He Yongqiang commented on PIG-833:

Thanks Jing.
Yeah, i know the design of column groups and projection.
The reason i was asking is that i saw an usage in line 251 TestBasicTable.java:
doReadWrite(path, 2, 100, "SF_a,SF_b,SF_c,SF_d,SF_e", 
"[SF_a,SF_b,SF_c];[SF_d,SF_e]", "SF_f,SF_a,SF_c,SF_d", true, false);
where  "SF_f,SF_a,SF_c,SF_d" is passed as projection, but is there a column 
"SF_f" defined?

btw, can you give more detail about the design of Partition? ColumnGroup is 
much like projection in C-Store, so it can be more easily to be understood.

> Storage access layer
> --------------------
>                 Key: PIG-833
>                 URL: https://issues.apache.org/jira/browse/PIG-833
>             Project: Pig
>          Issue Type: New Feature
>            Reporter: Jay Tang
>         Attachments: hadoop20.jar.bz2, PIG-833-zebra.patch, 
> PIG-833-zebra.patch.bz2, PIG-833-zebra.patch.bz2, 
> TEST-org.apache.hadoop.zebra.pig.TestCheckin1.txt, test.out, zebra-javadoc.tgz
> A layer is needed to provide a high level data access abstraction and a 
> tabular view of data in Hadoop, and could free Pig users from implementing 
> their own data storage/retrieval code.  This layer should also include a 
> columnar storage format in order to provide fast data projection, 
> CPU/space-efficient data serialization, and a schema language to manage 
> physical storage metadata.  Eventually it could also support predicate 
> pushdown for further performance improvement.  Initially, this layer could be 
> a contrib project in Pig and become a hadoop subproject later on.

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