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https://issues.apache.org/jira/browse/HIVE-20489?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16615413#comment-16615413
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Janaki Lahorani commented on HIVE-20489:
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A test case that covers this issue is pushed into master using HIVE-20526.
> Explain plan of query hangs
> ---------------------------
>
> Key: HIVE-20489
> URL: https://issues.apache.org/jira/browse/HIVE-20489
> Project: Hive
> Issue Type: Bug
> Reporter: Janaki Lahorani
> Assignee: Janaki Lahorani
> Priority: Major
> Fix For: 4.0.0
>
> Attachments: HIVE-20489.1.patch, HIVE-20489.2.patch,
> HIVE-20489.3.patch, HIVE-20489.4.patch
>
>
> Explain on a query that joins 47 views, in effect around 94 joins after view
> expansion seems to take forever. The case here tries to generate a plan
> using map join with conditional tasks.
> When the task graph is huge with many paths, there can be a performance issue
> during compilation. This is caused by recursive traversal of task graph in
> internTableDesc and deriveFinalExplainAttributes. The use of recursion is
> inefficient in a couple of ways.
> * For large graphs the recursion was filling up the stack
> * Instead of finding the map works, the traversal was walking all possible
> paths from root causing a huge performance problem.
> The fix is to replace the traversal from recursive to an iterative one,
> keeping track of the nodes already visited. The fix uses getMRTasks,
> getSparkTasks and getTezTasks to do iterative traversal. These calls were
> changed to using iterative calls through HIVE-17195. When pushing this patch
> to an older release, please make sure HIVE-17195 is also pushed to that
> release.
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