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https://issues.apache.org/jira/browse/IMPALA-7769?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul Rogers updated IMPALA-7769:
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Summary: Handle CAST(NULL AS type) in rewrites (was: Constant folding
sometimes introduces an unnecessary cast)
> Handle CAST(NULL AS type) in rewrites
> -------------------------------------
>
> Key: IMPALA-7769
> URL: https://issues.apache.org/jira/browse/IMPALA-7769
> Project: IMPALA
> Issue Type: Improvement
> Components: Frontend
> Affects Versions: Impala 3.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> Consider the following query:
> {code:sql}
> SELECT IFNULL(NULL + 1, id) FROM alltypessmall;
> {code}
> Visualize the rewritten query after analysis (using the new
> {{FullRewriteTest}}.) We get:
> {code:sql}
> SELECT CASE WHEN CAST(NULL AS INT) IS NULL THEN id ELSE NULL END FROM
> alltypessmall
> {code}
> (This is what the expression actually contains. The {{toSql()}} method lies
> and says that the statement is:
> {code:sql}
> SELECT CASE WHEN NULL IS NULL THEN id ELSE NULL END FROM alltypessmall
> {code}
> which causes confusion when debugging.)
> Expected:
> {code:sql}
> SELECT id FROM alltypessmall
> {code}
> The reason is that there are multiple rewrites, one of which has a flaw.
> # Rewrite IFNULL to CASE
> # Rewrite NULL + 1 to CAST(NULL AS SMALLINT)
> # Try to rewrite CAST(NULL AS SMALLINT) IS NULL, but fail because CAST is not
> a literal.
> The code in question in {{FoldConstantsRule.apply()}} is:
> {code:java}
> for (Expr child: expr.getChildren()) if (!child.isLiteral()) return expr;
> {code}
> In fact, this check is too restrictive. Need a new {{isLiteralLike()}} which
> should work like {{IsNullLiteral()}}:
> * True if the node is a literal.
> * True if the node is a cast of a literal.
> (Can't change {{isLiteral()}} since there are places that assume that this
> existing predicate indicates that the node is exactly a {{LiteralExpr}}.)
> Impala already has a predicate that does what is needed: {{isConstant()}}.
> However, the code in {{FoldConstantsRule.apply()}} specifically excludes
> calling it:
> {code:java}
> // Avoid calling Expr.isConstant() because that would lead to repeated
> traversals
> // of the Expr tree. Assumes the bottom-up application of this rule.
> Constant
> // children should have been folded at this point.
> {code}
> The new method solves the repeated traversal problem. With it, the test query
> now simplifies to the expected result.
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