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https://issues.apache.org/jira/browse/HIVE-19501?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16472950#comment-16472950
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Gopal V commented on HIVE-19501:
--------------------------------
The short and int versions are nearly identical, except they need to account
for the sign-bit during the cast up from int -> long by doing
{code}
long k = Integer.reverseBytes(data) & (-1L >>> 32);
int length = Integer.BYTES;
{code}
> Fix HyperLogLog to be threadsafe
> --------------------------------
>
> Key: HIVE-19501
> URL: https://issues.apache.org/jira/browse/HIVE-19501
> Project: Hive
> Issue Type: Bug
> Reporter: Zoltan Haindrich
> Priority: Major
>
> not sure if this is an issue in reality or not; but there are 3 static fields
> in HyperLogLog which are rewritten during working; if there are multiple
> threads are calculating HLL in the same JVM, there is a theoretical chance
> that they might overwrite eachothers value...
> static fields:
> https://github.com/apache/hive/blob/8028ce8a4cf5a03e2998c33e032a511fae770b47/standalone-metastore/src/main/java/org/apache/hadoop/hive/common/ndv/hll/HyperLogLog.java#L65
> usage:
> https://github.com/apache/hive/blob/8028ce8a4cf5a03e2998c33e032a511fae770b47/standalone-metastore/src/main/java/org/apache/hadoop/hive/common/ndv/hll/HyperLogLog.java#L216
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