The Query Compiler

Section 16.3

DATABASE SYSTEMS – The Complete Book

Presented By: Under the supervision of:

Deepti Kundu Dr. T.Y.Lin

Topics to be coveredTopics to be covered

From Parse to Logical Query Plans Conversion to Relational Algebra Removing Subqueries From Conditions Improving the Logical Query Plan Grouping Associative/ Commutative Operators

16.3 From Parse to Logical Query Plans ►

ReviewReview

Query

Preferred logical query plan

Parser

Preprocessor

Logical query plan generator

Query Rewriter

Section 16.1

Section 16.3

Two steps to turn Parse tree into Preferred Two steps to turn Parse tree into Preferred Logical Query PlanLogical Query Plan

Replace the nodes and structures of the parse tree, in appropriate groups, by an operator or operators of relational algebra.

Take the relational algebra expression and turn it into an expression that we expect can be converted to the most efficient physical query plan.

ReferenceReference RelationsRelations

StarsIn (movieTitle, movieYear, starName) MovieStar (name, address, gender, birthdate)

Conversion to Relational AlgebraConversion to Relational Algebra

If we have a <Query> with a <Condition> that has no subqueries, then we may replace the entire construct – the select-list, from-list, and condition – by a relational-algebra expression.

The relational-algebra expression consists of the following from bottom to top: The products of all the relations mentioned in the

<FromList>, which Is the argument of: A selection σC, where C is the <Condition> expression in

the construct being replaced, which in turn is the argument of:

A projection πL , where L is the list of attributes in the <SelList>

A query : ExampleA query : Example

SELECT movieTitle

FROM Starsin, MovieStar

WHERE starName = name AND

birthdate LIKE ‘%1960’;

SELECT movieTitleSELECT movieTitleFROM Starsin, MovieStarFROM Starsin, MovieStarWHERE starName = name AND WHERE starName = name AND birthdate LIKE ‘%1960’; birthdate LIKE ‘%1960’;

Translation to an algebraic expression treeTranslation to an algebraic expression tree

Removing Subqueries From ConditionsRemoving Subqueries From Conditions

For parse trees with a <Condition> that has a subquery

Intermediate operator – two argument selection It is intermediate in between the syntactic

categories of the parse tree and the relational-algebra operators that apply to relations.

Using a two-argument Using a two-argument σσ

πmovieTitle

σ

StarsIn <Condition>

MovieStar

IN πname<Tuple>

starName

σ birthdate LIKE ‘%1960'

<Attribute>

Two argument selection with condition Two argument selection with condition involving INinvolving IN

Now say we have, two arguments – some relation and the second argument is a <Condition> of the form t IN S. ‘t’ – tuple composed of some attributes of R ‘S’ – uncorrelated subquery

Steps to be followed:1. Replace the <Condition> by the tree that is the expression for S ( δ is

used to remove duplicates)

2. Replace the two-argument selection by a one-argument selection σC.

3. Give σC an argument that is the product of R and S.

Two argument selection with condition Two argument selection with condition involving INinvolving IN

σ

R <Condition>

t IN S

σC

X

R  δ

S

The effectThe effect

Improving the Logical Query PlanImproving the Logical Query Plan

Algebraic laws to improve logical query plans: Selections can be pushed down the expression tree as

far as they can go. Similarly, projections can be pushed down the tree, or

new projections can be added. Duplicate eliminations can sometimes be removed, or

moved to a more convenient position in the tree. Certain selections can be combined with a product

below to turn the pair of operations into an equijoin.

Grouping Associative/ Commutative Grouping Associative/ Commutative OperatorsOperators

An operator that is associative and commutative operators may be though of as having any number of operands.

We need to reorder these operands so that the multiway join is executed as sequence of binary joins.

Its more time consuming to execute them in the order suggested by parse tree.

For each portion of subtree that consists of nodes with the same associative and commutative operator (natural join, union, and intersection), we group the nodes with these operators into a single node with many children.

The effect of query rewritingThe effect of query rewriting

Π movieTitle

Starname = name

StarsIn σbirthdate LIKE ‘%1960’

MovieStar

Final step in producing logical query planFinal step in producing logical query plan

=>

U

U

U

W

R

S T

VU

U V W

R S T

An Example to summarizeAn Example to summarize

“find movies where the average age of the stars was at most 40 when the movie was made”

SELECT distinct m1.movieTitle, m1,movieYearFROM StarsIn m1WHERE m1.movieYear – 40 <= (

SELECT AVG (birthdate)FROM StartsIn m2, MovieStar sWHERE m2.starName = s.name AND

m1.movieTitle = m2.movieTitle ANDm1.movieYear = m2.movieyear

);

SELECT distinct m1.movieTitle, m1,movieYearFROM StarsIn m1WHERE m1.movieYear – 40 <= ( SELECT AVG (birthdate) FROM StartsIn m2, MovieStar s WHERE m2.starName = s.name AND m1.movieTitle = m2.movieTitle AND m1.movieYear = m2.movieyear );

Selections combined with a product to turn Selections combined with a product to turn the pair of operations into an equijoin…the pair of operations into an equijoin…

Condition pushed up the expression tree…

`

Selections combined…Selections combined…