1. CS 4432query processing1 CS4432: Database Systems II

2. CS 4432query processing2 Query Processing Query in SQL  Query Plan in Algebra

3. CS 4432query processing3 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

4. CS 4432query processing4 RABC S CDE a11010x2 b12020y2 c21030z2 d23540x1 e34550y3 AnswerB D 2 x SELECT B, D FROM R, S WHERE R.A = “c” and S.E = 2 and R.C=S.C

5. CS 4432query processing5 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

6. CS 4432query processing6 R X SR.AR.BR.CS.CS.DS.E a 1 10 10 x 2 a 1 10 20 y 2. C 2 10 10 x 2. Bingo! Got one...

7. CS 4432query processing7 But ? Performance would be unacceptable! We need a better approach to reasoning about queries, their execution orders and their respective costs

8. CS 4432query processing8 Formal Relational Query Languages Two mathematical Query Languages form basis for “real” languages (e.g. SQL), and for implementation: –Relational Calculus: Lets users describe what they want, rather than how to compute it. (Non-operational, declarative.) –Relational Algebra: More operational, very useful for representing execution plans.

9. CS 4432query processing9 Relational Algebra Tuple : ordered set of data values Relation: a set of tuples Algebra: formal mathematical system consisting of a set of objects and operations on those objects Relational algebra: Algebra whose objects are relations and operators transform relations into other relations

10. CS 4432query processing10 Relational Algebra ? Query: SELECT B, D FROM R, S WHERE R.A = “c” and S.E = 2 and R.C=S.C

11. CS 4432query processing11 Relational Algebra - can be used to describe plans... 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)]

12. CS 4432query processing12 Example Instances R1 S1 S2 “Sailors” and “Reserves” relations

13. CS 4432query processing13 Relational Algebra Basic operations: – Selection ( ) Selects a subset of rows from relation. – Projection ( ) Deletes unwanted columns from relation. – Cross-product ( ) Allows us to combine two relations. – Set-difference ( ) Tuples in reln. 1, but not in reln. 2. – Union ( ) Tuples in reln. 1 and in reln. 2. Additional operations: – Intersection, join, division, renaming: Not essential ! Algebra is “closed”: Since each operation returns a relation, operations can be composed !

14. CS 4432query processing14 Projection

15. CS 4432query processing15 Selection

16. CS 4432query processing16 Union, Intersection, Set Difference Operate on two union- compatible relations: – Same number of fields. – `Corresponding’ fields have same type.

17. CS 4432query processing17 Cross-Product Each row of S1 is paired with each row of R1.  Conflict: Both S1 and R1 have a field called sid.  Renaming operator :

18. CS 4432query processing18 Joins Condition Join : Result schema same as that of cross-product.

19. CS 4432query processing19 Joins Equi-Join: condition contains only equalities. Result schema only one copy of fields for which equality is specified. Natural Join: Equijoin on all common fields.

20. CS 4432query processing20 Division Not primitive operator, but useful: Find sailors who have reserved all boats. A has 2 fields x and y; B has only field y: – A/B = – i.e., A/B contains all x tuples (sailors) such that for every y tuple (boat) in B, there is an xy tuple in A.

21. CS 4432query processing21 Examples of Division A/B A B1 B2 B3 A/B1A/B2A/B3

22. CS 4432query processing22 Expressing A/B Using Basic Operators Division is useful shorthand. Idea: For A/B, compute all x values that are not `disqualified’ by some y value in B. Disqualified x values: A/B: all disqualified tuples

23. CS 4432query processing23 Find names of sailors who’ve reserved boat #103  Solution 2 :  Solution 3 : Solution 1 :

24. CS 4432query processing24 Find names of sailors who’ve reserved a red boat Information about boat color only available in Boats; so need an extra join:  A more efficient solution: A query optimizer can find this, given the first solution!

25. CS 4432query processing25 Find sailors who’ve reserved a red or a green boat Can identify all red or green boats, then find sailors who’ve reserved one of these boats:  Can also define Tempboats using union! (How?)  What happens if is replaced by in this query?

26. CS 4432query processing26 Find sailors who’ve reserved a red and a green boat Must identify sailors who’ve reserved red boats, sailors who’ve reserved green boats, then find the intersection (sid is a key for Sailors):

27. CS 4432query processing27 Find names of sailors who’ve reserved all boats Uses division; schemas of input relations to / must be carefully chosen:  To find sailors who’ve reserved all ‘Interlake’ boats:.....

28. CS 4432query processing28 Relational Algebra representation used to describe plans...

29. CS 4432query processing29 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

30. CS 4432query processing30 RABC S CDE a11010x2 b12020y2 c21030z2 d23540x1 e34550y3 AnswerB D 2 x

31. CS 4432query processing31 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)]

32. CS 4432query processing32 Another idea:  B,D  R.A = “c”  S.E = 2 R S Plan II natural join

33. CS 4432query processing33 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 2 10 10 x 2 20 y 2 c 2 10 20 y 2 30 z 2 d 2 35 30 z 2 40 x 1 e 3 45 50 y 3 SELECT B,D FROM R,S WHERE R.A = “c” and S.E = 2 and R.C=S.C

34. CS 4432query processing34 Yet another idea:  B,D  S.E = 2  R.A = “c” R S Plan III natural join

35. CS 4432query processing35 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

36. CS 4432query processing36 R S A B C C D E a 1 10 10 x 2 b 1 20 20 y 2 c 2 10 30 z 2 d 2 35 40 x 1 e 3 45 50 y 3 AC I1I1 I2I2 =“c” check=2? output: next tuple:

37. CS 4432query processing37 Overview of Query Optimization

38. CS 4432query processing38 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

39. CS 4432query processing39 Example: SQL query Query : Find the movies with stars born in 1960 SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE birthdate LIKE ‘%1960’ );

40. CS 4432query processing40 Example: Parse Tree SELECT FROM WHERE IN title StarsIn ( ) starName SELECT FROM WHERE LIKE name MovieStar birthDate ‘%1960’

41. CS 4432query processing41 Example: Generating Relational Algebra  title  StarsIn IN  name  birthdate LIKE ‘%1960’ starName MovieStar Fig. 16.14: An expression using a two-argument , midway between a parse tree and relational algebra

42. CS 4432query processing42 Example: Logical Query Plan  title  starName=name StarsIn  name  birthdate LIKE ‘%1960’ MovieStar Fig. 16.16: Applying the rule for IN conditions 

43. CS 4432query processing43 Example: Improved Logical Query Plan  title starName=name StarsIn  name  birthdate LIKE ‘%1960’ MovieStar Fig. 16.16: An improvement on prev fig Question: Push project to StarsIn?

44. CS 4432query processing44 Example: Estimate Result Sizes Need expected size StarsIn MovieStar 

45. CS 4432query processing45 Example: One Physical Plan Parameters: join order, memory size, project attributes,... Hash join SEQ scanindex scan Parameters: Select Condition,... StarsInMovieStar

46. CS 4432query processing46 Example: Estimate costs L.Q.P P1 P2 …. Pn C1 C2 …. Cn Pick best!

47. CS 4432query processing47 Query Optimization Relational algebra level … Detailed query plan level … –Estimate costs –Generate and compare plans