Sql Server query taking too much time - sql-server

Question

Below query execution takes 45 secs. How to improve the speed?
SELECT
u.i_UserID,
u.vch_LoginName,
ea.vch_EmailAddress,
u.vch_DisplayName,
'Current' As 'vch_RecordStatus'
FROM
tblUser u
LEFT JOIN [User].dbo.tblEmailAddress ea
ON u.i_EmailAddressID = ea.i_EmailAddressID
WHERE
IsNull(u.vch_LoginName, '') Like '%'
AND u.vch_DisplayName LIKE 'kala' -- 14 secs
UNION ALL -- 29 secs
SELECT
DISTINCT l.i_UserID
,l.vch_OldLoginName as vch_LoginName
,l.vch_OldEmailAddress as vch_EmailAddress
,u.vch_DisplayName as vch_DisplayName,
'Old' As 'vch_RecordStatus'
FROM tblUserStatusLog l
INNER JOIN tblUser u on u.i_UserID = l.i_UserID
WHERE
l.vch_OldLoginName Like '%'
AND u.vch_DisplayName LIKE 'kala' -- 28 secs
ORDER BY
u.vch_LoginName

Answer 1

First of all, why are you using LIKE to compare strings? LIKE is used to compare partially strings, you should use = operator to search for exact match.
I also don't understand the first condition on both of the where clauses, are you trying to check they are not empty strings and not null? better of use IS NOT NULL and <> operator:
SELECT
u.i_UserID,
u.vch_LoginName,
ea.vch_EmailAddress,
u.vch_DisplayName,
'Current' As 'vch_RecordStatus'
FROM
tblUser u
LEFT JOIN [User].dbo.tblEmailAddress ea ON u.i_EmailAddressID = ea.i_EmailAddressID
WHERE
u.vch_LoginName is not null and u.vch_loginname <> ''
AND u.vch_DisplayName = 'kala'
UNION ALL
SELECT
DISTINCT l.i_UserID
,l.vch_OldLoginName as vch_LoginName
,l.vch_OldEmailAddress as vch_EmailAddress
,u.vch_DisplayName as vch_DisplayName,
'Old' As 'vch_RecordStatus'
FROM tblUserStatusLog l
INNER JOIN tblUser u on u.i_UserID = l.i_UserID
WHERE
l.vch_OldLoginName <> ''
AND u.vch_DisplayName = 'kala'
ORDER BY
u.vch_LoginName
Other then that, I assume you can significantly improve the performance by adding the proper indexes to the tables, but we will need your table structures for that to see what already exists .
But you should have indexes on i_EmailAddressID on first two tables, i_userId on the second ones , and an index on vch_OldLoginName,vch_displayName might also improve the run.

Answer 2

You'd want to take a look at indexing. When you run this query with the actual execution plan switched on do you get any missing query hints? Make sure all indexes are optimised for this query and you'll get much better performance.
You're including the full path for tblEmailAddress, is this stored in another database called 'User'? If so, you'll want to reduce the amount of data you're pulling from the linked server. Again, an index should help you here.
You also don't want to be doing calculations in your where clauses. Check out the SARGability of queries. https://en.wikipedia.org/wiki/Sargable This will tell you why your LIKE statements are a bad idea too.