我有一个使用游标的数据存储查询(Objectify v5(，我想在结果列表中的每个项目之后获取游标。代码如下：
public List<Puzzle> queryWithCursor(String cursor, String order, int limit) {
Query<Puzzle> query = ObjectifyService.ofy()
.load()
.type(Puzzle.class)
.order(order)
.limit(limit);
query = query.startAt(Cursor.fromWebSafeString(cursor));
List<Puzzle> puzzles = new ArrayList<>();
QueryResultIterator<Puzzle> iterator = query.iterator();
while (iterator.hasNext()) {
Puzzle puzzle = iterator.next();
puzzle.setCursor(iterator.getCursor().toWebSafeString());
puzzles.add(puzzle);
}
return puzzles;
}
虽然该方法工作正常，但它会在后台触发大量数据存储查询。基本上，每次迭代器.getCursor((运行时，都会触发一个额外的查询。我从Stackdriver Trace中了解到，如果限制为20，该方法总共会触发19个查询(似乎最后一个.getCursor((不会触发额外的查询(。因此，这种方法比使用偏移量的类似查询更慢，成本更高。
这真的是个bug吗？有没有办法避免性能受到影响？
这实际上是数据存储的基本行为，至少在旧的sdk中是这样(与Objectify 6使用的新sdk相反，后者可能相同，也可能不同(。在非批边界调用getCursor((将重新启动查询。您可以使用低级API进行尝试。
有一个解决方法：制作自己的Cursor类。它应该包括低级别的游标和一个偏移。显式地设置一个chunk()大小，然后您的光标应该由索引0处的Cursor加上块的偏移量组成。
然后，当您想在该光标处重新启动查询时，请使用.cursor(batchStartCursor).offset(offsetIntoBatch)。
import com.google.appengine.api.datastore.Cursor;
import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.FetchOptions;
import com.google.appengine.api.datastore.PreparedQuery;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Query.SortDirection;
import com.google.appengine.api.datastore.QueryResultList;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ListPeopleServlet extends HttpServlet {
static final int PAGE_SIZE = 15;
private final DatastoreService datastore;
public ListPeopleServlet() {
datastore = DatastoreServiceFactory.getDatastoreService();
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
FetchOptions fetchOptions = FetchOptions.Builder.withLimit(PAGE_SIZE);
// If this servlet is passed a cursor parameter, let's use it.
String startCursor = req.getParameter("cursor");
if (startCursor != null) {
fetchOptions.startCursor(Cursor.fromWebSafeString(startCursor));
}
Query q = new Query("Person").addSort("name", SortDirection.ASCENDING);
PreparedQuery pq = datastore.prepare(q);
QueryResultList<Entity> results;
try {
results = pq.asQueryResultList(fetchOptions);
} catch (IllegalArgumentException e) {
// IllegalArgumentException happens when an invalid cursor is used.
// A user could have manually entered a bad cursor in the URL or there
// may have been an internal implementation detail change in App Engine.
// Redirect to the page without the cursor parameter to show something
// rather than an error.
resp.sendRedirect("/people");
return;
}
resp.setContentType("text/html");
resp.setCharacterEncoding("UTF-8");
PrintWriter w = resp.getWriter();
w.println("<!DOCTYPE html>");
w.println("<meta charset="utf-8">");
w.println("<title>Cloud Datastore Cursor Sample</title>");
w.println("<ul>");
for (Entity entity : results) {
w.println("<li>" + entity.getProperty("name") + "</li>");
}
w.println("</ul>");
String cursorString = results.getCursor().toWebSafeString();
// This servlet lives at '/people'.
w.println("<a href='/people?cursor=" + cursorString + "'>Next page</a>");
}
}