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From John Sichi <jsi...@facebook.com>
Subject Re: Question regarding region scans in HBase integration
Date Mon, 13 Sep 2010 02:51:11 GMT
I see.  My changes are starting out super-simple, addressing only the case of an equality predicate
and a simple key.  Once I get those committed, we can talk about how to add support for compound
keys and range predicates, which is where your code could come in.

JVS

On Sep 11, 2010, at 7:26 PM, Daniel Einspanjer wrote:

> Okay, that getSplits part is specifically where my code was involved.
> 
> My use case was one of salted rowkeys.  We are storing documents that have a guid as
the id and the creation date of the document is important for scanning.  When we tested having
a rowkey format of <creationtimestamp>+<guid>, the RegionServer hotspots became
problematic, so we decided to salt the rowkey by using the first digit of the guid: <hexchar>+<creationtimestamp>+<guid>.
 This gives us nice distribution of inserts throughout the cluster, but of course, it makes
scanning a contiguous date range much more complicated.
> 
> The code I have allows us to write a MR that takes a list of prefixes (e.g. the hexchar)
and a list of ranges (e.g. the desired timestamps) and construct a master Scan object that
contains any configuration such as filters or cache settings, and a series of Scan objects
that constitute the Cartesian product of the ranges.  Then, it passes those in to a custom
getSplits that ensures only the needed regions participate in the Map.
> 
> If this sounds like it might be useful, I'll work on getting it cleaned up and posted
somewhere so you can review it and maybe glean it for ideas.  If you are already past that
point then I apologize for not checking into this sooner. :)
> 
> -Daniel
> 
> On 9/11/10 7:09 PM, John Sichi wrote:
>> Hi Daniel,
>> 
>> I'm almost done with this for HIVE-1226; the remaining step I need to finish is to
get the filter passed down during getSplits, since the HBase getSplits implementation takes
care of figuring out which regions contain the row in question.
>> 
>> JVS
>> 
>> On Sep 11, 2010, at 7:00 PM, Daniel Einspanjer wrote:
>> 
>>> I was trying to spend a little time this weekend catching up with the current
state of HBase integration for Hive.  One thing that I haven't seen mentioned is how exactly
Hive scans an HBase table during a SELECT.
>>> 
>>> Does Hive have logic that allows it to intelligently scan only the participating
regions during a SELECT query that uses the rowkey?  If not, I recently wrote some code that
allows a MapReduce job to effectively select the regions based on a list of start/end rowkey
ranges.  If this might be useful to the Hive integration, I could create a Jira and take a
look at trying to set up a patch.
>>> 
>>> Daniel Einspanjer
>>> Metrics Architect
>>> Mozilla Corporation


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