Paul Rogers created IMPALA-7769:
-----------------------------------
Summary: Constant folding sometimes introduces an unnecessary cast
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
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 NULL IS NULL THEN id ELSE NULL END FROM alltypessmall
{code}
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()}} 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 problem and the test query now simplifies to the
expected result.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
