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https://issues.apache.org/jira/browse/ASTERIXDB-1736?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15698708#comment-15698708
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Taewoo Kim commented on ASTERIXDB-1736:
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Removed Grace Hash Join.
> Grace Hash Join and Hybrid Hash Join are not being used.
> --------------------------------------------------------
>
> Key: ASTERIXDB-1736
> URL: https://issues.apache.org/jira/browse/ASTERIXDB-1736
> Project: Apache AsterixDB
> Issue Type: Improvement
> Reporter: Taewoo Kim
> Assignee: Taewoo Kim
>
> As the title says, Grace Hash Join and Hybrid Hash Join are not being used. I
> suggest that we remove these two join methods. Here are my findings for these
> two joins.
> 1) Grace Hash Join
> GraceHashJoinOperatorDescriptor is only called from two places:
> org.apache.hyracks.examples.tpch.client.join and
> TPCHCustomerOrderHashJoinTest.
> One is a Hyracks example (tpch.client) and the other is a unit test. This
> join is not used currently (not chosen during the compilation).
> 2) Hybrid Hash Join
> During the compilation, the optimizer decides whether it will use Hybrid Hash
> Join or Optimized Hybrid Hash Join.
> If the hash function family for each key variable is set, then we use the
> optimized hybrid hash join.
> If not, we use the hybrid hash join. However, in fact, this path - hybrid
> hash join path will never be chosen. Let's check the code.
> {code:title=HybridHashJoinPOperator.java|borderStyle=solid}
> IBinaryHashFunctionFamily[] hashFunFamilies =
> JobGenHelper.variablesToBinaryHashFunctionFamilies(keysLeftBranch,
> env, context);
>
> ...
>
> boolean optimizedHashJoin = true;
> for (IBinaryHashFunctionFamily family : hashFunFamilies) {
> if (family == null) {
> optimizedHashJoin = false;
> break;
> }
> }
> if (optimizedHashJoin) {
> opDesc = generateOptimizedHashJoinRuntime(context, inputSchemas,
> keysLeft, keysRight, hashFunFamilies,
> comparatorFactories, predEvaluatorFactory, recDescriptor,
> spec);
> } else {
> opDesc = generateHashJoinRuntime(context, inputSchemas, keysLeft,
> keysRight, hashFunFactories,
> comparatorFactories, predEvaluatorFactory, recDescriptor,
> spec);
> }
> {code}
>
> As we can see, optimizedHashJoin is set to false only when the hash family is
> null.
> Then, how do we assign the hashfamily for each key variable?
> {code:title=JobGenHelper.java|borderStyle=solid}
> public static IBinaryHashFunctionFamily[]
> variablesToBinaryHashFunctionFamilies(
> Collection<LogicalVariable> varLogical, IVariableTypeEnvironment
> env, JobGenContext context)
> throws AlgebricksException {
> IBinaryHashFunctionFamily[] funFamilies = new
> IBinaryHashFunctionFamily[varLogical.size()];
> int i = 0;
> IBinaryHashFunctionFamilyProvider bhffProvider =
> context.getBinaryHashFunctionFamilyProvider();
> for (LogicalVariable var : varLogical) {
> Object type = env.getVarType(var);
> funFamilies[i++] = bhffProvider.getBinaryHashFunctionFamily(type);
> }
> return funFamilies;
> }
> {code}
> For each variable type, we try to get hash function family. In the current
> codebase, AqlBinaryHashFunctionFamilyProvider is the only class that
> implements IBinaryHashFunctionFamilyProvider.
> And for any type, it returns AMurmurHash3BinaryHashFunctionFamily.
> So, there is no way that the hash function family is null.
> {code:title= AqlBinaryHashFunctionFamilyProvider.java|borderStyle=solid}
> public class AqlBinaryHashFunctionFamilyProvider implements
> IBinaryHashFunctionFamilyProvider, Serializable {
> private static final long serialVersionUID = 1L;
> public static final AqlBinaryHashFunctionFamilyProvider INSTANCE = new
> AqlBinaryHashFunctionFamilyProvider();
> private AqlBinaryHashFunctionFamilyProvider() {
> }
> @Override
> public IBinaryHashFunctionFamily getBinaryHashFunctionFamily(Object type)
> throws AlgebricksException {
> // AMurmurHash3BinaryHashFunctionFamily converts numeric type to
> double type before doing hash()
> return AMurmurHash3BinaryHashFunctionFamily.INSTANCE;
> }
> }
> {code}
>
>
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