saffron.rel
Class ExpressionReader

java.lang.Object
  |
  +--saffron.rel.Rel
        |
        +--saffron.rel.ExpressionReader

Direct Known Subclasses:

PlanArrayReader

public class ExpressionReader

extends Rel

An ExpressionReader is a relational expression node which evaluates an expression and returns the results as a relation. Cases:

1. If the expression is an array T[], the each result row is of type T.
2. If the expression implements Map or derives from Hashtable, each result row contains fields {Object key; Object value;}.
3. If the expression implements Iterator or derives from Enumeration, each result row is an Object.

NOTE: We support Hashtable, Enumeration and Vector explicitly because Map, Iterator and Map does not exist until JDK 1.2.

Example accessing an array:

Emp[] emps;
 Emp[] males = (select from emps as emp where emp.gender.equals("M"));

The following example shows how you can cast the values returned from a Hashtable:

Hashtable nameToDeptno = new Hashtable();
 nameToDeptno.put("Fred", new Integer(20));
 nameToDeptno.put("Eric", new Integer(10));
 nameToDeptno.put("Bill", new Integer(10));
 for (i in (select {(String) key, ((Integer) value).intValue() as value}
            from nameToDeptno)) {
   print(i.key + " is in dept " + i.value);
 }

Here we access a value in a Hashtable by key:

Object fredDeptno =
    (select value from nameToDeptno where key.equals("Fred");

Because the Hashtable is being accessed via its key, the optimizer will probably optimize this code to nameToDeptno.get("Fred").

Since:

8 December, 2001

Version:

$Id: //open/saffron/src/main/saffron/rel/ExpressionReader.java#5 $

Author:

jhyde

Nested Class Summary

Nested classes inherited from class saffron.rel.Rel

Field Summary

protected CallingConvention	convention
private boolean	distinct Whether the rows are distinct; set by deriveRowType().
protected Expression	exp

Fields inherited from class saffron.rel.Rel

cluster, digest, emptyArray, id, nextId, rowType

Constructor Summary

ExpressionReader(Cluster cluster, Expression exp, Type rowType)
Creates an ExpressionReader.

Method Summary

private CallingConvention	chooseConvention(Expression exp)
Object	clone()
Cost	computeSelfCost(Planner planner) Returns the cost of this plan (not including children).
protected Type	deriveRowType()
void	explain(PlanWriter pw)
Expression[]	getChildExps() Returns an array of this Rel's child expressions (not including the inputs returned by Rel.getInputs().
CallingConvention	getConvention() Returns a value from CallingConvention.
Expression	getExp()
Object	implement(Implementor implementor, int ordinal) Create a plan for this expression according to a calling convention.
boolean	isDistinct() Returns whether the same value will not come out twice.
private static boolean	isDistinct(ArrayAllocationExpression arrayAlloc) Returns true if the array allocation expression consists of distinct literals, false otherwise.
static boolean	isRelational(OJClass clazz) Returns whether an expression clazz can be read as a relation.

Methods inherited from class saffron.rel.Rel

childrenAccept, computeDigest, getCluster, getCorelVariable, getEnvironment, getId, getInput, getInputs, getOrCreateCorelVariable, getParentEnv, getQualifier, getQuery, getRelTypeName, getRows, getRowType, getTable, getType, getVariablesStopped, implementExp, implementExps, implementFieldAccess, implementSelf, isAccessTo, onRegister, recomputeDigest, register, registerCorelVariable, replaceInput, setCorelVariable, toString

Methods inherited from class java.lang.Object

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

Field Detail

exp

protected Expression exp

convention

protected CallingConvention convention

distinct

private boolean distinct

Whether the rows are distinct; set by deriveRowType().

Constructor Detail

ExpressionReader

public ExpressionReader(Cluster cluster,
                        Expression exp,
                        Type rowType)

Creates an ExpressionReader.

Parameters:

cluster - Cluster this relational expression belongs to

exp - expression to evaluate

rowType - row type of the expression; if null, the row type is deduced from the expression (for example, the row type of a Iterator is Object.

Method Detail

getExp

public Expression getExp()

chooseConvention

private CallingConvention chooseConvention(Expression exp)

clone

public Object clone()

Specified by:

clone in class Rel

getConvention

public CallingConvention getConvention()

Description copied from class: Rel

Returns a value from CallingConvention.

Overrides:

getConvention in class Rel

getChildExps

public Expression[] getChildExps()

Description copied from class: Rel

Returns an array of this Rel's child expressions (not including the inputs returned by Rel.getInputs(). If there are no child expressions, returns an empty array, not null.

Overrides:

getChildExps in class Rel

isRelational

public static boolean isRelational(OJClass clazz)

Returns whether an expression clazz can be read as a relation.

deriveRowType

protected Type deriveRowType()

Specified by:

deriveRowType in class Rel

isDistinct

private static boolean isDistinct(ArrayAllocationExpression arrayAlloc)

Returns true if the array allocation expression consists of distinct literals, false otherwise. For example, isDistinct(new String[] { "a", "b"}) returns true, isDistinct(new int[] {1, 1+1}) returns false.

isDistinct

public boolean isDistinct()

Description copied from class: Rel

Returns whether the same value will not come out twice. Default value is false, derived classes should override.

Overrides:

isDistinct in class Rel

computeSelfCost

public Cost computeSelfCost(Planner planner)

Description copied from class: Rel

Returns the cost of this plan (not including children). The base implementation throws an error; derived classes should override.

Overrides:

computeSelfCost in class Rel

explain

public void explain(PlanWriter pw)

Overrides:

explain in class Rel

implement

public Object implement(Implementor implementor,
                        int ordinal)

Description copied from class: Rel

Create a plan for this expression according to a calling convention.

Overrides:

implement in class Rel

Parameters:

implementor - implementor

ordinal - indicates our position in the pre-, in- and postfix walk over the tree; ordinal is -1 when called from the parent, and i when called from the i^th child.