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

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

All Implemented Interfaces:

Transform

public class GlobalLimitOptimizer
extends Object
implements Transform

This optimizer is used to reduce the input size for the query for queries which are specifying a limit.

For eg. for a query of type:

select expr from T where limit 100;

Most probably, the whole table T need not be scanned. Chances are that even if we scan the first file of T, we would get the 100 rows needed by this query. This optimizer step populates the GlobalLimitCtx which is used later on to prune the inputs.

Constructor Summary

GlobalLimitOptimizer()

Method Summary

ParseContext

transform(ParseContext pctx) All transformation steps implement this interface.

Methods inherited from class java.lang.Object

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

Constructor Detail

GlobalLimitOptimizer

public GlobalLimitOptimizer()

Method Detail

transform

public ParseContext transform(ParseContext pctx)
                       throws SemanticException

Description copied from interface: Transform

All transformation steps implement this interface.

Specified by:

transform in interface Transform

Parameters:

pctx - input parse context

Returns:

ParseContext

Throws:

SemanticException