simpledb.query
Class Term

java.lang.Object
  simpledb.query.Term

public class Term
extends Object

A term is a comparison between two expressions.

Author:

Edward Sciore

Constructor Summary

Term(Expression lhs, Expression rhs)
Creates a new term that compares two expressions for equality.

Method Summary

boolean	appliesTo(Schema sch) Returns true if both of the term's expressions apply to the specified schema.
Constant	equatesWithConstant(String fldname) Determines if this term is of the form "F=c" where F is the specified field and c is some constant.
String	equatesWithField(String fldname) Determines if this term is of the form "F1=F2" where F1 is the specified field and F2 is another field.
boolean	isSatisfied(Scan s) Returns true if both of the term's expressions evaluate to the same constant, with respect to the specified scan.
int	reductionFactor(Plan p) Calculates the extent to which selecting on the term reduces the number of records output by a query.
String	toString()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Term

public Term(Expression lhs,
            Expression rhs)

Creates a new term that compares two expressions for equality.

Parameters:

lhs - the LHS expression

rhs - the RHS expression

Method Detail

reductionFactor

public int reductionFactor(Plan p)

Calculates the extent to which selecting on the term reduces the number of records output by a query. For example if the reduction factor is 2, then the term cuts the size of the output in half.

Parameters:

p - the query's plan

Returns:

the integer reduction factor.

equatesWithConstant

public Constant equatesWithConstant(String fldname)

Determines if this term is of the form "F=c" where F is the specified field and c is some constant. If so, the method returns that constant. If not, the method returns null.

Parameters:

fldname - the name of the field

Returns:

either the constant or null

equatesWithField

public String equatesWithField(String fldname)

Determines if this term is of the form "F1=F2" where F1 is the specified field and F2 is another field. If so, the method returns the name of that field. If not, the method returns null.

Parameters:

fldname - the name of the field

Returns:

either the name of the other field, or null

appliesTo

public boolean appliesTo(Schema sch)

Returns true if both of the term's expressions apply to the specified schema.

Parameters:

sch - the schema

Returns:

true if both expressions apply to the schema

isSatisfied

public boolean isSatisfied(Scan s)

Returns true if both of the term's expressions evaluate to the same constant, with respect to the specified scan.

Parameters:

s - the scan

Returns:

true if both expressions have the same value in the scan

toString

public String toString()

Overrides:

toString in class Object