可以将6MB的文本文件直接导入到我的数据库中。但是，由于CursorWindow的限制为2MB，因此无法提取文本。 （我应该使用过文件，但是一些用户已经遇到了这个问题，我需要阅读整个文本才能将其放入文件中）
我使用substr（一种特殊的SQL函数）仅请求了1 MB的内存，并且可以正常工作。但是，以下while循环在第二次迭代后不起作用（这意味着即使我调用cursor.close（），也不会清空CursorWindow，因此对于第一次迭代，它只有1MB，但是在第二次迭代之后，有2MB并抛出SQLiteBlobTooBigException异常）：
//Load in chunks
BookDbHelper bookDbHelper = new BookDbHelper(GlobalContext.get());
SQLiteDatabase readableDatabase = bookDbHelper.getReadableDatabase();
//Query length
int chunk_size = (int) Math.pow(2, 20);//mb
String query_length = "SELECT _id, length(text) FROM " + BookContract.TABLE_NAME + " WHERE _id=" + id;
Cursor cursor_length = readableDatabase.rawQuery(query_length, null);
cursor_length.moveToFirst();
int length = cursor_length.getInt(1);
cursor_length.close();
bookDbHelper.close();
readableDatabase.close();
//Query text
int numSteps = length / chunk_size + 1;
int i = 0;
while(i < numSteps) {
BookDbHelper bookDbHelper2 = new BookDbHelper(GlobalContext.get());
SQLiteDatabase readableDatabase2 = bookDbHelper2.getReadableDatabase();
int from = i * chunk_size + 1;
int to = (i + 1) * chunk_size + 1;
//L.v(from + ", " + to);
String query = "SELECT _id, substr(text," + from + "," + to + ") FROM " + BookContract.TABLE_NAME + " WHERE _id=" + id;
Cursor cursor = readableDatabase2.rawQuery(query, null);
//Read
cursor.moveToFirst();
String string = cursor.getString(1);
cursor.close();
bookDbHelper2.close();
readableDatabase2.close();
//stringBuilder.append(string);
i++;
}
相关的列是_id和text（包含一个非常大的字符串），相关的sql函数是length（）（以了解必要的迭代次数）和substr（）（这样就不会立即发生SQLiteBlobTooBigException，因为尚未达到2MB的上限）。
我尝试关闭bookDbHelper和可读数据库，但没有帮助。
如何强制关闭CursorWindow，以便发出1MB的请求，清空CursorWinow，然后继续发出其他请求？
最佳答案
如何强制关闭CursorWindow，以便发出1MB的请求，清空CursorWinow，然后继续发出其他请求？
我不认为关闭游标是您的问题，好像通过不关闭游标会追加到游标并扩展一样。
而是您的问题与您生成的查询有关。
简而言之，substr函数不是from到from，而是from for（因为它是返回字符串的大小/长度。您的计算基于第二个值，即字符的偏移量）。这样，提取的字符串的长度将减小块大小，直到超过字符串的末尾（在此之前先吹除CursorWindow）。
见substr(X,Y,Z) substr(X,Y)
因此，使用1MB的第二个块（如果将其视为使用偏移量）注定要在第二次运行中失败，因为它实际上是要提取的长度（2MB）。减小到小于1MB会留出一些余地，但可能会破坏CursorWindow（但会获取其他数据）。
但是，也可以使用单个光标，每个块作为伸出的行。姿态可能是：-
//Load in chunks
BookDbHelper bookDbHelper = new BookDbHelper(/*GlobalContext.get()*/this);
SQLiteDatabase readableDatabase = bookDbHelper.getReadableDatabase();
//Query length
StringBuilder wholeBookText = new StringBuilder();
int chunk_size = (int) Math.pow(2, 20);//mb
String query_length = "SELECT length(text) FROM " + BookContract.TABLE_NAME + " WHERE _id=?";
Cursor cursor = readableDatabase.rawQuery(query_length, new String[]{String.valueOf(id)});
int length = 0;
if (cursor.moveToFirst()) {
length = cursor.getInt(0);
}
int numSteps = length / chunk_size + 1;
int i = 0;
Log.d("BOOKINFO", "Length of Text is " + length + " Number of Chunks = " + numSteps + " Chunk Size = " + chunk_size);
StringBuilder sb = new StringBuilder();
for (i=1; i < length + 1; i+= chunk_size) {
if (sb.length() > 1) sb.append(" UNION ALL ");
sb.append("SELECT substr(text,")
.append(String.valueOf(i)).append(",").append(String.valueOf(chunk_size))
.append(") FROM ").append(BookContract.TABLE_NAME)
.append(" WHERE _id=").append(String.valueOf(id));
}
sb.append(";");
Log.d("BOOKINFOV2","SQL generated :-\n\t" + sb.toString());
cursor = readableDatabase.rawQuery(sb.toString(),null);
wholeBookText = new StringBuilder();
while (cursor.moveToNext()) {
wholeBookText.append(cursor.getString(0));
Log.d("BOOKINFO","Obtained String who's length is " + cursor.getString(0).length() + "\n\tTotal Extracted = " + wholeBookText.length());
}
而不是单个查询循环运行。这将生成一个查询，该查询将每个块提取为一行。那就是在所有查询之间建立一个UNION。例如
SELECT substr(text,1,1048576) FROM book WHERE _id=4
UNION ALL SELECT substr(text,1048577,1048576) FROM book WHERE _id=4
UNION ALL SELECT substr(text,2097153,1048576) FROM book WHERE _id=4
