Vincent Poon commented on PHOENIX-4704:

[~jamestaylor] mind doing a review?  This enhances IndexTool with an option to 
use TABLESAMPLE to sample the data table and presplit the index table.  There's 
also an option to only split if the data table has > N regions.


> Presplit index tables when building asynchronously
> --------------------------------------------------
>                 Key: PHOENIX-4704
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-4704
>             Project: Phoenix
>          Issue Type: Improvement
>            Reporter: Vincent Poon
>            Assignee: Vincent Poon
>            Priority: Major
>         Attachments: PHOENIX-4704.master.v1.patch
> For large data tables with many regions, if we build the index asynchronously 
> using the IndexTool, the index table will initial face a hotspot as all data 
> region mappers attempt to write to the sole new index region.  This can 
> potentially lead to the index getting disabled if writes to the index table 
> timeout during this hotspotting.
> We can add an optional step (or perhaps activate it based on the count of 
> regions in the data table) to the IndexTool to first do a MR job to gather 
> stats on the indexed column values, and then attempt to presplit the index 
> table before we do the actual index build MR job.

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