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https://issues.apache.org/jira/browse/HIVE-12391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15002860#comment-15002860
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Lefty Leverenz commented on HIVE-12391:
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Excuse my ignorance, but what does TS mean? (Best guess: table scan.)
> SkewJoinOptimizer might not kick in if columns are renamed after TS
> -------------------------------------------------------------------
>
> Key: HIVE-12391
> URL: https://issues.apache.org/jira/browse/HIVE-12391
> Project: Hive
> Issue Type: Bug
> Components: Logical Optimizer
> Affects Versions: 1.3.0, 2.0.0
> Reporter: Jesus Camacho Rodriguez
> Assignee: Jesus Camacho Rodriguez
> Attachments: HIVE-12391.patch
>
>
> SkewJoinOptimizer will not kick in if the columns are just renamed after the
> TS e.g. by the creation of a derived table.
> To reproduce, consider the following example:
> {code}
> set hive.optimize.skewjoin.compiletime = true;
> CREATE TABLE T1(key STRING, val STRING)
> SKEWED BY (key) ON ((2)) STORED AS TEXTFILE;
> CREATE TABLE T2(key STRING, val STRING)
> SKEWED BY (key) ON ((3)) STORED AS TEXTFILE;
> {code}
> For this query, SkewJoinOptimizer kicks in:
> {code}
> SELECT a.*, b.*
> FROM T1 a JOIN T2 b
> ON a.key = b.key
> {code}
> For this one, it does not:
> {code}
> SELECT a.*, b.*
> FROM
> (SELECT key as k, val as v FROM T1) a
> JOIN
> (SELECT key as k, val as v FROM T2) b
> ON a.k = b.k;
> {code}
> The reason is that SkewJoinOptimizer does not backtrack the origin of the
> column. Instead it just uses its name to know if it is produced by a certain
> TS.
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