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From John Sichi <jsi...@facebook.com>
Subject Re: Question regarding region scans in HBase integration
Date Mon, 13 Sep 2010 19:33:48 GMT
I've uploaded a patch for review:


And updated the design doc:


CC'ing a few people who've been working on storage handlers.


On Sep 12, 2010, at 7:51 PM, John Sichi wrote:

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.


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. :)


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.


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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