Background material for Chapter 3 taken from CS 245

Transcription

1 Background material for Chapter 3 taken from C 245 contributed by Hector Garcia-Molina selected/cut by tefan Böttcher Query Processing Q Query Plan Focus: elational ystem Others? Background C 245 material taken from C245 contributed Notes by Hector 4 Garcia-Molina (tanford University, UA) 1 / 1 Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 2 / 2 Example elect B,D From, Where A = c E = 2 C=C A B C C D E a x 2 b y 2 c z 2 d x 1 e y 3 Answer B D 2 x Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 3 / 3 Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 4 / 4 How do we execute query? One idea -Do Cartesian product - elect tuples - Do projection X A B C C D E a x 2 a y 2 Bingo! C x 2 Got one Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 5 / 5 Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 6 / 6

2 elational Algebra -can be used to describe plans Ex: Plan I Π B,D Another idea: Plan II Π B,D σ A= c E=2 C=C natural join X σ A = c σ E = 2 O: Π B,D [ σ A= c E=2 C = C (X)] Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 7 / 7 Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 8 / 8 A B C σ () σ() C D E a 1 10 A B C C D E 10 x 2 b 1 20 c x 2 20 y 2 c y 2 30 z 2 d z 2 40 x 1 e y 3 Plan III Use A and C Indexes (1) Use A index to select tuples with A = c (2) For each C value found, use C index to find matching tuples (3) Eliminate tuples E 2 (4) Join matching, tuples, project B,D attributes and place in result Background C 245 material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 9 / 9 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 10 / 10 A= c C A B C I1 I2 C D E a x 2 b 1 20 <c,2,10> <10,x,2> 20 y 2 c 2 10 check=2? 30 z 2 d 2 35 output: <2,x> 40 x 1 e y 3 next tuple: <c,7,15> parse convert QL query parse tree logical query plan apply laws improved lqp estimate result sizes lqp +sizes consider physical plans Overview of Query Optimization statistics execute pick best {(P1,C1),(P2,C2)} answer Pi estimate costs Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 11 / 11 {P1,P2, } Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 12 / 12

3 QL query ELECT title FOM WHEE starname IN ( ELECT name FOM WHEE birthdate LIKE %1960 ); (Find the movies with stars born in 1960) Parse Tree <Query> <FW> ELECT <ellist> FOM <FromList> WHEE <Condition> <Attribute> <elname> <Tuple> IN <Query> title <Attribute> ( <Query> ) starname <FW> ELECT <ellist> FOM <FromList> WHEE <Condition> <Attribute> <elname> <Attribute> LIKE <Pattern> name birthdate %1960 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 13 / 13 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 14 / 14 Generating elational Algebra Logical Query Plan Πtitle Πtitle σ <condition> σstarname=name <tuple> IN Πname Πname <attribute> σbirthdate LIKE %1960 σbirthdate LIKE %1960 starname Fig 715: An expression using a two-argument σ, midway between a parse tree and relational algebra Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 15 / 15 Fig 718: Applying the rule for IN conditions Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 16 / 16 Improved Logical Query Plan (LQP) Estimate esult izes Πtitle starname=name Πname σbirthdate LIKE %1960 Question: Push project to? Need expected size Π σ Fig 720: An improvement on fig 718 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 17 / 17 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 18 / 18

4 One Physical Plan (Pi) Estimate costs Ci of Pi Hash join Parameters: join order, memory size, project attributes, EQ scan index scan Parameters: elect Condition, LQP P1 P2 Pn C1 C2 Cn depending on costs, pick best physical plan! Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 19 / 19 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 20 / 20 Textbook: Chapteres 6 and 7 - outline 61 Algebra for queries 62 Physical operators (can,sort, ) Implementing operators +estimating their cost 71 Parsing 72 Algebraic laws 73 Parse tree -> logical query plan 74 Estimating result sizes Cost based optimization elational algebra optimization Transformation rules (preserve equivalence) What are good transformations? Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 21 / 21 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 22 / 22 ules: Natural joins & cross products & union = ( ) T = ( T) Note: Carry attribute names in results, so order is not important Can also write as trees, eg: T T Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 23 / 23 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 24 / 24

5 ules: Natural joins & cross products & union = ( ) T = ( T) x = x ( x ) x T = x ( x T) U = U U ( U T) = ( U ) U T ules: elects σ p1 p2 () = σ p1vp2 () = σ p1 [ σ p2 ()] [ σ p1 ()] U [ σ p2 ()] ( applies only to set-union, not to bag-union ) Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 25 / 25 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 26 / 26 ules: Project ules: σ + combined Let: X = set of attributes Y = set of attributes XY = X U Y Let p = predicate with only attribs q = predicate with only attribs m = predicate with only, attribs πxy () = πx [πy ()] σ p ( ) = σ q ( ) = [σ p ()] [σ q ()] Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 27 / 27 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 28 / 28 ules: σ + combined (continued) Do one, others for homework: ome ules can be Derived: σp q ( ) = σp q m ( ) = σpvq ( ) = Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 29 / 29 σp q ( ) = [σp ()] [σq ()] σp q m ( ) = σm [(σp ) σpvq ( ) = (σq )] [(σp ) ] U [ (σq )] Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 30 / 30

6 --> Derivation for first one: ules: π,σ combined σp q ( ) = σp [σq ( ) ] = σp [ σq () ] = [σp ()] [σq ()] Let x = subset of attributes z = attributes in predicate P (subset of attributes) πxz πx[σp () ] = πx {σp [ πx () ]} Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 31 / 31 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 32 / 32 ules: π, combined Let x = subset of attributes y = subset of attributes z = intersection of, attributes πxy ( ) = πxy{[πxz () ] [πyz () ]} πxy {σp ( )} = πxy {σp [πxz () πyz ()]} z = z U {attributes used in P } Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 33 / 33 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 34 / 34 ules for σ, π combined with X ules σ, U combined: similar eg, σp ( X ) =? σp( U ) = σp() U σp() σp( - ) = σp() - = σp() - σp() Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 35 / 35 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 36 / 36

7 Which are good transformations? σp1 p2 () σp1 [σp2 ()] σp ( ) [σp ()] πx [σp ()] πx {σp [πxz ()]} Conventional wisdom: do projects early (A,B,C,D,E) x={e} P: (A=3) (B= cat ) πx {σp ()} vs πe {σp{πabe()}} Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 37 / 37 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 38 / 38 But What if we have A, B indexes? Bottom line: B = cat A=3 No transformation is always good Usually good: early selections Intersect pointers to get pointers to matching tuples Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 39 / 39 Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 40 / 40 In textbook: more transformations Eliminate common sub-expressions Other operations: duplicate elimination Background C 245material taken from C245 contributed Notes by Hector 6 Garcia-Molina (tanford University, UA) 41 / 41