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From Bejoy Ks <bejoy.had...@gmail.com>
Subject Re: has bzip2 compression been deprecated?
Date Mon, 09 Jan 2012 18:59:58 GMT
Hi Tony
       As  I understand your requirement, your mapreduce job produces a
Sequence File as ouput and you need to use this file as an input to hive
table.
        When you CREATE and EXTERNAL Table in hive you specify a location
where your data is stored and also what is the format of that data( like
the field delimiter,row delimiter, file type etc of your data). You are
actually not loading data any where when you create a hive external
table(issue DDL), just specifying where the data lies in file system in
fact there is not even any validation performed that time to check on the
data quality. When you Query/Retrive your data  through Hive QLs the
parameters specified along with CREATE TABLE as ROW FORMAT,FILEDS
TERMINATED, STORED AS etc are used to execute the right MAP REDUCE job(s).

     In short STORED AS refer to the type of files that a table's data
directory holds.

For details
https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL#LanguageManualDDL-CreateTable

Hope it helps!..

Regards
Bejoy.K.S

On Mon, Jan 9, 2012 at 11:32 PM, Tony Burton <TBurton@sportingindex.com>wrote:

> Thanks Bejoy - I'm fairly new to Hive so may be wrong here, but I was
> under the impression that the STORED AS part of a CREATE TABLE in Hive
> refers to how the data in the table will be stored once the table is
> created, rather than the compression format of the data used to populate
> the table. Can you clarify which is the correct interpretation? If it's the
> latter, how would I read a sequence file into a Hive table?
>
> Thanks,
>
> Tony
>
>
>
>
> -----Original Message-----
> From: Bejoy Ks [mailto:bejoy.hadoop@gmail.com]
> Sent: 09 January 2012 17:33
> To: common-user@hadoop.apache.org
> Subject: Re: has bzip2 compression been deprecated?
>
> Hi Tony
>       Adding on to Harsh's comments. If you want the generated sequence
> files to be utilized by a hive table. Define your hive table as
>
> CREATE EXTERNAL TABLE tableNAme(col1 INT, c0l2 STRING)
> ...
> ...
> ....
> STORED AS SEQUENCEFILE;
>
>
> Regards
> Bejoy.K.S
>
> On Mon, Jan 9, 2012 at 10:32 PM, alo.alt <wget.null@googlemail.com> wrote:
>
> > Tony,
> >
> > snappy is also available:
> > http://code.google.com/p/hadoop-snappy/
> >
> > best,
> >  Alex
> >
> > --
> > Alexander Lorenz
> > http://mapredit.blogspot.com
> >
> > On Jan 9, 2012, at 8:49 AM, Harsh J wrote:
> >
> > > Tony,
> > >
> > > * Yeah, SequenceFiles aren't human-readable, but "fs -text" can read it
> > out (instead of a plain "fs -cat"). But if you are gonna export your
> files
> > into a system you do not have much control over, probably best to have
> the
> > resultant files not be in SequenceFile/Avro-DataFile format.
> > > * Intermediate (M-to-R) files use a custom IFile format these days,
> > which is built purely for that purpose.
> > > * Hive can use SequenceFiles very well. There is also documented info
> on
> > this in the Hive's wiki pages (Check the DDL pages, IIRC).
> > >
> > > On 09-Jan-2012, at 9:44 PM, Tony Burton wrote:
> > >
> > >> Thanks for the quick reply and the clarification about the
> > documentation.
> > >>
> > >> Regarding sequence files: am I right in thinking that they're a good
> > choice for intermediate steps in chained MR jobs, or for file transfer
> > between the Map and the Reduce phases of a job; but they shouldn't be
> used
> > for human-readable files at the end of one or more MapReduce jobs? How
> > about if the only use a job's output is analysis via Hive - can Hive
> create
> > tables from sequence files?
> > >>
> > >> Tony
> > >>
> > >>
> > >>
> > >> -----Original Message-----
> > >> From: Harsh J [mailto:harsh@cloudera.com]
> > >> Sent: 09 January 2012 15:34
> > >> To: common-user@hadoop.apache.org
> > >> Subject: Re: has bzip2 compression been deprecated?
> > >>
> > >> Bzip2 is pretty slow. You probably do not want to use it, even if it
> > does file splits (a feature not available in the stable line of
> 0.20.x/1.x,
> > but available in 0.22+).
> > >>
> > >> To answer your question though, bzip2 was removed from that document
> > cause it isn't a native library (its pure Java). I think bzip2 was added
> > earlier due to an oversight, as even 0.20 did not have a native bzip2
> > library. This change in docs does not mean that BZip2 is deprecated -- it
> > is still fully supported and available in the trunk as well. See
> > https://issues.apache.org/jira/browse/HADOOP-6292 for the doc update
> > changes that led to this.
> > >>
> > >> The best way would be to use either:
> > >>
> > >> (a) Hadoop sequence files with any compression codec of choice (best
> > would be lzo, gz, maybe even snappy). This file format is built for HDFS
> > and MR and is splittable. Another choice would be Avro DataFiles from the
> > Apache Avro project.
> > >> (b) LZO codecs for Hadoop, via
> https://github.com/toddlipcon/hadoop-lzo(and hadoop-lzo-packager for
> packages). This requires you to run indexing
> > operations before the .lzo can be made splittable, but works great with
> > this extra step added.
> > >>
> > >> On 09-Jan-2012, at 7:17 PM, Tony Burton wrote:
> > >>
> > >>> Hi,
> > >>>
> > >>> I'm trying to work out which compression algorithm I should be using
> > in my MapReduce jobs.  It seems to me that the best solution is a
> > compromise between speed, efficiency and splittability. The only
> > compression algorithm to handle file splits (according to Hadoop: The
> > Definitive Guide 2nd edition p78 etc) is bzip2, at the expense of
> > compression speed.
> > >>>
> > >>> However, I see from the documentation at
> > http://hadoop.apache.org/common/docs/current/native_libraries.html that
> > the bzip2 library is no longer mentioned, and hasn't been since version
> > 0.20.0, see
> > http://hadoop.apache.org/common/docs/r0.20.0/native_libraries.html -
> > however the bzip2 Codec is still in the API at
> >
> http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/io/compress/BZip2Codec.html
> > .
> > >>>
> > >>> Has bzip2 support been removed from Hadoop, or will it be removed
> soon?
> > >>>
> > >>> Thanks,
> > >>>
> > >>> Tony
> > >>>
> > >>>
> > >>>
> > >>>
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