Notes06

# Notes06 - CS245:DatabaseSystem Principles...

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CS 245 Notes 6 1 CS 245: Database System  Principles Notes 6: Query Processing Steven Whang

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CS 245 Notes 6 2 Query Processing Q      Query Plan Focus:  Relational System Others?
CS 245 Notes 6 3 Example Select B,D From R,S Where R.A = “c”     S.E = 2     R.C=S.C

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CS 245 Notes 6 4 R A B C S 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 Answer B D 2 x
CS 245 Notes 6 5  How do we execute query? - Do Cartesian product - Select tuples - Do projection One idea

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CS 245 Notes 6 6 RXS R.A R.B R.C S.C S.D S.E a 1 10 10 x 2 a 1 10 20 y 2 . . C 2 10 10 x 2 . . Bingo! Got one...
CS 245 Notes 6 7 Relational Algebra  - can be used to    describe plans... Ex:  Plan I Π B,D   σ R.A =“c”  S.E=2   R.C=S.C X R S OR:    Π B,D  [   σ R.A=“c”  S.E=2   R.C = S.C  (RXS)]

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CS 245 Notes 6 8 Another idea: Π B,D    σ R.A = “c”   σ S.E = 2 R   S Plan II natural join
CS 245 Notes 6 9    R         S A  B  C σ ( R ) σ ( S )      C  D  E a  1  10        A   B  C       C  D  E     10  x  2 b  1  20 c   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

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CS 245 Notes 6 10 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 245 Notes 6 11    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 A C I 1 I 2 =“c” <c,2,10> <10,x,2> check=2? output: <2,x> next tuple: <c,7,15>

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CS 245 Notes 6 12 Overview of Query Optimization
CS 245 Notes 6 13 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

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CS 245 Notes 6 14 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 245 Notes 6 15 Example:    Parse Tree <Query> <SFW> SELECT   <SelList>    FROM    <FromList>     WHERE     <Condition> <Attribute>              <RelName>                 <Tuple>  IN  <Query>

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