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23-qo

# 23-qo - Announcements(Thu Dec 1 Query Optimization CPS 116...

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1 Query Optimization CPS 116 Introduction to Database Systems Announcements (Thu. Dec. 1) Extra credit (20 points) due next Tuesday Homework #4 deadline extended—due next Thursday (Dec. 8) Sign up (via email) for a 30-minute slot in the project demo period, Dec. 12-14 Two “public” demo slots available right after final exam Final exam 2-4pm Dec. 13 Open book, open notes Focus on the second half of the course Sample final available 2 3 Query optimization One logical plan ! “best” physical plan Questions How to enumerate possible plans How to estimate costs How to pick the “best” one Often the goal is not getting the optimum plan, but instead avoiding the horrible ones 1 second 1 hour 1 minute Any of these will do 4 Plan enumeration in relational algebra Apply relational algebra equivalences ) Join reordering: £ and ! are associative and commutative (except column ordering, but that is unimportant) ! ! R S T ! ! S R T ! ! R T S = = = 5 More relational algebra equivalences Convert ¾ p - £ to/from ! p : ¾ p ( R £ S ) = R ! p S Merge/split ¾ ’s: ¾ p 1 ( ¾ p 2 R ) = ¾ p 1 Æ p 2 R Merge/split ¼ ’s: ¼ L 1 ( ¼ L 2 R ) = ¼ L 1 R , where L 1 μ L 2 Push down/pull up ¾ : ¾ p Æ pr Æ ps ( R ! p’ S ) = ( ¾ pr R ) ! p Æ p’ ( ¾ ps S ), where pr is a predicate involving only R columns ps is a predicate involving only S columns p and p’ are predicates involving both R and S columns Push down ¼ : ¼ L ( ¾ p R ) = ¼ L ( ¾ p ( ¼ L L’ R )), where L’ is the set of columns referenced by p that are not in L Many more (seemingly trivial) equivalences… Can be systematically used to transform a plan to new ones 6 Relational query rewrite example ¼ title ¾ Student . name =“Bart” Æ Student . SID = Enroll . SID Æ Enroll . CID = Course.CID £ Enroll Course £ Student ¼ title ¾ Enroll . CID = Course.CID £ Enroll Course £ Student ¾ Student . SID = Enroll.SID ¾ Student . name = “Bart” Push down ¾ ¼ title ! Enroll . CID = Course . CID Enroll Course Student ! Student . SID = Enroll . SID ¾ name = “Bart” Convert ¾ p - £ to ! p

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2 7 Heuristics-based query optimization Start with a logical plan Push selections/projections down as much as possible Why? Reduce the size of intermediate results Why not? May be expensive; maybe joins filter better Join smaller relations first, and avoid cross product Why? Reduce the size of intermediate results Why not? Size depends on join selectivity too Convert the transformed logical plan to a physical plan (by choosing appropriate physical operators) 8 SQL query rewrite More complicated—subqueries and views divide a query into nested “blocks” Processing each block separately forces particular join methods and join order Even if the plan is optimal for each block, it may not be
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