[jira] [Commented] (DRILL-4122) Create unit test suite for checking quality of hashing for hash based operators
[ https://issues.apache.org/jira/browse/DRILL-4122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15183278#comment-15183278 ] Sudheesh Katkam commented on DRILL-4122: This is actually a bad measure because there are distributions (other than uniform) with skewness of 0. > Create unit test suite for checking quality of hashing for hash based > operators > --- > > Key: DRILL-4122 > URL: https://issues.apache.org/jira/browse/DRILL-4122 > Project: Apache Drill > Issue Type: Bug > Components: Functions - Drill >Affects Versions: 1.3.0 >Reporter: Aman Sinha >Assignee: Sudheesh Katkam > > We have encountered substantial skew in the hash based operators (hash > distribution, hash aggregation, hash join) for certain data sets. Two such > issues are DRILL-2803, DRILL-4119. > It would be very useful to have a unit test suite to test the quality of > hashing. > The number of combinations is large: num_data_types x nullability x > num_hash_function_types (32bit, 64bit, AsDouble variations). Plus, the nature > of the data itself. We would have to be judicious about picking a > reasonable subset of this space. We should also look at open source test > suites in this area. -- This message was sent by Atlassian JIRA (v6.3.4#6332)
[jira] [Commented] (DRILL-4122) Create unit test suite for checking quality of hashing for hash based operators
[
https://issues.apache.org/jira/browse/DRILL-4122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15130729#comment-15130729
]
Sudheesh Katkam commented on DRILL-4122:
[Skewness|http://mathworld.wolfram.com/Skewness.html] is another measure. This
method could be implemented using a Drill UDAF based on the [online skewness
algorithm|https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Higher-order_statistics].
The following query checks the skewness of 10^7 pseudorandom double values
(between 0 inclusive and 1 exclusive):
{code}
WITH x AS (SELECT * FROM (VALUES 1,1,1,1,1,1,1,1,1,1) AS MyTable(key))
SELECT skewness(mod(abs(hash(random())), 10))
FROM x AS x1
INNER JOIN x AS x2 ON x2.key = x1.key
INNER JOIN x AS x3 ON x3.key = x2.key
INNER JOIN x AS x4 ON x4.key = x3.key
INNER JOIN x AS x5 ON x5.key = x4.key
INNER JOIN x AS x6 ON x6.key = x5.key
INNER JOIN x AS x7 ON x7.key = x6.key;
{code}
> Create unit test suite for checking quality of hashing for hash based
> operators
> ---
>
> Key: DRILL-4122
> URL: https://issues.apache.org/jira/browse/DRILL-4122
> Project: Apache Drill
> Issue Type: Bug
> Components: Functions - Drill
>Affects Versions: 1.3.0
>Reporter: Aman Sinha
>Assignee: Sudheesh Katkam
>
> We have encountered substantial skew in the hash based operators (hash
> distribution, hash aggregation, hash join) for certain data sets. Two such
> issues are DRILL-2803, DRILL-4119.
> It would be very useful to have a unit test suite to test the quality of
> hashing.
> The number of combinations is large: num_data_types x nullability x
> num_hash_function_types (32bit, 64bit, AsDouble variations). Plus, the nature
> of the data itself. We would have to be judicious about picking a
> reasonable subset of this space. We should also look at open source test
> suites in this area.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
[jira] [Commented] (DRILL-4122) Create unit test suite for checking quality of hashing for hash based operators
[ https://issues.apache.org/jira/browse/DRILL-4122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15023091#comment-15023091 ] Parth Chandra commented on DRILL-4122: -- https://code.google.com/p/smhasher/(Unfortunately in C) It might also be a good idea to evaluate other hash methods. > Create unit test suite for checking quality of hashing for hash based > operators > --- > > Key: DRILL-4122 > URL: https://issues.apache.org/jira/browse/DRILL-4122 > Project: Apache Drill > Issue Type: Bug > Components: Functions - Drill >Affects Versions: 1.3.0 >Reporter: Aman Sinha > > We have encountered substantial skew in the hash based operators (hash > distribution, hash aggregation, hash join) for certain data sets. Two such > issues are DRILL-2803, DRILL-4119. > It would be very useful to have a unit test suite to test the quality of > hashing. > The number of combinations is large: num_data_types x nullability x > num_hash_function_types (32bit, 64bit, AsDouble variations). Plus, the nature > of the data itself. We would have to be judicious about picking a > reasonable subset of this space. We should also look at open source test > suites in this area. -- This message was sent by Atlassian JIRA (v6.3.4#6332)
