org.basex.query.xpath.expr
Class Expr

java.lang.Object
  org.basex.query.xpath.expr.Expr

Direct Known Subclasses:

ArrayExpr, DualExpr, Filter, Func, InternalExpr, Item, LocPath, Path, Unary

public abstract class Expr
extends java.lang.Object

Common interface for all expressions.

Author:

Workgroup DBIS, University of Konstanz 2005-07, ISC License, Tim Petrowsky

Constructor Summary

Expr()

Method Summary

abstract Expr	compile(XPContext ctx) Tries to optimize the Expression.
abstract Item	eval(XPContext ctx) Evaluates the expression with the specified context set.
Expr	indexEquivalent(XPContext ctx, Step step) For predicate optimization.
int	indexSizes(XPContext ctx, Step step, int min) For predicate optimization.
java.lang.Class<?>	returnedValue() Gets the expected return type.
boolean	sameAs(Expr cmp) Checks current and specified expression for equivalence.
abstract java.lang.String	toString()
abstract boolean	usesPos() Checks whether this Expression (or its children) make use of the position parameter.
abstract boolean	usesSize() Checks whether this Expression (or its children) make use of the setsize parameter.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Expr

public Expr()

Method Detail

compile

public abstract Expr compile(XPContext ctx)
                      throws EvalException

Tries to optimize the Expression.

Parameters:

ctx - expression context

Returns:

optimized Expression

Throws:

EvalException - evaluation exception

eval

public abstract Item eval(XPContext ctx)
                   throws EvalException

Evaluates the expression with the specified context set. Additionally provides a context

Parameters:

ctx - query context

Returns:

resulting XPathValue

Throws:

EvalException - evaluation exception

indexEquivalent

public Expr indexEquivalent(XPContext ctx,
                            Step step)
                     throws EvalException

For predicate optimization. If possible return an expression yielding the same results using the index. This may not do any changes to the current expression.

Parameters:

ctx - root

step - the LocationStep this Expression is a predicate of

Returns:

Equivalent index-expression or null

Throws:

EvalException - evaluation exception

indexSizes

public int indexSizes(XPContext ctx,
                      Step step,
                      int min)

For predicate optimization. If possible return the number of entries an index returns. This may not do any changes to the current expression.

Parameters:

ctx - root

step - the LocationStep this Expression is a predicate of

min - current minimum size

Returns:

Equivalent index-expression or null

returnedValue

public final java.lang.Class<?> returnedValue()

Gets the expected return type. This may be XPathValue itself.

Returns:

the expected class returned by the eval() method

sameAs

public boolean sameAs(Expr cmp)

Checks current and specified expression for equivalence.

Parameters:

cmp - expression to be compared

Returns:

result of check

toString

public abstract java.lang.String toString()

Overrides:

toString in class java.lang.Object

usesPos

public abstract boolean usesPos()

Checks whether this Expression (or its children) make use of the position parameter. If not this allows some nice optimizations.

Returns:

whether position is used

usesSize

public abstract boolean usesSize()

Checks whether this Expression (or its children) make use of the setsize parameter. If not this allows early predicate evaluation.

Returns:

whether setsize is used