[jira] [Commented] (HIVE-4005) Column truncation

["Namit Jain (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-4005?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13640186#comment-13640186 ] 

Namit Jain commented on HIVE-4005:
----------------------------------

Sorry, this one slipped under the radar.
Can you refresh one more time ?
                
> Column truncation
> -----------------
>
>                 Key: HIVE-4005
>                 URL: https://issues.apache.org/jira/browse/HIVE-4005
>             Project: Hive
>          Issue Type: New Feature
>          Components: CLI
>    Affects Versions: 0.11.0
>            Reporter: Kevin Wilfong
>            Assignee: Kevin Wilfong
>         Attachments: HIVE-4005.1.patch.txt, HIVE-4005.2.patch.txt, HIVE-4005.3.patch.txt, HIVE-4005.4.patch.txt, HIVE-4005.5.patch.txt
>
>
> Column truncation allows users to remove data for columns that are no longer useful.
> This is done by removing the data for the column and setting the length of the column data and related lengths to 0 in the RC file header.
> RC file was fixed to recognize columns with lengths of zero to be empty and are treated as if the column doesn't exist in the data, a null is returned for every value of that column in every row. This is the same thing that happens when more columns are selected than exist in the file.
> A new command was added to the CLI
> TRUNCATE TABLE ... PARTITION ... COLUMNS ...
> This launches a map only job where each mapper rewrites a single file without the unnecessary column data and the adjusted headers. It does not uncompress/deserialize the data so it is much faster than rewriting the data with NULLs.

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