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https://issues.apache.org/jira/browse/HIVE-26722?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Work on HIVE-26722 started by Alessandro Solimando.
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> HiveFilterSetOpTransposeRule incorrectly prune UNION ALL operands
> -----------------------------------------------------------------
>
> Key: HIVE-26722
> URL: https://issues.apache.org/jira/browse/HIVE-26722
> Project: Hive
> Issue Type: Bug
> Components: CBO
> Affects Versions: 4.0.0-alpha-1
> Reporter: Alessandro Solimando
> Assignee: Alessandro Solimando
> Priority: Major
>
> h1. Reproducer
> Consider the following query:
> {code:java}
> set hive.cbo.rule.exclusion.regex=ReduceExpressionsRule\(Project\);
> CREATE EXTERNAL TABLE t (a string, b string);
> INSERT INTO t VALUES ('1000', 'b1');
> INSERT INTO t VALUES ('2000', 'b2');
> SELECT * FROM (
> SELECT
> a,
> b
> FROM t
> UNION ALL
> SELECT
> a,
> CAST(NULL AS string)
> FROM t) AS t2
> WHERE a = 1000;EXPLAIN CBO
> SELECT * FROM (
> SELECT
> a,
> b
> FROM t
> UNION ALL
> SELECT
> a,
> CAST(NULL AS string)
> FROM t) AS t2
> WHERE a = 1000; {code}
>
> The expected result is:
> {code:java}
> 1000 b1
> 1000 NULL{code}
> An example of correct plan is as follows:
> {noformat}
> CBO PLAN:
> HiveUnion(all=[true])
> HiveProject(a=[$0], b=[$1])
> HiveFilter(condition=[=(CAST($0):DOUBLE, 1000)])
> HiveTableScan(table=[[default, t]], table:alias=[t])
> HiveProject(a=[$0], _o__c1=[null:VARCHAR(2147483647) CHARACTER SET
> "UTF-16LE"])
> HiveFilter(condition=[=(CAST($0):DOUBLE, 1000)])
> HiveTableScan(table=[[default, t]], table:alias=[t]){noformat}
>
> Consider now a scenario where expression reduction in projections is disabled
> by setting the following property{_}:{_}
> {noformat}
> set hive.cbo.rule.exclusion.regex=ReduceExpressionsRule\(Project\);
> {noformat}
> In this case, the simplification of _CAST(NULL)_ into _NULL_ does not happen,
> and we get the following (invalid) result:
> {code:java}
> 1000 b1{code}
> produced by the following invalid plan:
> {code:java}
> CBO PLAN:
> HiveProject(a=[$0], b=[$1])
> HiveFilter(condition=[=(CAST($0):DOUBLE, 1000)])
> HiveTableScan(table=[[default, t]], table:alias=[t]) {code}
> h1. Problem Analysis
> At
> [HiveFilterSetOpTransposeRule.java#L112|https://github.com/apache/hive/blob/297f510d3b581c9d4079e42caa28aa84f8486012/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveFilterSetOpTransposeRule.java#L112]
> the _RelMetadataQuery::getPulledUpPredicates_ method infers the following
> predicate due to the CAST(NULL) in the projection:
> {code:java}
> (=($1, CAST(null:NULL):VARCHAR(2147483647) CHARACTER SET "UTF-16LE")){code}
> When the CAST is simplified to the NULL literal, the IS_NULL($1) predicate is
> inferred.
> In
> [HiveFilterSetOpTransposeRule.java#L114-L122|https://github.com/apache/hive/blob/297f510d3b581c9d4079e42caa28aa84f8486012/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveFilterSetOpTransposeRule.java#L114-L122],
> the rule checks if the conjunction of the predicate coming from the filter
> (here =(CAST($0):DOUBLE, 1000)) and the inferred predicates is satisfiable or
> not, under the _UnknownAsFalse_ semantics.
> To summarize, the following expression is simplified under the
> _UnknownAsFalse_ semantics:
> {code:java}
> AND((=($1, CAST(null:NULL):VARCHAR(2147483647) CHARACTER SET "UTF-16LE")),
> =(CAST($0):DOUBLE, 1000))
> {code}
> Under In such semantics, (=($1, CAST(null:NULL):...) evaluates to
> {_}FALSE{_}, because no value is equal to NULL (even NULL itself), AND(FALSE,
> =(CAST($0):DOUBLE, 1000)) necessarily evaluates to _FALSE_ altogether, and
> the UNION ALL operand is pruned.
> Only by chance, when _CAST(NULL)_ is simplified to _NULL,_ we avoid the
> issue, due to the _IS_NULL($1)_ inferred predicate, see
> [HiveRelMdPredicates.java#L153-L156|https://github.com/apache/hive/blob/297f510d3b581c9d4079e42caa28aa84f8486012/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/HiveRelMdPredicates.java#L153-L156]
> for understanding how the NULL literal is treated differently during
> predicate inference.
> The problem lies in the fact that, depending on the input _RelNode_ that we
> infer predicates from, the semantics is not necessarily {_}UnknownAsFalse{_},
> but it might be {_}UnknownAsUnknown{_}, like for {_}Project{_}, as in this
> case.
> h1. Solution
> In order to correctly simplify a predicate and test if it's always false or
> not, we should build RexSimplify with _predicates_ as the list of predicates
> known to hold in the context. In this way, the different semantics are
> correctly taken into account.
> The code at
> [HiveFilterSetOpTransposeRule.java#L114-L121|https://github.com/apache/hive/blob/297f510d3b581c9d4079e42caa28aa84f8486012/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveFilterSetOpTransposeRule.java#L114-L121]
> should be replaced by the following:
> {code:java}
> final RexExecutor executor =
> Util.first(filterRel.getCluster().getPlanner().getExecutor(),
> RexUtil.EXECUTOR);
> final RexSimplify simplify = new RexSimplify(rexBuilder, predicates,
> executor);
> final RexNode x = simplify.simplifyUnknownAs(newCondition,
> RexUnknownAs.FALSE);{code}
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