INTRODUCTION TO INDEXES AND SARGABILITY

Transcription

1 INTRODUCTION TO INDEXES AND SARGABILITY Jared

2 WHAT ARE INDEXES AND WHY DO WE CARE? Database objects we create Make queries run faster Make server run better Reduce locking and blocking

3 WHAT KINDS OF INDEXES ARE THERE? Clustered Non-Clustered Full-Text (Not discussed today) Spatial (Not discussed today) Columnstore (Not discussed today)

4 CLUSTERED INDEXES Clustered Index (CI) Table/Data Can only be 1 per table (See above) A table without a CI is a heap Clustered Key Column(s) the table is ordered by Can be more than 1

5 CI PSEUDO-EXAMPLES 1. CREATE TABLE WhitePages (LastName, FirstName, Street, Address, City, State, PhoneNumber) 2. CREATE CLUSTERED INDEX ON WhitePages (LastName, FirstName ASCENDING) Data is logically ordered by LastName and FirstName The combination of these 2 values is called the clustered key How is data retrieved? (ORDER BY)

6 DEMO

7 NON-CLUSTERED INDEXES Non-Clustered Index (NCI) Stored in a structure separate from the rows of data, but part of the table Can have more than 1 (up to 999) Contain a copy of the clustered key Non-Clustered Index Key Columns that the index is ordered by Maximum of 16 key columns Included columns Columns that are in the index, but no particular order Along for the ride

8 NCI PSEUDO-EXAMPLES 1. CREATE NONCLUSTERED INDEX ON WhitePages (Address ASCENDING) 2. CREATE NONCLUSTERED INDEX ON WhitePages (FirstName, LastName) INCLUDE(PhoneNumber) Each non-clustered index has to have a way to find its row(s) in the table, so a copy of the clustered key is stored with each entry in the non-clustered index; Key Lookup Only the data specified is stored in the non-clustered index

9 DEMO

10 PROPERTIES OF A GOOD CI Narrow or Small Unique Unchanging Ever increasing

11 SEEKS AND SCANS A Table Scan or Index Scan means that the query has to read every row in the table or index A Seek means that the query can use an index to pinpoint the row(s) of data

12 SARGABILITY (CAN I USE AN INDEX?) SARGable refers to whether or not an index can be used to satisfy all or part of a query Things that are not SARGable Function in a WHERE clause on the column being looked at LIKE operator in WHERE clause that starts with a wildcard (%) Some predicates are SARGable, but will not work without the correct index; searching on a predicate of the index that is not the first.

13 SARGABILITY EXAMPLES NOT SARGable WHERE YEAR(createdDate) = 2013 WHERE ISNULL(ID, ) = WHERE DATEDIFF(mm, UpdatedDate, GETDATE()) > = 30 SARGable WHERE createddate >= AND createddate < WHERE ID = OR ID IS NULL WHERE UpdatedDate < DATEADD(mm,-30, GETDATE())

14 COVERING INDEXES 1. Contains all predicates that are being filtered on 2. Includes all columns needed for the result set Ex. I need the StreetAddress, City, State and PhoneNumber, but I am searching by City only. CREATE NONCLUSTERED INDEX ix_whitepages_city ON WhitePages (City) INCLUDE (StreetAddress, State, PhoneNumber)

15 DEMO

16 TEST WHERE LastName = Ross WHERE FirstName = Jennifer WHERE StreetAddress LIKE 4% WHERE LastName LIKE %s WHERE FirstName LIKE J% WHERE PostalCode > WHERE SUBSTRING(LastName, 1, 1) = J WHERE FirstName LIKE J%D

17 DUPLICATE OR REDUNDANT INDEXES CREATE INDEX IX_WhitePages_FirstName ON WhitePages (FirstName ASC) CREATE INDEX IX_WhitePages_FirstName_LastName ON WhitePages(FirstName ASC, LastName ASC) CREATE INDEX IX_WhitePages_FirstName_LastName_Includes ON WhitePages(FirstName ASC, LastName ASC) INCLUDE (PhoneNumber)

18 SUMMARY Clustered index is the data and there can only be 1 Non-clustered index is separate from the data, but contains a subset of it. Data in indexes is logically ordered by the Index Key A SARGable predicate can use an index to find the data more efficiently using a SEEK Be on the lookout for duplicate or redundant indexes

19 QUESTIONS

20 LINKS!!! Introduction to Indexes Gail Shaw Stairway to SQL Server Indexes or just go to SQLServerCentral.com, click on Stairways, and scroll down to find Stairway to SQL Server Indexes Something cool from Aaron Bertrand

21 THANKS!!! Jared