org.basex.query.xpath.expr
Class Expr

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

Direct Known Subclasses:

Arr, Filter, FTArrayExpr, Func, InternalExpr, Item, LocPath, Pos, Unary

public abstract class Expr
extends ExprInfo

Common interface for all expressions.

Author:

Workgroup DBIS, University of Konstanz 2005-08, ISC License, Tim Petrowsky

Constructor Summary

Expr()

Method Summary

abstract Expr	comp(XPContext ctx) Optimizes the expression.
abstract Item	eval(XPContext ctx) Evaluates the expression with the specified context set.
Expr	indexEquivalent(XPContext ctx, Step step, boolean seq) Returns an equivalent expression which accesses an index structure.
int	indexSizes(XPContext ctx, Step step, int min) Returns the number of results if this query is evaluated by an index.
java.lang.Class<?>	returnedValue() Gets the expected return type.
boolean	sameAs(Expr cmp) Checks current and specified expression for equivalence.
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 org.basex.query.ExprInfo

color, info, name, plan, toString

Methods inherited from class java.lang.Object

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

Constructor Detail

Expr

public Expr()

Method Detail

comp

public abstract Expr comp(XPContext ctx)
                   throws QueryException

Optimizes the expression.

Parameters:

ctx - expression context

Returns:

optimized Expression

Throws:

QueryException - evaluation exception

eval

public abstract Item eval(XPContext ctx)
                   throws QueryException

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

Parameters:

ctx - query context

Returns:

resulting XPathValue

Throws:

QueryException - evaluation exception

returnedValue

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

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

Returns:

the expected class returned by the eval(org.basex.query.xpath.XPContext) method

sameAs

public boolean sameAs(Expr cmp)

Checks current and specified expression for equivalence.

Parameters:

cmp - expression to be compared

Returns:

result of check

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

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

indexEquivalent

public Expr indexEquivalent(XPContext ctx,
                            Step step,
                            boolean seq)
                     throws QueryException

Returns an equivalent expression which accesses an index structure.

Parameters:

ctx - root

step - location step

seq - flag for sequential evaluation

Returns:

Equivalent index-expression or null

Throws:

QueryException - evaluation exception

indexSizes

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

Returns the number of results if this query is evaluated by an index. If Integer.MAX_VALUE is returned, no index access is possible.

Parameters:

ctx - root

step - the current location step

min - current minimum index hits

Returns:

number of expected results