优化ClickHouse查询：根据PlatformType条件选择计数字段

下面是修改后的代码：

SELECT toStartOfDay(toDateTime(Ds)) AS datetime_day_Ds,
    PlatformType AS PlatformType,
    CASE
        WHEN PlatformType in (3,4) THEN count(DISTINCT UrlContent2)
        ELSE count(DISTINCT ViewContent2)
    END AS index_mergedviewcount
FROM log_iMonkey_Wechat_overview
WHERE 
    Debug = 1 AND 
    PlatformType not IN (0) AND
    Ds >= toDateTime(#day_wechat_views_time.start#) AND
    Ds <= toDateTime(#day_wechat_views_time.end#) + INTERVAL 1 DAY
GROUP BY datetime_day_Ds, PlatformType
ORDER BY datetime_day_Ds DESC, PlatformType DESC

修改后的代码使用了CASE语句来根据PlatformType的值选择不同的计数字段。当PlatformType为3或4时，index_mergedviewcount取值为count(DISTINCT UrlContent2)，否则取值为count(DISTINCT ViewContent2)。