[ 
https://issues.apache.org/jira/browse/SPARK-14393?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15564350#comment-15564350
 ] 

Takeshi Yamamuro commented on SPARK-14393:
------------------------------------------

Since coalesce() just after monotonicallyIncreasingId() breaks the semantics, 
it's okay not to do that.
For example, coalesce() before monotonicallyIncreasingId() is okay;
{code}
scala> 
spark.range(10).repartition(5).coalesce(1).select(monotonicallyIncreasingId()).show
warning: there was one deprecation warning; re-run with -deprecation for details
+-----------------------------+
|monotonically_increasing_id()|
+-----------------------------+
|                            0|
|                            1|
|                            2|
|                            3|
|                            4|
|                            5|
|                            6|
|                            7|
|                            8|
|                            9|
+-----------------------------+
{code}

> monotonicallyIncreasingId not monotonically increasing with downstream 
> coalesce
> -------------------------------------------------------------------------------
>
>                 Key: SPARK-14393
>                 URL: https://issues.apache.org/jira/browse/SPARK-14393
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 1.6.0
>            Reporter: Jason Piper
>
> When utilising monotonicallyIncreasingId with a coalesce, it appears that 
> every partition uses the same offset (0) leading to non-monotonically 
> increasing IDs.
> See examples below
> {code}
> >>> sqlContext.range(10).select(monotonicallyIncreasingId()).show()
> +---------------------------+
> |monotonicallyincreasingid()|
> +---------------------------+
> |                25769803776|
> |                51539607552|
> |                77309411328|
> |               103079215104|
> |               128849018880|
> |               163208757248|
> |               188978561024|
> |               214748364800|
> |               240518168576|
> |               266287972352|
> +---------------------------+
> >>> sqlContext.range(10).select(monotonicallyIncreasingId()).coalesce(1).show()
> +---------------------------+
> |monotonicallyincreasingid()|
> +---------------------------+
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> |                          0|
> +---------------------------+
> >>> sqlContext.range(10).repartition(5).select(monotonicallyIncreasingId()).coalesce(1).show()
> +---------------------------+
> |monotonicallyincreasingid()|
> +---------------------------+
> |                          0|
> |                          1|
> |                          0|
> |                          0|
> |                          1|
> |                          2|
> |                          3|
> |                          0|
> |                          1|
> |                          2|
> +---------------------------+
> {code}



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