12-03 07:48:23.494 I/System.out: 1 {
2019-12-03 07:48:23.494 I/System.out:    cid=1
2019-12-03 07:48:23.494 I/System.out:    name=co
2019-12-03 07:48:23.494 I/System.out:    type=TEXT
2019-12-03 07:48:23.494 I/System.out:    notnull=0
2019-12-03 07:48:23.494 I/System.out:    dflt_value=null
2019-12-03 07:48:23.494 I/System.out:    pk=0
2019-12-03 07:48:23.494 I/System.out: }
2019-12-03 07:48:23.495 I/System.out: 2 {
2019-12-03 07:48:23.495 I/System.out:    cid=2
2019-12-03 07:48:23.495 I/System.out:    name=pass
2019-12-03 07:48:23.495 I/System.out:    type=TEXT
2019-12-03 07:48:23.495 I/System.out:    notnull=0
2019-12-03 07:48:23.495 I/System.out:    dflt_value=null
2019-12-03 07:48:23.495 I/System.out:    pk=0
2019-12-03 07:48:23.495 I/System.out: }
2019-12-03 07:48:23.495 I/System.out: <<<<<
2019-12-03 07:48:23.498 D/rajas: Count  - 3 Alt count = 3
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