This is day three of the SQL Advent 2012 series of blog posts. Today we are going to look at sargable queries. You might ask yourself, what is this weird term sargable. Sargable comes from searchable argument, sometimes also referred as Search ARGument ABLE. What that means is that the query will be able to use an index, a seek will be performed instead of a scan. In general any time you have a function wrapped around a column, an index won’t be used
Some examples that are not sargable
WHERE LEFT(Name,1) = 'S'
WHERE Year(SomeDate) = 2012
WHERE OrderID * 3 = 33000
Those three should be rewritten like this in order to become sargable
WHERE Name LIKE 'S%'
WHERE SomeDate >= '20120101' AND SomeDate < '20130101'
WHERE OrderID = 33000/3
Let’s create a table, insert some data so that we can look at the execution plan
Create this simple table
CREATE TABLE Test(SomeID varchar(100))
Let’s insert some data that will start with a letter followed by some digits
INSERT Test
SELECT LEFT(v2.type,1) +RIGHT('0000' + CONVERT(varchar(4),v1.number),4)
FROM master..spt_values v1
CROSS JOIN (SELECT DISTINCT LEFT(type,1) AS type
FROM master..spt_values) v2
WHERE v1.type = 'p'
That insert should have generated 32768 rows
Now create this index on that table
CREATE CLUSTERED INDEX cx_test ON Test(SomeID)
Let’s take a look at the execution plan, hit CTRL + M, this will add the execution plan once the query is done running
SELECT * FROM Test
WHERE SomeID LIKE 's%'
SELECT * FROM Test
WHERE LEFT(SomeID,1) = 's'
Here is what the plans looks like
As you can see it is 9% versus 91% between the two queries, that is a big difference
Hit CTRL + M again to disable the inclusion of the plan
Run this codeblock, it will give you the plans in a text format
SET SHOWPLAN_TEXT ON
GO
SELECT * FROM Test
WHERE SomeID LIKE 's%'
SELECT * FROM Test
WHERE LEFT(SomeID,1) = 's'
GO
SET SHOWPLAN_TEXT OFF
GO
Here are the two plans
|–Clustered Index Seek(OBJECT:([master].[dbo].[Test].[cx_test]),
SEEK:([master].[dbo].[Test].[SomeID] >= ‘Rþ’ AND [master].[dbo].[Test].[SomeID] < ’T’), WHERE:([master].[dbo].[Test].[SomeID] like ’s%‘) ORDERED FORWARD) |–Clustered Index Scan(OBJECT:([master].[dbo].[Test].[cx_test]), WHERE:(substring([master].[dbo].[Test].[SomeID],(1),(1))=’s’))
As you can see the top one while looking more complicated is actually giving you a seek
Making a case sensitive search sargable
Now let’s take a look at how we can make a case sensitive search sargable as well
In order to do a search and make it case sensitive, you have to have a case sensitive collation, if your table is not created with a case sensitive collation then you can supply it as part of the query
Here is an example to demonstrate what I mean
This is a simple table created without a collation
CREATE TABLE TempCase1 (Val CHAR(1))
INSERT TempCase1 VALUES('A')
INSERT TempCase1 VALUES('a')
Running this select statement will return both rows
SELECT * FROM TempCase1
WHERE Val = 'A'
Val
—–
A
a
Now create the same kind of table but with a case sensitive collation
CREATE TABLE TempCase2 (Val CHAR(1) COLLATE SQL_Latin1_General_CP1_CS_AS)
INSERT TempCase2 VALUES('A')
INSERT TempCase2 VALUES('a')
Run the same query
SELECT * FROM TempCase2
WHERE Val = 'A'
Val
—–
A
As you can see you only get the one row now that matches the case
SELECT * FROM TempCase1
WHERE Val = 'A' COLLATE SQL_Latin1_General_CP1_CS_AS
Val
—–
A
a
Now let’s take a look at how we can make the case sensitive search sargable
First create this table and insert some data
CREATE TABLE TempCase (Val CHAR(1))
