我有一个非常简单的表，其中包含2列，message_id和server_timestamp，但是当我查看此查询的计划时：
i+1其中index_messages_id_server_timestamp是（message_id，server_timestamp）上的索引。
为什么此查询需要使用临时b树进行排序？
答案 0 :(得分：1)
由于GROUP BY，表的多行可能会在输出中产生一行。这打破了import UIKit
import AVFoundation
class ViewController: UIViewController {
let captureSession = AVCaptureSession()
let stillImageOutput = AVCaptureStillImageOutput()
var error: NSError?
override func viewDidLoad() {
super.viewDidLoad()
let devices = AVCaptureDevice.devices().filter{ $0.hasMediaType(AVMediaTypeVideo) && $0.position == AVCaptureDevicePosition.Back }
if let captureDevice = devices.first as? AVCaptureDevice {
captureSession.addInput(AVCaptureDeviceInput(device: captureDevice, error: &error))
captureSession.sessionPreset = AVCaptureSessionPresetPhoto
captureSession.startRunning()
stillImageOutput.outputSettings = [AVVideoCodecKey:AVVideoCodecJPEG]
if captureSession.canAddOutput(stillImageOutput) {
captureSession.addOutput(stillImageOutput)
}
if let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession) {
previewLayer.bounds = view.bounds
previewLayer.position = CGPointMake(view.bounds.midX, view.bounds.midY)
previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
let cameraPreview = UIView(frame: CGRectMake(0.0, 0.0, view.bounds.size.width, view.bounds.size.height))
cameraPreview.layer.addSublayer(previewLayer)
cameraPreview.addGestureRecognizer(UITapGestureRecognizer(target: self, action:"saveToCamera:"))
view.addSubview(cameraPreview)
}
}
}
func saveToCamera(sender: UITapGestureRecognizer) {
if let videoConnection = stillImageOutput.connectionWithMediaType(AVMediaTypeVideo) {
stillImageOutput.captureStillImageAsynchronouslyFromConnection(videoConnection) {
(imageDataSampleBuffer, error) -> Void in
let imageData = AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(imageDataSampleBuffer)
UIImageWriteToSavedPhotosAlbum(UIImage(data: imageData), nil, nil, nil)
}
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
}
和message_id值之间的关系，因此不再可能证明使用索引对它们进行排序是有效的。