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https://issues.apache.org/jira/browse/HIVE-16600?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16014389#comment-16014389
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Xuefu Zhang commented on HIVE-16600:
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Hi [~lirui], For your example, my understanding is that the target table isn't
expected to contained top 5, ordered result from {{A}}. Assuming {{A}} is a
regular table that happens to be ordered,{{select A.k from A limit 5}} can
choose any 5 records from A. Both orderby and limit clause is to control the
query output. In your example, the orderby clause is to control the output of
the subquery, which is {{A}}. That means that {{A}} is expected to contain all
ordered rows in {{src}}. The limit clause is to control the output of the
second select statement, which has no confusion. Even if {{A}} is ordered, the
second {{select ... from}} can completely ignore that order and return an
unordered set of A, which is further limited to any 5 rows by the limit clause.
I don't think {{select}} is equivalent to {{fetch first}} introduced in SQL2008
or {{select first}} supported by some DB vendors.
If a user wants the top-5 records from src, your example can be rewritten as {{
FROM src insert overwrite table target select src.k order by src.k limit 5}},
which may further clarify why in your example the target table is not expected
to have top-5 records from src.
> Refactor SetSparkReducerParallelism#needSetParallelism to enable parallel
> order by in multi_insert cases
> --------------------------------------------------------------------------------------------------------
>
> Key: HIVE-16600
> URL: https://issues.apache.org/jira/browse/HIVE-16600
> Project: Hive
> Issue Type: Sub-task
> Reporter: liyunzhang_intel
> Assignee: liyunzhang_intel
> Attachments: HIVE-16600.1.patch, HIVE-16600.2.patch,
> HIVE-16600.3.patch, HIVE-16600.4.patch, HIVE-16600.5.patch, mr.explain,
> mr.explain.log.HIVE-16600
>
>
> multi_insert_gby.case.q
> {code}
> set hive.exec.reducers.bytes.per.reducer=256;
> set hive.optimize.sampling.orderby=true;
> drop table if exists e1;
> drop table if exists e2;
> create table e1 (key string, value string);
> create table e2 (key string);
> FROM (select key, cast(key as double) as keyD, value from src order by key) a
> INSERT OVERWRITE TABLE e1
> SELECT key, value
> INSERT OVERWRITE TABLE e2
> SELECT key;
> select * from e1;
> select * from e2;
> {code}
> the parallelism of Sort is 1 even we enable parallel order
> by("hive.optimize.sampling.orderby" is set as "true"). This is not
> reasonable because the parallelism should be calcuated by
> [Utilities.estimateReducers|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SetSparkReducerParallelism.java#L170]
> this is because SetSparkReducerParallelism#needSetParallelism returns false
> when [children size of
> RS|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SetSparkReducerParallelism.java#L207]
> is greater than 1.
> in this case, the children size of {{RS[2]}} is two.
> the logical plan of the case
> {code}
> TS[0]-SEL[1]-RS[2]-SEL[3]-SEL[4]-FS[5]
> -SEL[6]-FS[7]
> {code}
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