INSERT TempCase VALUES('A')
INSERT TempCase VALUES('B')
INSERT TempCase VALUES('C')
INSERT TempCase VALUES('D')
INSERT TempCase VALUES('E')
INSERT TempCase VALUES('F')
INSERT TempCase VALUES('G')
INSERT TempCase VALUES('H')
Now we will insert some lowercase characters
INSERT TempCase
SELECT LOWER(Val) FROM TempCase
Now we will create our real table which will have 65536 rows
CREATE TABLE CaseSensitiveSearch (Val VARCHAR(50))
We will do a couple of cross joins to generate the data for our queries
INSERT CaseSensitiveSearch
SELECT t1.val + t2.val + t3.val + t4.val
FROM TempCase t1
CROSS JOIN TempCase t2
CROSS JOIN TempCase t3
CROSS JOIN TempCase t4
Create an index on the table
CREATE INDEX IX_SearchVal ON CaseSensitiveSearch(Val)
Just like before, if we run this we will get back the exact value we passed in and also all the upper case and lower case variations
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD'
Here are the results of that query
Val
—–
AbCd
ABcd
Abcd
ABCd
aBCd
abCd
aBcd
abcd
abCD
aBcD
abcD
aBCD
ABCD
AbCD
ABcD
AbcD
If you add the collation to the query, you will get only what matches your value
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
Here is the result, it maches what was passed in
Val
—
ABCD
The problem with the query above is that it will cause a scan. So what can we do, how can we make it perform better? It is simple combine the two queries
First grab all case sensitive and case insensitive values and then after that filter out the case insensitive values
Here is what that query will look like
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
AND Val LIKE 'ABCD'
AND Val LIKE ‘ABCD’ will result in a seek, now when it also does the Val = ‘ABCD’ COLLATE SQL_Latin1_General_CP1_CS_AS part, it only returns the row that matches your value
If you run both queries, you can look at the plan difference (hit CTRL + M so that the plan is included)
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
AND Val LIKE 'ABCD'
Here is the plan
As you can see, there is a big difference between the two
Here is the plan in text as well
SET SHOWPLAN_TEXT ON
GO
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
SELECT * FROM CaseSensitiveSearch
WHERE Val = 'ABCD' COLLATE SQL_Latin1_General_CP1_CS_AS
AND Val LIKE 'ABCD'
GO
SET SHOWPLAN_TEXT OFF
GO
|–Table Scan(OBJECT:([tempdb].[dbo].[CaseSensitiveSearch]),
WHERE:(CONVERT_IMPLICIT(varchar(50),[tempdb].[dbo].[CaseSensitiveSearch].[Val],0)=CONVERT(varchar(8000),[@1],0)))
|–Index Seek(OBJECT:([tempdb].[dbo].[CaseSensitiveSearch].[IX_SearchVal]), SEEK:([tempdb].[dbo].[CaseSensitiveSearch].[Val] >= ‘ABCD’
AND [tempdb].[dbo].[CaseSensitiveSearch].[Val] <= ‘ABCD’), WHERE:(CONVERT_IMPLICIT(varchar(50),[tempdb].[dbo].[CaseSensitiveSearch].[Val],0)=‘ABCD’ AND [tempdb].[dbo].[CaseSensitiveSearch].[Val] like ‘ABCD’) ORDERED FORWARD)
Also take a look at Only In A Database Can You Get 1000% + Improvement By Changing A Few Lines Of Code to see how this works with dates
That is all for day three of the SQL Advent 2012 series, come back tomorrow for the next one, you can also check out all the posts from last year here: SQL Advent 2011 Recap
Denis has been working with SQL Server since version 6.5. Although he worked as an ASP/JSP/ColdFusion developer before the dot com bust, he has been working exclusively as a database developer/architect since 2002. In addition to English, Denis is also fluent in Croatian and Dutch, but he can curse in many other languages and dialects (just ask the SQL optimizer) He lives in Princeton, NJ with his wife and three kids.
