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Query handlingbytheserver
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- 2. OPTIMIZER’S ROLE • Theoptimizer should generate an efficient plan to access the data that we need to work with.
- 3. WHY DO WENEED IT? • SQL Statement defines what we are looking for. It doesn’t define how to get the data. • The optimizer has to generate a query plan which tells the server how to get the data.
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- 5. NUMBER OF JOINORDERS
- 6. OPTIMIZER • The optimizerdoesn't necessarily look for the best query plan. • The optimizer searches for a good enough plan.
- 7. WORK STAGES (HIGHLEVEL) • Create a logical plan • Apply transformation rules • Apply heuristics • Select the cheapest one
- 8. STEPS OF QUERYPROCESSING
- 9. PARSING • Checks thatthe syntax is correct • Creates a logical tree that represents the query.
- 10. DEMO 1 • Seelogical tree that was produced by parse step.
- 11. BINDIND • Gets thelogical tree that was produced by the parsing step • Makes sure that all objects referenced by the logical tree exist and that the user can see them.
- 12. OPTIMIZATION • The mostimportant step. Generates the query plan that will be executed
- 13. TRIVIAL PLAN • Verysimple queries with very simple logical tree won’t get full optimization • The optimizer will generate a plan that is called a trivial plan • The trivial plan is very cheap to generate and it won’t be inserted into the plan cache
- 14. TRIVIAL PLAN • Ifthe trivial plan’s cost is more then the value that is configured as “cost threshold for parallelism”, the query will get full optimization • You can disable the trivial plan by using trace flag 8757
- 15. SIMPLIFICATION • The OPTIMIZERrewrites the tree to make it more simple. There are few simplification methods: – Constant folding – Domain simplification – Contradiction detection
- 16. DEMO 2 • Simplificationand trivial plan
- 17. OPTIMIZATION LEVELS • Ifno trivial plan was found, SQL Server starts optimizing the query. • SQL Server has 3 stages that are called: – Search 0 (Transaction processing) – Search 1 (Quick plan) – Search 2 (Full optimization)
- 18. OPTIMIZATION LEVELS • Eachlevel has an entry and termination condition. • Termination condition can be good enough plan was found, or to much time passed • Optimization can begin at a low search step and get to a higher search step.
- 19. PROPERTIES • Each nodein the logical tree has properties attached to it. • There are 2 types of properties – Logical properties (Node cost, output columns, Type information and nullability, etc) – Physical properties (Sort order, partition information etc’)
- 20. RULES • The optimizerhas set of more then 350 rules that is using to optimize the query. • The rules help the optimizer to modify the logical tree in a way that doesn’t effect the query’s results • The rules also dictate the physical implementation of the logical tree
- 21. RULES • There arefour types of rules that can be used: – Simplification rules – Exploration rules – Implementation rules – Physical properties enforcement rules
- 22. MEMO • All thetrees are stored in memory in a structure that is called Memo • Each optimization has its own memo • A single memo can get to the size of 1.6 GB
- 23. OPTIMIZATION PROBLEMS • Onrare occasions there can be a timeout for the optimization process • Most times when it happens, it will be on search 2 stage, and the server will use the query plan that was produced on search 1 stage. • Sometime the server stops optimization because of memory pressure
- 24. DMVs • There are2 DMVs that can give us information about the optimizer: – sys.dm_exec_query_optimizer_info, shows information about optimization process – sys.dm_exec_query_transformation_stats, sho ws information about rules usage
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- 26. QUSTIONS? • Additional reading: – http://sqlblog.com/blogs/paul_white/archive/ 2012/04/28/query-optimizer-deep-dive-part- 1.aspx – http://www.red- gate.com/community/books/inside-sql-server- query-optimizer