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Query Translation

Overview

Converts SQL statement text to RA expression.

Processes:

  • Lexer/parser.
  • Mapping rules.
  • Rewriting rules.

Mapping from SQL to RA may include some optimisations.

Parsing SQL

Parsing task is similar to that for programming languages.

Expression Rewriting Rules

Since RA is a well-defined formal system:

  • There exist many algebraic laws on RA expressions.
  • Which can be used as a basis for expression rewriting.
  • In order to produce equivalent (potentially more efficient) expressions.

Expression transformation based on such rules can be used:

  • To simplify/improve SQL to RA mapping results.
  • To generate new plan variations to check in query optimisation.

Relational Algebra Laws

Commutative and Associative Laws:

  • R ⨝ S = S ⨝ R, (R ⨝ S) ⨝ T = R ⨝ (S ⨝ T) (natural join).
  • R ∪ S = S ∪ R, (R ∪ S) ∪ T = R ∪ (S ∪ T) (normal set algebra rules).
  • R ⨝_Cond S = S ⨝_Cond R (theta join).
  • σ_c(σ_d(R)) = σ_d(σ_c(R)).

Selection splitting:

  • σ_c∧d(R) = σ_c(σ_d(R)).
  • σ_c∨d(R) = σ_c(R) ∪ σ_d(R).

Selection pushing:

  • σ_c(R ∪ S) = σ_c(R) ∪ σ_c(S).
  • σ_c(R ∩ S) = σ_c(R) ∩ σ_c(S).

Selection pushing with join:

  • σ_c(R ⨝ S) = σ_c(R) ⨝ S (if c refers only to attributes from R).
  • σ_c(R ⨝ S) = R ⨝ σ_c(S) (if c refers only to attributes from S).
  • If condition contains attributes from both R and S:
    • σ_c′∧c″ (R ⨝ S) = σ_c′(R) ⨝ σ_c″(S).
    • Where c' contains only R attributes and c'' contains only S attributes.

Projection:

  • All but last project can be ignored:
    • π_L1(π_L2(...π_Ln (R))) ==> π_L1(R).
  • Projections can be pushed into joins:
    • π_L(R ⨝c S) = π_L(π_M(R) ⨝c π_N(S))
    • Where M and N must contain all attributes needed for c and M and N must contain all attributes used in L (L ⊆ M ∪ N).

Query Rewriting

Subqueries are converted to joins when possible.

PostgreSQL Views

In PostgreSQL, views are implemented via rewrite rules.

  • A reference to view in SQL expands its definition in RA.

For example:

create view COMP9315studes as
select stu,mark from Enrolments where course='COMP9315';
-- students who passed
select stu from COMP9315studes where mark >= 50;

is represented as:

COMP9315studes
  = Proj[stu,mark](Sel[course=COMP9315](Enrolments))
-- with query ...
Proj[stu](Sel[mark>=50](COMP9315studes))
-- becomes ...
Proj[stu](Sel[mark>=50](
  Proj[stu,mark](Sel[course=COMP9315](Enrolments)))
)
-- which could be rewritten as ...
Proj[stu](Sel[mark>=50 & course=COMP9315]Enrolments)