UNION ALL SELECT substr(text,3145729,1048576) FROM book WHERE _id=4;
取自上述测试。
可以看出to（应该是）是块大小。最后一块将根据剩余数据被截断。
测试运行的完整输出：-
2019-12-16 14:21:35.546 D/BOOKINFOV2: SQL generated :-
SELECT substr(text,1,1048576) FROM book WHERE _id=4 UNION ALL SELECT substr(text,1048577,1048576) FROM book WHERE _id=4 UNION ALL SELECT substr(text,2097153,1048576) FROM book WHERE _id=4 UNION ALL SELECT substr(text,3145729,1048576) FROM book WHERE _id=4;
2019-12-16 14:21:35.555 W/CursorWindow: Window is full: requested allocation 1048577 bytes, free space 1048128 bytes, window size 2097152 bytes
2019-12-16 14:21:35.585 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 1048576
2019-12-16 14:21:35.599 W/CursorWindow: Window is full: requested allocation 1048577 bytes, free space 1048128 bytes, window size 2097152 bytes
2019-12-16 14:21:35.616 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 2097152
2019-12-16 14:21:35.653 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 3145728
2019-12-16 14:21:35.654 D/BOOKINFO: Obtained String who's length is 51
Total Extracted = 3145779
如您所见，CursorWindow会溢出，但是未添加该行，下次将添加该行并且可以访问该行。
当然，您可以采用多种查询方法，在这种情况下，代码可能是：-
//Load in chunks
BookDbHelper bookDbHelper = new BookDbHelper(/*GlobalContext.get()*/this);
SQLiteDatabase readableDatabase = bookDbHelper.getReadableDatabase();
//Query length
StringBuilder wholeBookText = new StringBuilder();
int chunk_size = (int) Math.pow(2, 19);//mb
chunk_size = (1024 * 1024);
String query_length = "SELECT length(text) FROM " + BookContract.TABLE_NAME + " WHERE _id=?";
Cursor cursor = readableDatabase.rawQuery(query_length, new String[]{String.valueOf(id)});
int length = 0;
if (cursor.moveToFirst()) {
length = cursor.getInt(0);
}
int numSteps = length / chunk_size + 1;
int i = 0;
Log.d("BOOKINFO", "Length of Text is " + length + " Number of Chunks = " + numSteps + " Chunk Size = " + chunk_size);
int from = 1, to = chunk_size;
while (i < numSteps && length > 0) {
if (to > length) to = length;
String query = "SELECT substr(text," + from + "," + (chunk_size) + ") FROM " + BookContract.TABLE_NAME + " WHERE _id=?";
Log.d("BOOKINFOSQL",query);
cursor.close();
cursor = readableDatabase.rawQuery(query, new String[]{String.valueOf(id)});
//Read
if (cursor.moveToFirst()) {
wholeBookText.append(cursor.getString(0));
Log.d("BOOKINFO","Obtained String who's length is " + cursor.getString(0).length() + "\n\tTotal Extracted = " + wholeBookText.length());
}
cursor.close();
i++;
from = (i * chunk_size) + 1;
to = from + chunk_size;
}
if (!cursor.isClosed()) {
cursor.close();
}
Log.d("BOOKINFO", "The length of the extracted data is " + wholeBookText.length());
上面的结果是：
2019-12-16 14:16:15.336 D/BOOKINFO: Length of Text is 3145779 Number of Chunks = 4 Chunk Size = 1048576
2019-12-16 14:16:15.336 D/BOOKINFOSQL: SELECT substr(text,1,1048576) FROM book WHERE _id=?
2019-12-16 14:16:15.358 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 1048576
2019-12-16 14:16:15.358 D/BOOKINFOSQL: SELECT substr(text,1048577,1048576) FROM book WHERE _id=?
2019-12-16 14:16:15.382 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 2097152
2019-12-16 14:16:15.383 D/BOOKINFOSQL: SELECT substr(text,2097153,1048576) FROM book WHERE _id=?
2019-12-16 14:16:15.409 D/BOOKINFO: Obtained String who's length is 1048576
Total Extracted = 3145728
2019-12-16 14:16:15.409 D/BOOKINFOSQL: SELECT substr(text,3145729,1048576) FROM book WHERE _id=?
2019-12-16 14:16:15.418 D/BOOKINFO: Obtained String who's length is 51
Total Extracted = 3145779
2019-12-16 14:16:15.418 D/BOOKINFO: The length of the extracted data is 3145779
关于android - 将请求分为1 MB的块(和cursor.close())后，仍然会发生SQLiteBlobTooBigException，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/59348784/