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https://issues.apache.org/jira/browse/DRILL-4237?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15231433#comment-15231433
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ASF GitHub Bot commented on DRILL-4237:
---------------------------------------
Github user laurentgo commented on a diff in the pull request:
https://github.com/apache/drill/pull/430#discussion_r58967457
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/DrillHash.java
---
@@ -0,0 +1,50 @@
+/**
+ * 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.drill.exec.expr.fn.impl;
+
+import io.netty.buffer.DrillBuf;
+import io.netty.util.internal.PlatformDependent;
+
+/**
+ * The base class of hash classes used in Drill.
--- End diff --
it says base class, but it doesn't provide any method to implement or
override. Is there any thought about having an interface with hashXX methods
instead?
> Skew in hash distribution
> -------------------------
>
> Key: DRILL-4237
> URL: https://issues.apache.org/jira/browse/DRILL-4237
> Project: Apache Drill
> Issue Type: Bug
> Components: Functions - Drill
> Affects Versions: 1.4.0
> Reporter: Aman Sinha
> Assignee: Chunhui Shi
>
> Apparently, the fix in DRILL-4119 did not fully resolve the data skew issue.
> It worked fine on the smaller sample of the data set but on another sample of
> the same data set, it still produces skewed values - see below the hash
> values which are all odd numbers.
> {noformat}
> 0: jdbc:drill:zk=local> select columns[0], hash32(columns[0]) from `test.csv`
> limit 10;
> +-----------------------------------+--------------+
> | EXPR$0 | EXPR$1 |
> +-----------------------------------+--------------+
> | f71aaddec3316ae18d43cb1467e88a41 | 1506011089 |
> | 3f3a13bb45618542b5ac9d9536704d3a | 1105719049 |
> | 6935afd0c693c67bba482cedb7a2919b | -18137557 |
> | ca2a938d6d7e57bda40501578f98c2a8 | -1372666789 |
> | fab7f08402c8836563b0a5c94dbf0aec | -1930778239 |
> | 9eb4620dcb68a84d17209da279236431 | -970026001 |
> | 16eed4a4e801b98550b4ff504242961e | 356133757 |
> | a46f7935fea578ce61d8dd45bfbc2b3d | -94010449 |
> | 7fdf5344536080c15deb2b5a2975a2b7 | -141361507 |
> | b82560a06e2e51b461c9fe134a8211bd | -375376717 |
> +-----------------------------------+--------------+
> {noformat}
> This indicates an underlying issue with the XXHash64 java implementation,
> which is Drill's implementation of the C version. One of the key difference
> as pointed out by [~jnadeau] was the use of unsigned int64 in the C version
> compared to the Java version which uses (signed) long. I created an XXHash
> version using com.google.common.primitives.UnsignedLong. However,
> UnsignedLong does not have bit-wise operations that are needed for XXHash
> such as rotateLeft(), XOR etc. One could write wrappers for these but at
> this point, the question is: should we think of an alternative hash function
> ?
> The alternative approach could be the murmur hash for numeric data types that
> we were using earlier and the Mahout version of hash function for string
> types
> (https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/HashHelper.java#L28).
> As a test, I reverted to this function and was getting good hash
> distribution for the test data.
> I could not find any performance comparisons of our perf tests (TPC-H or DS)
> with the original and newer (XXHash) hash functions. If performance is
> comparable, should we revert to the original function ?
> As an aside, I would like to remove the hash64 versions of the functions
> since these are not used anywhere.
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