索引也是需要时间的

   以前编程的时候，对于索引的计算时间基本忽略，但是当数据量很大的时候，这些索引计算的时间累加起来也是很大的时间开销,下面用实例分析

#include <iostream>
#include <time.h>
#include <map>

using namespace std;

struct KillEvent
{
    KillEvent()
        :killerId(0)
        ,bekillerId(0)
    {
    }
    KillEvent(int killerid, int bekillerid)
        :killerId(killerid)
        ,bekillerId(bekillerid)
    {
    }

    int killerId;
    int bekillerId;
};

struct PlayerScore
{
    PlayerScore()
        :userId(0)
        ,score1(0)
        ,score2(0)
        ,score3(0)
    {
    }

    PlayerScore(int userid)
        :userId(userid)
        ,score1(0)
        ,score2(0)
        ,score3(0)
    {
    }

    int userId;
    int score1;
    int score2;
    int score3;
};

map<int, KillEvent>        g_killEvent;
map<int, PlayerScore>    g_playerScoreList;

int main()
{
    clock_t start, end;

    for(int i = 0; i < 10000; ++i)
    {
        g_playerScoreList[i] = PlayerScore(i);
    }

    for(int i = 0; i < 1000000; ++i)
    {
        g_killEvent[i] = KillEvent(i,(i + 100) %1000000);
    }

    start = clock();

    for(int i = 0; i < g_killEvent.size(); ++i)
    {
        g_playerScoreList[g_killEvent[i].killerId].score1 += 10;
        g_playerScoreList[g_killEvent[i].killerId].score2 += 5;
        g_playerScoreList[g_killEvent[i].killerId].score3 += 3;

        /*int index = g_killEvent[i].killerId;

        g_playerScoreList[index].score1 += 10;
        g_playerScoreList[index].score2 += 5;
        g_playerScoreList[index].score3 += 3;*/
    }

    end = clock();

    cout << "total use " << (end - start) /CLOCKS_PER_SEC << " s" << endl;

    system("pause");

    return 0;
}

直接使用索引的结果是

：total use 45 s

将索引计算的结果保存下来的计算结果是

：total use 34 s

所以在一些大数据量处理的时候，我们千万不要忽略索引本身的计算开销