How to reduce image descriptor dimension to speed up image retrival [closed]

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This is a image retrival example, get image sift descritprs which is n rows 128 cols, then call flann function to match, I want to speed it up, so i use PCA to reduce vector dimension such as 64 cols, but after PCA it can't find any match, Is then PCA code is wrong or can't call flann function to match after PCA processing

code:

#include <stdio.h>
#include <unistd.h>
#include <iostream>

#include <opencv2 opencv.hpp="">
#include <opencv2 xfeatures2d.hpp="">

using namespace cv;
using namespace cv::xfeatures2d;
using namespace std;

Mat PcaDecColumn(const Mat &src, int dstColNum)
{
    Mat beforePCA = src.clone();

    PCA pca(beforePCA, Mat(), CV_PCA_DATA_AS_ROW, dstColNum);
    Mat afterProject = pca.project(beforePCA);
    //Mat dst = pca.backProject(afterProject);
    Mat dst = afterProject;
    return dst;
}

    int
main( int argc, char **argv )
{
    cv::Mat queryImage = cv::imread(argv[1]);
    cv::Mat frameImage = cv::imread(argv[2]);
    if (frameImage.empty() || queryImage.empty())
    {
        printf( "%s %d err\n", __func__, __LINE__ );
        return -1;
    }

    vector<keypoint> queryKeypoints;
    vector<keypoint> frameKeypoints;
    Mat queryDescriptors;
    Mat frameDescriptors;

    //Ptr<orb> orb = ORB::create(1000);
    Ptr<feature2d> orb = xfeatures2d::SIFT::create();
    orb->detectAndCompute(queryImage, noArray(), queryKeypoints, queryDescriptors);
    orb->detectAndCompute(frameImage, noArray(), frameKeypoints, frameDescriptors);

    queryDescriptors = PcaDecColumn(queryDescriptors, 64);
    frameDescriptors = PcaDecColumn(frameDescriptors, 64);

    if (frameDescriptors.empty() || queryDescriptors.empty())
    {
        printf( "%s %d err\n", __func__, __LINE__ );
        return -1;
    }

    vector<dmatch> matches;
    vector<vector<dmatch> > totalMatches;
    FlannBasedMatcher matcher;
    matcher.knnMatch(frameDescriptors, queryDescriptors, totalMatches, 2);

    const float minRatio = 1.f / 1.5f;
    for (size_t i = 0; i < totalMatches.size(); i++) {
        const DMatch& bestMatch = totalMatches[i][0];
        const DMatch& betterMatch = totalMatches[i][1];

        float  distanceRatio = bestMatch.distance / betterMatch.distance;
        if (distanceRatio < minRatio)
            matches.push_back(bestMatch);
    }

    printf( "matches size:%d\n", matches.size() );

    Mat img_match;
    drawMatches(frameImage, frameKeypoints, queryImage, queryKeypoints, matches, img_match);
    imshow("match", img_match);
    waitKey(0);

    return 0;
}