Raghu Angadi commented on PIG-993:

Deletion procedure : 

   # Check if a column group with the given name exists and throw an error if 
there is no such group.
   # If the column group is already deleted return normally.
      ** If a column group is already marked deleted and the corresponding 
physical directory still 
        exists, try to remove the the column group data again. An earlier 
attempt might not have
        removed the directory.
   # Create a an empty file ".deleted-CGNAME" in the top level directory. 
   # If the creation fails, check if the file already exists. This can happen 
when two users concurrently
      try to delete the same column group. If CG is marked deleted after this, 
return success. Exception is 
      thrown for any other error.
   # Delete the column group directory. 
   # An exception is thrown if deletion fails. Note that, column group is 
already marked deleted even though 
      the deletion of a directory failed. A subsequent deletion of such a 
column group will again try to to delete the directory.

> [zebra] Abitlity to drop a column group in a table
> --------------------------------------------------
>                 Key: PIG-993
>                 URL: https://issues.apache.org/jira/browse/PIG-993
>             Project: Pig
>          Issue Type: Bug
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>             Fix For: 0.5.0
> A Zebra table is stored as multiple sub tables each containing a set of 
> columns called column group (CG). The user specifies how these columns are 
> grouped while creating a table through the _storage hint_.
> For some of the large tables, it might be necessary for users to remove a set 
> of columns and retain the rest. This jira provides a way for users to delete 
> an entire column group. 
> The following comments will have more details on API and the semantics. 

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