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https://issues.apache.org/jira/browse/HIVE-4005?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13574923#comment-13574923
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Kevin Wilfong commented on HIVE-4005:
-------------------------------------
https://reviews.facebook.net/D8487
(Portions of this diff were written by [~dikanggu], [~sxyuan], and myself)
> 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
>
> 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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