[jira] [Comment Edited] (HIVE-18608) ORC should allow selectively disabling dictionary-encoding on specified columns

["Mithun Radhakrishnan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-18608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16349660#comment-16349660 ] 

Mithun Radhakrishnan edited comment on HIVE-18608 at 2/2/18 2:03 AM:
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I've attached an initial implementation, where dictionary encoding might be disabled via a table-property ({{'orc.skip.dictionary.for.columns'}}).

Note: I've only added support for top-level columns. Specifying this on a {{STRUCT}} will disable dictionary encoding for the entire sub-tree (i.e. all members of the {{STRUCT}}, recursively).

It might be good to support selection at an arbitrary depth, at a later date.
E.g. {{myInfoArray.__elem__.emailBody}}.


was (Author: mithun):
I've attached an initial implementation, where dictionary encoding might be disabled via a table-property ({{'orc.skip.dictionary.for.columns'}}).

Note: I've only added support for top-level columns. Specifying this on a {{STRUCT}} will disable dictionary encoding for the entire sub-tree (i.e. all members of the STRUCT), recursively.

It might be good to support selection at an arbitrary depth, at a later date.
E.g. {{myInfoArray.__elem__.emailBody}}.

> ORC should allow selectively disabling dictionary-encoding on specified columns
> -------------------------------------------------------------------------------
>
>                 Key: HIVE-18608
>                 URL: https://issues.apache.org/jira/browse/HIVE-18608
>             Project: Hive
>          Issue Type: New Feature
>          Components: ORC
>    Affects Versions: 3.0.0, 2.4.0, 2.2.1
>            Reporter: Mithun Radhakrishnan
>            Assignee: Mithun Radhakrishnan
>            Priority: Major
>         Attachments: HIVE-18608.1-branch-2.2.patch
>
>
> Just as ORC allows the choice of columns to enable bloom-filters on, it would be nice to have a way to specify which columns {{DICTIONARY_V2}} encoding should be disabled on.
> Currently, the choice of dictionary-encoding depends on the results of sampling the first row-stride within a stripe. If the user knows that a column's cardinality is bound to prevent an effective dictionary, she might choose to simply disable it on just that column, and avoid the cost of sampling in the first row-stride.



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