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Indy pass writing efficient queries – part 1 - indexing
1. Writing Efficient Queries – Part 1 Using SQL Server Internals to Improve Data Access Eddie Wuerch MCT, MCITP Principal, Data Management ExactTarget eddie@indydba.com
2. Disk I/O – Key Points Disk I/O (reads and writes) is usually the slowest component of a system Compare: Memory and CPU speeds are reported in GHz: billions of actions per second Disk I/O rates are simply measured in IOPS: Input/output Operations Per Second Fast disks on mostly sequential workloads can get 100-150 IOPS Because of disk access rates, our first tuning goal is to reduce overall I/O …so let’s look at those data pages, then examine how to process less of them
3. Disk I/O – Key Points - II SQL Server is not a black box Most data is in structured storage There are many ways to access the data, some ways are significantly faster – and cause less impact on other processes - than others Understanding SQL Server data storage internals will guide you to the faster ways
4. Data Storage in SQL Server The base unit of SQL Server data storage is the page All data – system and user – is stored in pages Each page is 8KB (8192 bytes) Pages are allocated from files in 64KB (8-page) extents
5. Page Processing Pages are read from disk and processed in memory as an entire 8KB unit Extents are often read in from disk as a single block to reduce I/O All data is processed in memory, pulled from disk first (processing put on hold) if data is not in memory
6. Page Types System Page Types Space Management: File Header, PFS, GAM, SGAM Change Management: DCM, BCM Data Page Types In-row data Index LOB data and Row-overflow data All pages are 8KB
12. Disk Access Methods Think of a phone book, with each entry as a record Ordered by Last Name, First Name, MI Two ways to find a record: Use Last Name, First Name to find a number (Index Seek) Look through the entire phone book, one page at a time, scanning each row for data (Table Scan)
13. Clustered Index Represents the table itself Index specifies the physical ordering of that data Only 1 allowed per table May be unique, does not have to be the primary key Non-clustered index Additional index of data Over 200 allowed May be unique Index Types
14. The phone book example If a table has a clustered index, the pointer to each row in the table is the clustered index key The leaf level of the nonclustered index contains the nonclustered keys and the clustered index keys Nonclustered indexes may also include additional non-indexed columns, will be stored at the leaf level of the index
15. Index Pages A-K : Page 4 L-U : Page 5 V-Z : Page 6 4 5 6 A : Page 22 B : Page 23 C : Page 24 D…. Baa : Page 276 Baba : Page 277 Base : Page 278 Ba… 22 23 24 25 26 27 274 275 276 277 278 279 280 281 282 283
21. One trip through the other table for each entry in the first table
22. Generally the slowest of the three typesOne trip through each table Requires indexes on both sides, at least one of them must be unique Usually the fastest join type Works well for very large joins Builds join data in tempdb
23. Join and Indexing Tips When defining an index, if the data is unique, then declare the index as unique Join on keys Provide arguments in WHERE clauses to match available indexes Cluster tables on range scans Look for covering indexes
26. So How Do I Know? sys.dm_db_index_usage_stats User_seeks User_scans User_lookups User_updates Sys.dm_db_missing_index_* Not magic, has limitations Many similar index entries with different INCLUDE statements may indicate a need to revisit the clustered index design
27. So How Do I Know? Scan-indicating waits Lots of PAGEIOLATCH_SH and PAGEIOLATCH_EX waits are generated by tables scans that read from disk CX_PACKET waits – related to parallellism often caused by scanning large tables (don’t reduce MAXDOP: fix the scan!) Other processes with SOS_SCHEDULER_YIELD or high signal wait times may be mitigated by reducing CPU load of scans
28. So How Do I Know? TempDB activity in instances without much use of temp tables or table variables SELECT * FROM sys.dm_io_virtual_file_stats(DB_ID(‘TempDB’), NULL) Must track over time, perform time-slice analysis May indicate additional worktable sort and hash-match activity Tracking this for all of your databases shows the amount of I/O your systems are performing, and if the disk systems are keeping up
29. Resources Microsoft White Papers SQL Server 2000 I/O Basics (http://technet.microsoft.com/en-us/library/cc966500.aspx) SQL Server I/O Basics, Chapter 2 (http://technet.microsoft.com/en-us/library/cc917726.aspx) SQL Server Waits and Queues (download) (http://technet.microsoft.com/en-us/library/cc966413.aspx) The Waits and Queues document is highly recommended tuning or analyzing workloads
30. Resources Inside SQL Server Book Series SQL 2005 The Storage Engine (Kalen Delaney) Query Tuning and Optimization (Delany, et. al.) T-SQL Querying (Ben-Gan, Kollar, Sarka) SQL 2008 Microsoft SQL Server 2008 Internals (Delaney, Randal, Tripp, Cunningham) T-SQL Querying (Ben-Gan, Kollar, Sarka)
