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From "Yin Huai (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-4113) Optimize select count(1) with RCFile and Orc
Date Thu, 19 Sep 2013 21:09:55 GMT

    [ https://issues.apache.org/jira/browse/HIVE-4113?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13772317#comment-13772317
] 

Yin Huai commented on HIVE-4113:
--------------------------------

please ignore those duplicated replies... 
                
> Optimize select count(1) with RCFile and Orc
> --------------------------------------------
>
>                 Key: HIVE-4113
>                 URL: https://issues.apache.org/jira/browse/HIVE-4113
>             Project: Hive
>          Issue Type: Bug
>          Components: File Formats
>            Reporter: Gopal V
>            Assignee: Yin Huai
>             Fix For: 0.12.0
>
>         Attachments: HIVE-4113-0.patch, HIVE-4113.1.patch, HIVE-4113.2.patch, HIVE-4113.3.patch,
HIVE-4113.patch, HIVE-4113.patch
>
>
> select count(1) loads up every column & every row when used with RCFile.
> "select count(1) from store_sales_10_rc" gives
> {code}
> Job 0: Map: 5  Reduce: 1   Cumulative CPU: 31.73 sec   HDFS Read: 234914410 HDFS Write:
8 SUCCESS
> {code}
> Where as, "select count(ss_sold_date_sk) from store_sales_10_rc;" reads far less
> {code}
> Job 0: Map: 5  Reduce: 1   Cumulative CPU: 29.75 sec   HDFS Read: 28145994 HDFS Write:
8 SUCCESS
> {code}
> Which is 11% of the data size read by the COUNT(1).
> This was tracked down to the following code in RCFile.java
> {code}
>       } else {
>         // TODO: if no column name is specified e.g, in select count(1) from tt;
>         // skip all columns, this should be distinguished from the case:
>         // select * from tt;
>         for (int i = 0; i < skippedColIDs.length; i++) {
>           skippedColIDs[i] = false;
>         }
> {code}

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