JOIN对多列时使用索引

Question

简化,我有两个表,contacts和donotcall

CREATE TABLE contacts
(
    id int PRIMARY KEY,
    phone1 varchar(20) NULL,
    phone2 varchar(20) NULL,
    phone3 varchar(20) NULL,
    phone4 varchar(20) NULL
);
CREATE TABLE donotcall
(
    list_id int NOT NULL,
    phone varchar(20) NOT NULL
);
CREATE NONCLUSTERED INDEX IX_donotcall_list_phone ON donotcall
(
    list_id ASC,
    phone ASC
);

我想看看哪些联系人与DoNotCall手机的特定列表中的电话号码相匹配.为了更快地查找,我已经编入索引donotcall的list_id和phone.

当我进行以下JOIN时需要很长时间(例如9秒):

SELECT DISTINCT c.id
FROM contacts c
JOIN donotcall d
    ON d.list_id = 1
    AND d.phone IN (c.phone1, c.phone2, c.phone3, c.phone4)  

Pastebin的执行计划

如果我在每个手机领域单独LEFT JOIN,它运行得更快(例如1.5秒):

SELECT c.id
FROM contacts c
LEFT JOIN donotcall d1
    ON d1.list_id = 1
    AND d1.phone = c.phone1
LEFT JOIN donotcall d2
    ON d2.list_id = 1
    AND d2.phone = c.phone2
LEFT JOIN donotcall d3
    ON d3.list_id = 1
    AND d3.phone = c.phone3
LEFT JOIN donotcall d4
    ON d4.list_id = 1
    AND d4.phone = c.phone4
WHERE
    d1.phone IS NOT NULL
    OR d2.phone IS NOT NULL
    OR d3.phone IS NOT NULL
    OR d4.phone IS NOT NULL

Pastebin的执行计划

我的假设是第一个代码片段运行缓慢,因为它没有使用索引donotcall.
那么,如何对多列进行连接并仍然使用索引？

Answer 1

SQL Server可能认为IN (c.phone1, c.phone2, c.phone3, c.phone4)使用索引进行解析太昂贵了.

您可以使用提示测试索引是否更快:

SELECT c.*
FROM contacts c
JOIN donotcall d with (index(IX_donotcall_list_phone))
    ON d.list_id = 1
    AND d.phone IN (c.phone1, c.phone2, c.phone3, c.phone4)

根据您发布的查询计划,它显示第一个计划估计会产生4万行,但它只返回21行.第二个计划估计1排(当然也会返回21排).

您的统计信息是最新的吗？过时的统计数据可以解释查询分析器做出错误选择的原因.统计数据应自动更新或每周更新一次.使用以下方法检查统计信息的年龄:

select  object_name(ind.object_id) as TableName
,       ind.name as IndexName
,       stats_date(ind.object_id, ind.index_id) as StatisticsDate
from    sys.indexes ind
order by 
        stats_date(ind.object_id, ind.index_id) desc

您可以手动更新它们:

EXEC sp_updatestats;