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https://issues.apache.org/jira/browse/DRILL-1691?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Arina Ielchiieva reassigned DRILL-1691:
---------------------------------------
Assignee: Arina Ielchiieva
> ConvertCountToDirectScan rule should be applicable for 2 or more COUNT
> aggregates
> ---------------------------------------------------------------------------------
>
> Key: DRILL-1691
> URL: https://issues.apache.org/jira/browse/DRILL-1691
> Project: Apache Drill
> Issue Type: Improvement
> Components: Query Planning & Optimization
> Affects Versions: 0.6.0
> Reporter: Aman Sinha
> Assignee: Arina Ielchiieva
> Fix For: 1.12.0
>
>
> The ConvertCountToDirectScan rule currently only applies if there is a single
> COUNT(*) or COUNT(column) aggregate without group-by. This rule should be
> extended to apply for multiple such aggregates since the rule depends on the
> underlying ParquetGroupScan providing it the correct column value count and
> retrieving that count for multiple columns should be fine. However, if even
> 1 such column does not have statistics, then we should not apply this rule.
> Here's an example sequence:
> First do a CTAS such that we ensure that statistics are present for the
> table (the original Parquet data may not have stats):
> {code:sql}
> 0: jdbc:drill:zk=local> create table nation3 as select * from
> cp.`tpch/nation.parquet`;
> +------------+---------------------------+
> | Fragment | Number of records written |
> +------------+---------------------------+
> | 0_0 | 25 |
> +------------+---------------------------+
> {code}
> The Explain below shows the count is retrieved directly from the Scan:
> {code:sql}
> 0: jdbc:drill:zk=local> explain plan for select count(n_regionkey) as x from
> nation3;
> +------------+------------+
> | text | json |
> +------------+------------+
> | 00-00 Screen
> 00-01 Project(x=[$0])
> 00-02
> Scan(groupscan=[org.apache.drill.exec.store.pojo.PojoRecordReader@5db6cb92])
> {code}
> The following query which does 2 aggregates causes the StreamAgg to be
> introduced in the plan which is not needed:
> {code:sql}
> 0: jdbc:drill:zk=local> explain plan for select count(n_regionkey) as x,
> count(n_nationkey) as y from nation3;
> +------------+------------+
> | text | json |
> +------------+------------+
> | 00-00 Screen
> 00-01 Project(x=[$0], y=[$1])
> 00-02 StreamAgg(group=[{}], x=[COUNT($0)], y=[COUNT($1)])
> 00-03 Project(n_regionkey=[$1], n_nationkey=[$0])
> 00-04 Scan(groupscan=[ParquetGroupScan [entries=[ReadEntryWithPath
> [path=file:/tmp/nation3]], selectionRoot=/tmp/nation3, numFiles=1,
> columns=[`n_regionkey`, `n_nationkey`]]])
> {code}
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