优化学习01-尽量不在循环里进行查询

背景：BI项目，查询导购的等级，前端访问调接口太慢了，排查了其他sql，发现是自己把查询放在循环里的原因

错误示范：

修改方案：

//最终的结果
        List<ResGuiderMonthSaleDTO> result = new ArrayList<>();
        //个人
        if (Objects.equals(groupType, "personal")) {
            //月销售
            result = biGuiderSaleMapper.getMonthSalePersonal(101L, startDate, endDate,lastStartDate, groupId, personalName,year,month,lastyear,lastmonth);
            //匹配粉丝数和日净销售
            List<ResFanNumDTO> fanNums = biGuiderSaleMapper.getFanNums(101L, startDate, endDate,lastStartDate, "guiderId");
            List<BiGetValueDTO> daySalePrice = biGuiderSaleMapper.getDaySalePrice(101L, startDate, endDate, "guiderId");
            //查询导购等级
            List<BiGetValueDTO> allGuiderLevel = this.biGuiderSaleMapper.getAllGuiderLevel();
            //根据id分
            Map<Long, ResFanNumDTO> fanNumsMap = fanNums.stream()
                    .collect(Collectors.toMap(ResFanNumDTO::getId, Function.identity()));
            Map<Long, BiGetValueDTO> daySalePriceMap = daySalePrice.stream()
                    .collect(Collectors.toMap(BiGetValueDTO::getId, Function.identity()));
            Map<Long, BiGetValueDTO> guiderLevelMap = allGuiderLevel.stream()
                    .collect(Collectors.toMap(BiGetValueDTO::getId, Function.identity()));
            for (ResGuiderMonthSaleDTO one : result) {
                if (fanNumsMap.containsKey(one.getGuiderId())) {
                    one.setTotalFanNums(fanNumsMap.get(one.getGuiderId()).getTotalFanNums());//总粉丝数 endDate之前加的所有好友
                    one.setLastTotalFanNums(fanNumsMap.get(one.getGuiderId()).getLastTotalFanNums());//上期总粉丝数
                    one.setLastLastTotalFanNums(fanNumsMap.get(one.getGuiderId()).getLastLastTotalFanNums());//环比上期总粉丝数
                    one.setFriendsCount(fanNumsMap.get(one.getGuiderId()).getFriendsCount());//总好友数 当月及当月之前加的好友
                    one.setTotalAddSum(fanNumsMap.get(one.getGuiderId()).getTotalAddSum());//总加粉数 当月一共加了多少粉丝
                }
                if (daySalePriceMap.containsKey(one.getGuiderId())) {
                    one.setDaySalePrice(daySalePriceMap.get(one.getGuiderId()).getDecimal());
                }
                if (guiderLevelMap.containsKey(one.getGuiderId())) {
                    one.setGuiderLevelName(guiderLevelMap.get(one.getGuiderId()).getValue());
                }
            }
        }

 总结：尽量减少访问数据库的请求，导购数据本就不多，几十条记录，一次性把数据全拿出来，把对数据的处理放到内存中，会快很多。