org.apache.hadoop.hive.ql.optimizer
Class ColumnPruner

java.lang.Object
  org.apache.hadoop.hive.ql.optimizer.ColumnPruner

All Implemented Interfaces:

Transform

public class ColumnPruner
extends Object
implements Transform

Implementation of one of the rule-based optimization steps. ColumnPruner gets the current operator tree. The \ tree is traversed to find out the columns used for all the base tables. If all the columns for a table are not used, a select is pushed on top of that table (to select only those columns). Since this changes the row resolver, the tree is built again. This can be optimized later to patch the tree.

Nested Class Summary

static class

ColumnPruner.ColumnPrunerWalker Walks the op tree in post order fashion (skips selects with file sink or script op children).

Field Summary

protected ParseContext

pGraphContext

Constructor Summary

ColumnPruner()
empty constructor.

Method Summary

ParseContext

transform(ParseContext pactx) Transform the query tree.

Methods inherited from class java.lang.Object

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

Field Detail

pGraphContext

protected ParseContext pGraphContext

Constructor Detail

ColumnPruner

public ColumnPruner()

empty constructor.

Method Detail

transform

public ParseContext transform(ParseContext pactx)
                       throws SemanticException

Transform the query tree. For each table under consideration, check if all columns are needed. If not, only select the operators needed at the beginning and proceed.

Specified by:

transform in interface Transform

Parameters:

pactx - the current parse context

Returns:

ParseContext

Throws:

SemanticException