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https://issues.apache.org/jira/browse/DRILL-1457?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14943540#comment-14943540
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Victoria Markman commented on DRILL-1457:
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I'm removing "disable parallel plan if no order is required" from the title:
this is not what was implemented and is a bit misleading.
> Limit operator optimization : push limit operator past exchange operator;
> disable parallel plan if no order is required.
> ------------------------------------------------------------------------------------------------------------------------
>
> Key: DRILL-1457
> URL: https://issues.apache.org/jira/browse/DRILL-1457
> Project: Apache Drill
> Issue Type: Bug
> Components: Query Planning & Optimization
> Reporter: Jinfeng Ni
> Assignee: Jinfeng Ni
> Priority: Critical
> Labels: no_verified_test
> Fix For: 1.2.0
>
> Attachments:
> 0001-DRILL-1457-Push-Limit-past-through-UnionExchange.patch
>
>
> When there is LIMIT clause in a query, we would want to push down the LIMIT
> operator as much as possible, so that the upstream operator will stop
> execution once the desired number of rows are fetched.
> Within one execution fragment, Drill applies a pull model. In many cases,
> there would be no performance impact if LIMIT operator is not pushed down,
> since LIMIT would inform the upstream operators to stop. However, in multiple
> fragments, Drill use a push model. if LIMIT is not pushed past the exchange
> operator, and the upstream fragment would continue the execution, until it
> receives a notice from downstream fragment, even if LIMIT operator has
> already got the required # of rows.
> For instance:
> explain plan for select * from
> dfs.`/Users/jni/work/tpch-data/tpch-sf10/lineitem` limit 1;
> +------------+------------+
> | 00-00 Screen
> 00-01 SelectionVectorRemover
> 00-02 Limit(fetch=[1])
> 00-03 UnionExchange
> 01-01 Scan(groupscan=[ParquetGroupScan [entries=[ReadEntryWithPath
> [path=file:/Users/jni/work/tpch-data/tpch-sf10/lineitem]],
> selectionRoot=/Users/jni/work/tpch-data/tpch-sf10/lineitem,
> columns=[SchemaPath [`*`]]]])
> The query profile shows Scan operator fetches much more records than desired:
> Minor Fragment Start End Total Time Max Records Max
> Batches
> 01-00-xx 0.507 1.059 0.552 43688 8
> 01-01-xx 0.570 1.054 0.484 27305 5
> 01-02-xx 0.617 1.038 0.421 16383 3
> 01-03-xx 0.668 1.056 0.388 10922 2
> 01-04-xx 0.740 1.055 0.315 10922 2
> 01-05-xx 0.813 1.057 0.244 5461 1
> In the above plan, there would be two choices for performance optimization:
> 1) push the LIMIT operator past through EXCHANGE operator, ideally into SCAN
> operator.
> 2) Disable the parallel plan by removing EXCHANGE operator.
>
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