simpledb

Class Aggregate

java.lang.Object

simpledb.Operator

simpledb.Aggregate

All Implemented Interfaces:

java.io.Serializable, DbIterator

public class Aggregate
extends Operator

The Aggregation operator that computes an aggregate (e.g., sum, avg, max, min). Note that we only support aggregates over a single column, grouped by a single column.

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
private int	aField
private Aggregator	aggregator
private DbIterator	child
private int	gbField
private DbIterator	iterator
private Aggregator.Op	op
private static long	serialVersionUID
private TupleDesc	tupleDesc

Constructor Summary

Constructors

Constructor and Description
Aggregate(DbIterator child, int afield, int gfield, Aggregator.Op aop) Constructor.

Method Summary

Modifier and Type	Method and Description
int	aggregateField()
java.lang.String	aggregateFieldName()
Aggregator.Op	aggregateOp()
void	close() Closes the iterator
protected Tuple	fetchNext() Returns the next tuple.
DbIterator[]	getChildren()
TupleDesc	getTupleDesc() Returns the TupleDesc of this Aggregate.
int	groupField()
java.lang.String	groupFieldName()
static java.lang.String	nameOfAggregatorOp(Aggregator.Op aop)
void	open() Opens the iterator
void	rewind() Rewinds the iterator
void	setChildren(DbIterator[] children) Set the children(child) of this operator.

Methods inherited from class simpledb.Operator

getEstimatedCardinality, hasNext, next, setEstimatedCardinality

Methods inherited from class java.lang.Object

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

Field Detail

serialVersionUID

private static final long serialVersionUID

See Also:

Constant Field Values

child

private DbIterator child

iterator

private DbIterator iterator

aField

private int aField

gbField

private int gbField

op

private Aggregator.Op op

tupleDesc

private TupleDesc tupleDesc

aggregator

private Aggregator aggregator

Constructor Detail

Aggregate

public Aggregate(DbIterator child,
                 int afield,
                 int gfield,
                 Aggregator.Op aop)

Constructor. Implementation hint: depending on the type of afield, you will want to construct an IntegerAggregator or StringAggregator to help you with your implementation of readNext().

Parameters:

child - The DbIterator that is feeding us tuples.

afield - The column over which we are computing an aggregate.

gfield - The column over which we are grouping the result, or -1 if there is no grouping

aop - The aggregation operator to use

Method Detail

groupField

public int groupField()

Returns:

If this aggregate is accompanied by a groupby, return the groupby field index in the INPUT tuples. If not, return Aggregator.NO_GROUPING

groupFieldName

public java.lang.String groupFieldName()

Returns:

If this aggregate is accompanied by a group by, return the name of the groupby field in the OUTPUT tuples If not, return null;

aggregateField

public int aggregateField()

Returns:

the aggregate field

aggregateFieldName

public java.lang.String aggregateFieldName()

Returns:

return the name of the aggregate field in the OUTPUT tuples

aggregateOp

public Aggregator.Op aggregateOp()

Returns:

return the aggregate operator

nameOfAggregatorOp

public static java.lang.String nameOfAggregatorOp(Aggregator.Op aop)

open

public void open()
          throws java.util.NoSuchElementException,
                 DbException,
                 TransactionAbortedException

Opens the iterator

Specified by:

open in interface DbIterator

Overrides:

open in class Operator

Throws:

java.util.NoSuchElementException - When no element is found

DbException

TransactionAbortedException

fetchNext

protected Tuple fetchNext()
                   throws TransactionAbortedException,
                          DbException

Returns the next tuple. If there is a group by field, then the first field is the field by which we are grouping, and the second field is the result of computing the aggregate, If there is no group by field, then the result tuple should contain one field representing the result of the aggregate. Should return null if there are no more tuples.

Specified by:

fetchNext in class Operator

Returns:

Tuple The next tuple

Throws:

DbException

TransactionAbortedException

rewind

public void rewind()
            throws DbException,
                   TransactionAbortedException

Rewinds the iterator

Throws:

DbException

TransactionAbortedException

getTupleDesc

public TupleDesc getTupleDesc()

Returns the TupleDesc of this Aggregate. If there is no group by field, this will have one field - the aggregate column. If there is a group by field, the first field will be the group by field, and the second will be the aggregate value column. The name of an aggregate column should be informative. For example: "aggName(aop) (child_td.getFieldName(afield))" where aop and afield are given in the constructor, and child_td is the TupleDesc of the child child.

Specified by:

getTupleDesc in interface DbIterator

Specified by:

getTupleDesc in class Operator

Returns:

TupleDesc The TupleDesc of Aggregrate

close

public void close()

Closes the iterator

Specified by:

close in interface DbIterator

Overrides:

close in class Operator

getChildren

public DbIterator[] getChildren()

Specified by:

getChildren in class Operator

Returns:

return the children DbIterators of this operator. If there is only one child, return an array of only one element. For join operators, the order of the children is not important. But they should be consistent among multiple calls.

setChildren

public void setChildren(DbIterator[] children)

Description copied from class: Operator

Set the children(child) of this operator. If the operator has only one child, children[0] should be used. If the operator is a join, children[0] and children[1] should be used.

Specified by:

setChildren in class Operator

Parameters:

children - the DbIterators which are to be set as the children(child) of this operator