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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-4113) select count(1) reads all columns with RCFile
Date Mon, 04 Mar 2013 22:03:13 GMT
Gopal V created HIVE-4113:
-----------------------------

             Summary: select count(1) reads all columns with RCFile
                 Key: HIVE-4113
                 URL: https://issues.apache.org/jira/browse/HIVE-4113
             Project: Hive
          Issue Type: Bug
            Reporter: Gopal V


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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