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https://issues.apache.org/jira/browse/HIVE-17342?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17597947#comment-17597947
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Aman Sinha commented on HIVE-17342:
-----------------------------------
A slightly different example shows the limit 0 optimization does not get
applied:
create table t1 (a1 int, b1 int);
explain cbo select y from (select a1 y from t1 where b1 > 10) q WHERE 1=0;
+----------------------------------------------------+
| Explain |
+----------------------------------------------------+
| CBO PLAN: |
| HiveProject(y=[$0]) |
| HiveSortLimit(fetch=[0]) |
| HiveProject(a1=[$0], b1=[$1]) |
| HiveFilter(condition=[>($1, 10)]) |
| HiveTableScan(table=[[db1, t1]], table:alias=[t1]) |
| |
{noformat}
> Where condition with 1=0 should be treated similar to limit 0
> -------------------------------------------------------------
>
> Key: HIVE-17342
> URL: https://issues.apache.org/jira/browse/HIVE-17342
> Project: Hive
> Issue Type: Improvement
> Reporter: Rajesh Balamohan
> Assignee: Krisztian Kasa
> Priority: Minor
>
> In some cases, queries may get executed with where condition mentioning to
> "1=0" to get schema. E.g
> {noformat}
> SELECT * FROM (select avg(d_year) as y from date_dim where d_year>1999) q
> WHERE 1=0
> {noformat}
> Currently hive executes the query; it would be good to consider this similar
> to "limit 0" which does not execute the query.
> {code}
> hive> explain SELECT * FROM (select avg(d_year) as y from date_dim where
> d_year>1999) q WHERE 1=0;
> OK
> Plan optimized by CBO.
> Vertex dependency in root stage
> Reducer 2 <- Map 1 (CUSTOM_SIMPLE_EDGE)
> Stage-0
> Fetch Operator
> limit:-1
> Stage-1
> Reducer 2 vectorized, llap
> File Output Operator [FS_13]
> Group By Operator [GBY_12] (rows=1 width=76)
> Output:["_col0"],aggregations:["avg(VALUE._col0)"]
> <-Map 1 [CUSTOM_SIMPLE_EDGE] vectorized, llap
> PARTITION_ONLY_SHUFFLE [RS_11]
> Group By Operator [GBY_10] (rows=1 width=76)
> Output:["_col0"],aggregations:["avg(d_year)"]
> Filter Operator [FIL_9] (rows=1 width=0)
> predicate:false
> TableScan [TS_0] (rows=1 width=0)
>
> default@date_dim,date_dim,Tbl:PARTIAL,Col:NONE,Output:["d_year"]
> {code}
> It does generate 0 splits, but does send a DAG plan to the AM and receive 0
> rows as output.
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