Judge, Jury, and Executioner. What happens when you run a query and how to read all about it.

Transcription

1 Judge, Jury, and Executioner What happens when you run a query and how to read all about it.

2 Por favor silenciar los Teléfonos Celulares

3 What s the big deal? Talk about how a query is processed and gets results back Common Operators Performance Detectives Not a here s how to fix a query session.

4 PASS La oferta de capacitación más completa Evento Mundial Evento Mundial Grupos Locales en todo Mundo Eventos On Line Gratuitos de SQL Server y BI Evento Gratuito de Día Completo Evento Regional Grabaciones de Sesiones Newsletter PASS Capacitación Técnica Virtual Gratuita

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6 Evaluaciones de Sesión

7 @carloslchacon

8 So you want to run a query? SELECT SOD.SalesOrderID, P.ProductID, P.name, SOD.OrderQty FROM Sales.SalesOrderDetail AS SOD INNER JOIN Production.Product P ON SOD.ProductID = P.ProductID WHERE SOD.OrderQty > 38

9 Ready for Inspection Have I seen this query before? No Yes Off to the query Optimizer and compile a plan to execute No Do I have a valid plan for this query? Yes Store the plan/update the stats for the plan Grab the existing plan and execute

10 SQL Server Optimizer

11 Estimate VS Actual

12 Play Detective

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14 Image courtesy of Richard Douglas

15 Table Scans only occur against heap tables, tables without clustered indexes.

16 This operation occurs when a Seek against the clustered index or some other index, can't satisfy the needs of the query. In that event, SQL Server must walk through, i.e. scan, the entire data set.

17 A Clustered Index Seek operator occurs when a query uses the index to access only one row, or a few contiguous rows. It's one of the faster ways to retrieve data from the system. A WHERE clause limits the amount of data being returned, which makes it more likely that indexes will be used to return the appropriate data.

18 Scan vs Seek Over simplification

19 A Key Lookup operator (there are two, RID and Key) is required to get data from the heap or the clustered index, respectively, when a non-clustered index is used, but is not a covering index.

20 Covering Index? SELECT SOD.SalesOrderID, P.ProductID, P.name, P.Manufacturer A covering index is query specific FROM Sales.SalesOrderDetail AS SOD INNER JOIN Production.Product P ON SOD.ProductID = P.ProductID WHERE SOD.OrderQty > 38

21 Indexes on Production.Product

22 A Nested Loops join functions by taking a set of data, referred to as the outer set, and comparing it, one row at a time to another set of data, called the inner set. This sounds like a cursor, and effectively, it is one but, with the appropriate data set, it can be a very efficient operation.

23 A Hash Match operator appears in the plan when SQL Server puts two data sets into temporary tables, hash tables, and then uses these structures to compare data and arrive at the matching set.

24 A spool reads the data and saves it in TempDB. This process is used whenever the Optimizer knows that the density of the column is high and the intermediate result is very complex to calculate.

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29 SET STATISTICS TIME ON; SET STATISTICS IO ON; Statistics Time returns information relating to the cpu time and elapsed time for execution as well as parse and compile times. Statistics IO returns information relating to the amount of disk activity generate by the t-sql statement.

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35 Dynamic Management Views Sys.dm_exec_query_stats Sys.dm_exec_procedure_stats Sys.dm_exec_cached_plans Sys.dm_exec_query_optimizer_info Sys.dm_exec_sql_text Sys.dm_exec_query_plan Sys.dm_exec_text_query_plan Sys.dm_exec_plan_attributes

36 Query Plans

37 SQLCruise

38 Plan Explorer

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41 Looking for High Estimated Costs

42 Top Operators

43 Thick Lines

44 Performance Tuning Get the actual numbers Are estimates far from actuals? Look for low-hanging fruit changes that can be made without affecting code Make a change and compare the result

45 Plan Cache is in memory Volatile and not persistent Past performance is not an indication of future results Parameterization Execution Plans are only as good as your stats Table Variables

46 References SQL Server Execution Plans By Grant Fritchey Adam Machanic 2014 TechEd Session hed/northamerica/2014/dbi- B489#fbid=

47 Questions? Husbands Love Your Wives Wives Nurture Your Children Children Honor Your Parents