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https://issues.apache.org/jira/browse/HIVE-4113?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13772163#comment-13772163
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Ashutosh Chauhan commented on HIVE-4113:
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[~yhuai] I left some comments on RB. But, it seems like you updated the patch
in meanwhile, so some of those you may have already addressed.
> 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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