像在SQL中查询一样花时间

Question

So I've looked around to try to find some posts on this and there are many Like Query 1 and Like Query 2 but none that address my specific question (that I could find).

I have two tables in which I have around 5000000+ records and I am returning Search result from these tables as :

    SELECT A.ContactFirstName, A.ContactLastName
    FROM   Customer.CustomerDetails AS A WITH (nolock)
    WHERE  (A.ContactFirstName + ' ' + A.ContactLastName LIKE '%' + 'a' + '%')
    UNION
    SELECT C.ContactFirstName, C.ContactLastName
    FROM   Customer.Contacts AS C WITH (nolock) 
    WHERE  (C.ContactFirstName + ' ' + C.ContactLastName LIKE '%' + 'a' + '%')

My problem is it is taking around 1 minute to execute.

For above query I am expecting result like :

Please suggest me the best practice to improve performance. Thanks in advance.

NOTE : No missing Indexes.

Answer 1

when you use "LIKE '%xxx%'" index are not used that why your query is slow i think. When you use "LIKE 'xxx%')" index is used (if an index exist on column of course. >Other proble you do a like on concatenante column, i dont knwo if index is used in this case. And why do a 'xxx' + ' ' + 'yyy' like 'z%', just do 'xxx' like 'z%' its the same. You can try to modify your query like this

SELECT A.ContactFirstName, A.ContactLastName
FROM   Customer.CustomerDetails AS A WITH (nolock)
WHERE  A.ContactFirstName LIKE '%a%' or A.ContactLastName LIKE '%a%'
UNION
SELECT C.ContactFirstName, C.ContactLastName
FROM   Customer.Contacts AS C WITH (nolock) 
WHERE  C.ContactFirstName LIKE 'a%'

Answer 2

Use Charindex which improves performance of the search ,Here it checks the string to match with first charcter of given search charecter and doesn't search for any more matches.

DECLARE @Search VARCHAR(10)='a'
SELECT A.ContactFirstName, A.ContactLastName
    FROM   Customer.CustomerDetails AS A WITH (NOLOCK)
    WHERE  CHARINDEX(@Search,(A.ContactFirstName + ' ' + A.ContactLastName),0)>1