CPS216: Advanced Database Systems Query Rewrite Rules for Subqueries Shivnath Babu

More Query Rewrite Rules Transform one logical plan into another –Do not use statistics Equivalences in relational algebra Push-down predicates Do projects early Avoid cross-products if possible Use left-deep trees Use of constraints, e.g., uniqueness Subqueries  Joins (we will study this rewrite rule after we do physical plan selection)

SQL Query with an Uncorrelated Subquery Find the movies with stars born in 1960 MovieStar(name, address, gender, birthdate) StarsIn(title, year, starName) SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE birthdate LIKE ‘%1960’ );

Parse Tree SELECT FROM WHERE IN title StarsIn ( ) starName SELECT FROM WHERE LIKE name MovieStar birthDate ‘%1960’

Generating Relational Algebra  title  StarsIn IN  name  birthdate LIKE ‘%1960’ starName MovieStar Two-argument selection

Rewrite Rule for Two-argument Selection with Conditions Involving IN  Lexp IN Rexp Two-argument selection  Lexp Rexp δ X

Applying the Rewrite Rule  title  StarsIn IN  name  birthdate LIKE ‘%1960’ starName MovieStar  title  starName=name StarsIn δ  birthdate LIKE ‘%1960’ MovieStar   name

Improving the Logical Query Plan  title starName=name StarsIn  name  birthdate LIKE ‘%1960’ MovieStar  title  starName=name StarsIn δ  birthdate LIKE ‘%1960’ MovieStar   name

SQL Query with an Correlated Subquery MovieStar(name, address, gender, birthdate) StarsIn(title, year, starName) SELECT title FROM StarsIn WHERE starName IN ( SELECT name FROM MovieStar WHERE name LIKE ‘Tom%’ and year = birthdate + 30 ); Can we rewrite this query as a single-block join query?