为什么INTERSECT和嵌套的JOIN一样慢？

Question

我正在使用MS SQL.

我有一个巨大的表与索引来快速查询:

select userid from IncrementalStatistics where
IncrementalStatisticsTypeID = 5 and
IncrementalStatistics.AssociatedPlaceID = 47828 and
IncrementalStatistics.Created > '12/2/2010

它在不到1秒的时间内返回.该表有数十亿行.只有大约10000个结果.

我希望这个查询也能在大约一秒钟内完成:

select userid from IncrementalStatistics where
IncrementalStatisticsTypeID = 5 and
IncrementalStatistics.AssociatedPlaceID = 47828 and
IncrementalStatistics.Created > '12/2/2010'

intersect

select userid from IncrementalStatistics where
IncrementalStatisticsTypeID = 5 and
IncrementalStatistics.AssociatedPlaceID = 40652 and
IncrementalStatistics.Created > '12/2/2010'

intersect

select userid from IncrementalStatistics where
IncrementalStatisticsTypeID = 5 and
IncrementalStatistics.AssociatedPlaceID = 14403 and
IncrementalStatistics.Created > '12/2/2010'

但它需要20秒.所有单个查询都需要<1秒,并返回大约10k的结果.

我希望SQL内部将每个子查询的结果抛出到哈希表中并进行哈希交集 - 应该是O(n).结果集足够大以适应内存,因此我怀疑这是一个IO问题.

我编写了一个备用查询,它只是一系列嵌套的JOIN,这也需要大约20秒,这是有道理的.

为什么INTERSECT这么慢？它是否在查询处理的早期阶段缩减为JOIN？

Answer 1

试试这个.显然未经测试,但我认为它会为您提供您想要的结果.

select userid 
    from IncrementalStatistics 
    where IncrementalStatisticsTypeID = 5 
        and IncrementalStatistics.AssociatedPlaceID in (47828,40652,14403)  
        and IncrementalStatistics.Created > '12/2/2010'
    group by userid
    having count(distinct IncrementalStatistics.AssociatedPlaceID) = 3