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https://issues.apache.org/jira/browse/HIVE-4113?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13773425#comment-13773425
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Yin Huai commented on HIVE-4113:
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[~ashutoshc] it is pretty easy. I just spent sometime on refactoring the code
to make sure we assign needed columns to all dummy TableScanOperators. However,
it seems in trunk, if we need to have an individual MR job for UNION ALL, we
always create a dummy TableScanOperator with a dummy conf. But, in other cases,
a dummy TableScanOperator does not have a conf. I think adding the conf is
better because those dummy TableScanOperators can be seen in the results of
EXPLAIN. So, a bug such as HIVE-4927 can be found in a easier way. The one time
cost of adding a dummy conf to a dummy TableScanOperator is that we may need to
update lots of golden files...
> 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-4113.4.patch, HIVE-4113.5.patch, HIVE-4113.6.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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