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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.hive.ql.ppd;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.Map;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.hadoop.hive.ql.exec.CommonJoinOperator;
import org.apache.hadoop.hive.ql.exec.FilterOperator;
import org.apache.hadoop.hive.ql.exec.LateralViewForwardOperator;
import org.apache.hadoop.hive.ql.exec.LateralViewJoinOperator;
import org.apache.hadoop.hive.ql.exec.LimitOperator;
import org.apache.hadoop.hive.ql.exec.Operator;
import org.apache.hadoop.hive.ql.exec.PTFOperator;
import org.apache.hadoop.hive.ql.exec.ReduceSinkOperator;
import org.apache.hadoop.hive.ql.exec.ScriptOperator;
import org.apache.hadoop.hive.ql.exec.TableScanOperator;
import org.apache.hadoop.hive.ql.exec.UDTFOperator;
import org.apache.hadoop.hive.ql.lib.DefaultGraphWalker;
import org.apache.hadoop.hive.ql.lib.DefaultRuleDispatcher;
import org.apache.hadoop.hive.ql.lib.Dispatcher;
import org.apache.hadoop.hive.ql.lib.GraphWalker;
import org.apache.hadoop.hive.ql.lib.Node;
import org.apache.hadoop.hive.ql.lib.NodeProcessor;
import org.apache.hadoop.hive.ql.lib.Rule;
import org.apache.hadoop.hive.ql.lib.RuleRegExp;
import org.apache.hadoop.hive.ql.optimizer.Transform;
import org.apache.hadoop.hive.ql.parse.ParseContext;
import org.apache.hadoop.hive.ql.parse.SemanticException;
/**
* Implements predicate pushdown. Predicate pushdown is a term borrowed from
* relational databases even though for Hive it is predicate pushup. The basic
* idea is to process expressions as early in the plan as possible. The default
* plan generation adds filters where they are seen but in some instances some
* of the filter expressions can be pushed nearer to the operator that sees this
* particular data for the first time. e.g. select a.*, b.* from a join b on
* (a.col1 = b.col1) where a.col1 > 20 and b.col2 > 40
*
* For the above query, the predicates (a.col1 > 20) and (b.col2 > 40), without
* predicate pushdown, would be evaluated after the join processing has been
* done. Suppose the two predicates filter out most of the rows from a and b,
* the join is unnecessarily processing these rows. With predicate pushdown,
* these two predicates will be processed before the join.
*
* Predicate pushdown is disabled by setting hive.optimize.ppd to false. It is
* enabled by default.
*
* The high-level algorithm is describe here - An operator is processed after
* all its children have been processed - An operator processes its own
* predicates and then merges (conjunction) with the processed predicates of its
* children. In case of multiple children, there are combined using disjunction
* (OR). - A predicate expression is processed for an operator using the
* following steps - If the expr is a constant then it is a candidate for
* predicate pushdown - If the expr is a col reference then it is a candidate
* and its alias is noted - If the expr is an index and both the array and index
* expr are treated as children - If the all child expr are candidates for
* pushdown and all of the expression reference only one alias from the
* operator's RowResolver then the current expression is also a candidate One
* key thing to note is that some operators (Select, ReduceSink, GroupBy, Join
* etc) change the columns as data flows through them. In such cases the column
* references are replaced by the corresponding expression in the input data.
*/
public class PredicatePushDown extends Transform {
private static final Logger LOG = LoggerFactory.getLogger(PredicatePushDown.class);
private ParseContext pGraphContext;
@Override
public ParseContext transform(ParseContext pctx) throws SemanticException {
pGraphContext = pctx;
// create a the context for walking operators
OpWalkerInfo opWalkerInfo = new OpWalkerInfo(pGraphContext);
Map opRules = new LinkedHashMap();
opRules.put(new RuleRegExp("R1",
FilterOperator.getOperatorName() + "%"),
OpProcFactory.getFilterProc());
opRules.put(new RuleRegExp("R2",
PTFOperator.getOperatorName() + "%"),
OpProcFactory.getPTFProc());
opRules.put(new RuleRegExp("R3",
CommonJoinOperator.getOperatorName() + "%"),
OpProcFactory.getJoinProc());
opRules.put(new RuleRegExp("R4",
TableScanOperator.getOperatorName() + "%"),
OpProcFactory.getTSProc());
opRules.put(new RuleRegExp("R5",
ScriptOperator.getOperatorName() + "%"),
OpProcFactory.getSCRProc());
opRules.put(new RuleRegExp("R6",
LimitOperator.getOperatorName() + "%"),
OpProcFactory.getLIMProc());
opRules.put(new RuleRegExp("R7",
UDTFOperator.getOperatorName() + "%"),
OpProcFactory.getUDTFProc());
opRules.put(new RuleRegExp("R8",
LateralViewForwardOperator.getOperatorName() + "%"),
OpProcFactory.getLVFProc());
opRules.put(new RuleRegExp("R9",
LateralViewJoinOperator.getOperatorName() + "%"),
OpProcFactory.getLVJProc());
opRules.put(new RuleRegExp("R10",
ReduceSinkOperator.getOperatorName() + "%"),
OpProcFactory.getRSProc());
// The dispatcher fires the processor corresponding to the closest matching
// rule and passes the context along
Dispatcher disp = new DefaultRuleDispatcher(OpProcFactory.getDefaultProc(),
opRules, opWalkerInfo);
GraphWalker ogw = new DefaultGraphWalker(disp);
// Create a list of topop nodes
ArrayList topNodes = new ArrayList();
topNodes.addAll(pGraphContext.getTopOps().values());
ogw.startWalking(topNodes, null);
if (LOG.isDebugEnabled()) {
LOG.debug("After PPD:\n" + Operator.toString(pctx.getTopOps().values()));
}
return pGraphContext;
}
}
