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Subject: Re: Review Request 28417: DRILL-1742 Use Hive stats when planning
 queries on Hive data sources
From: "abdelhakim deneche" <adeneche@gmail.com>
To: "Aman Sinha" <asinha@maprtech.com>,
 "abdelhakim deneche" <adeneche@gmail.com>,
 "drill" <drill-dev@incubator.apache.org>
Date: Wed, 11 Feb 2015 17:55:03 -0000
Message-ID: <20150211175503.29076.22859@reviews.apache.org>
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Reply-To: "abdelhakim deneche" <adeneche@gmail.com>

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> On Nov. 25, 2014, 1:23 a.m., Aman Sinha wrote:
> > contrib/storage-hive/core/src/main/java/org/apache/drill/exec/store/hive/HiveScan.java, lines 298-300
> > <https://reviews.apache.org/r/28417/diff/3/?file=775081#file775081line298>
> >
> >     This is not necessarily true;  if you have empty tables, the rowcount will be 0. So you need to distinguish between the case where the stats are not available (maybe use -1 as an indicator) from the case where it is available and has 0 rowcount.
> 
> abdelhakim deneche wrote:
>     The problem is that when numRows=0 in the stats can actually mean the stats have not been computed yet! so we still need to estimate the row count using the size of the input splits.
>     I made some tests using empty tables, and the estimated row count for those tables is 0 too, so it's correct.

in hive 0.13 numRows will contain -1 when the stats were never computed.


- abdelhakim


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On Nov. 25, 2014, 7:46 p.m., abdelhakim deneche wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/28417/
> -----------------------------------------------------------
> 
> (Updated Nov. 25, 2014, 7:46 p.m.)
> 
> 
> Review request for drill.
> 
> 
> Bugs: DRILL-1742
>     https://issues.apache.org/jira/browse/DRILL-1742
> 
> 
> Repository: drill-git
> 
> 
> Description
> -------
> 
> HiveScan.getSplits() already gets the table and partitions metadata using MetaStoreUtils.
> We compute the total number of rows using the numRows property and store the computed number of rows in rowCount attribute which is later returned by getScanStats().
> 
> 
> Diffs
> -----
> 
>   contrib/storage-hive/core/src/main/java/org/apache/drill/exec/store/hive/HiveScan.java ddbc100 
> 
> Diff: https://reviews.apache.org/r/28417/diff/
> 
> 
> Testing
> -------
> 
> created several partitioned and non-partitioned tables, loaded data in hive.
> 
> used explain plan to check the number of rows when the whole table is queried and also when specific partitions are queried (to make sure the row count takes hive partition pruning into account)
> 
> 
> Thanks,
> 
> abdelhakim deneche
> 
>


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