CS 4432query processing - lecture 121 CS4432: Database Systems II Lecture #12 Query Processing Professor Elke A. Rundensteiner

CS 4432query processing - lecture 122 Query Processing Query in SQL  Query Plan in Algebra

CS 4432query processing - lecture 123 Example Data: relation R (A, B, C) relation S (C, D, E) Query: SELECT B, D FROM R, S WHERE R.A = “c” and S.E = 2 and R.C=S.C

CS 4432query processing - lecture 124 RABC S CDE a11010x2 b12020y2 c21030z2 d23540x1 e34550y3 AnswerB D 2 x

CS 4432query processing - lecture 125 How do we execute query? - Form Cartesian product of all tables in FROM-clause - Select tuples that match WHERE-clause - Project columns that occur in SELECT-clause One idea

CS 4432query processing - lecture 126 Relational Algebra - to describe plan Ex: Plan I  B,D  R.A =“c”  S.E=2  R.C=S.C  X RS OR:  B,D [  R.A=“c”  S.E=2  R.C = S.C (RXS)]

CS 4432query processing - lecture 127 Another idea:  B,D  R.A = “c”  S.E = 2 R S Plan II natural join

CS 4432query processing - lecture 128 R S A B C  ( R )  ( S ) C D E a 1 10 A B C C D E 10 x 2 b 1 20c x 2 20 y 2 c y 2 30 z 2 d z 2 40 x 1 e y 3

CS 4432query processing - lecture 129 Plan III Use R.A and S.C Indexes (1) Use R.A index to select R tuples with R.A = “c” (2) For each R.C value found, use S.C index to find matching tuples (3) Eliminate S tuples S.E  2 (4) Join matching R,S tuples, project B,D attributes and place in result

CS 4432query processing - lecture 1210 R S A B C C D E a x 2 b y 2 c z 2 d x 1 e y 3 AC I1I1 I2I2 =“c” check=2? output: next tuple:

CS 4432query processing - lecture 1211 Overview of Query Optimization

CS 4432query processing - lecture 1212 parse convert apply laws estimate result sizes consider physical plans estimate costs pick best execute {P1,P2,…..} {(P1,C1),(P2,C2)...} Pi answer SQL query parse tree logical query plan “improved” l.q.p l.q.p. +sizes statistics

CS 4432query processing - lecture 1213 Example: SQL query SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE birthdate LIKE ‘%1960’ ); (Find the movies with stars born in 1960)

CS 4432query processing - lecture 1214 Example: Parse Tree SELECT FROM WHERE IN title StarsIn ( ) starName SELECT FROM WHERE LIKE name MovieStar birthDate ‘%1960’

CS 4432query processing - lecture 1215 Example: Generating Relational Algebra  title  StarsIn IN  name  birthdate LIKE ‘%1960’ starName MovieStar Fig. 7.15: An expression using a two-argument , midway between a parse tree and relational algebra

CS 4432query processing - lecture 1216 Example: Logical Query Plan  title  starName=name StarsIn  name  birthdate LIKE ‘%1960’ MovieStar Fig. 7.18: Applying the rule for IN conditions 

CS 4432query processing - lecture 1217 Example: Improved Logical Query Plan  title starName=name StarsIn  name  birthdate LIKE ‘%1960’ MovieStar Fig. 7.20: An improvement on fig Question: Push project to StarsIn?

CS 4432query processing - lecture 1218 Example: Estimate Result Sizes Need expected size StarsIn MovieStar 

CS 4432query processing - lecture 1219 Example: One Physical Plan Parameters: join order, memory size, project attributes,... Hash join SEQ scanindex scan Parameters: Select Condition,... StarsInMovieStar

CS 4432query processing - lecture 1220 Example: Estimate costs L.Q.P P1 P2 …. Pn C1 C2 …. Cn Pick best!

CS 4432query processing - lecture 1221 Query Optimization Relational algebra level (NEXT) … Detailed query plan level –Estimate Costs without indexes with indexes –Generate and compare plans