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

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

All Implemented Interfaces:

Transform

public class LimitPushdownOptimizer
extends Object
implements Transform

Make RS calculate top-K selection for limit clause. It's only works with RS for limit operation which means between RS and LITMIT, there should not be other operators which may change number of rows like FilterOperator. see Operator.acceptLimitPushdown() If RS is only for limiting rows, RSHash counts row with same key separately. But if RS is for GBY, RSHash should forward all the rows with the same key. Legend : A(a) --> key A, value a, row A(a) If each RS in mapper tasks is forwarded rows like this MAP1(RS) : 40(a)-10(b)-30(c)-10(d)-70(e)-80(f) MAP2(RS) : 90(g)-80(h)-60(i)-40(j)-30(k)-20(l) MAP3(RS) : 40(m)-50(n)-30(o)-30(p)-60(q)-70(r) OBY or GBY makes result like this, REDUCER : 10(b,d)-20(l)-30(c,k,o,p)-40(a,j,m)-50(n)-60(i,q)-70(e,r)-80(f,h)-90(g) LIMIT 3 for GBY: 10(b,d)-20(l)-30(c,k,o,p) LIMIT 3 for OBY: 10(b,d)-20(l) with the optimization, the amount of shuffling can be reduced, making identical result For GBY, MAP1 : 40(a)-10(b)-30(c)-10(d) MAP2 : 40(j)-30(k)-20(l) MAP3 : 40(m)-50(n)-30(o)-30(p) REDUCER : 10(b,d)-20(l)-30(c,k,o,p)-40(a,j,m)-50(n) LIMIT 3 : 10(b,d)-20(l)-30(c,k,o,p) For OBY, MAP1 : 10(b)-30(c)-10(d) MAP2 : 40(j)-30(k)-20(l) MAP3 : 40(m)-50(n)-30(o) REDUCER : 10(b,d)-20(l)-30(c,k,o)-40(j,m)-50(n) LIMIT 3 : 10(b,d)-20(l)

Constructor Summary

LimitPushdownOptimizer()

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

LimitPushdownOptimizer

public LimitPushdownOptimizer()

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