[
https://issues.apache.org/jira/browse/IMPALA-7769?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Paul Rogers updated IMPALA-7769:
--------------------------------
Description:
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}
Another case:
{code:sql}
CASE WHEN NULL + 1 THEN 10 ELSE 20 END
{code}
Ensure the {{NULL + 1}} is constant folded with a cast. Then, in {{CASE}}:
{code:java}
for (int i = loopStart; i < numChildren - 1; i += 2) {
if (expr.getChild(i).isLiteral()) {
canSimplify = true;
break;
}
}
{code}
The {{isLiteral()}} method won’t match the cast and the simplification won’t
fire.
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.
In addition to the {{CASE}} issue above, the {{FoldConstantsRule}} itself is
tripped up. It is supposed to simplify the {{... IS NULL}} expression above,
but does not.
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.
was:
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.
> 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
> 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}
> Another case:
> {code:sql}
> CASE WHEN NULL + 1 THEN 10 ELSE 20 END
> {code}
> Ensure the {{NULL + 1}} is constant folded with a cast. Then, in {{CASE}}:
> {code:java}
> for (int i = loopStart; i < numChildren - 1; i += 2) {
> if (expr.getChild(i).isLiteral()) {
> canSimplify = true;
> break;
> }
> }
> {code}
> The {{isLiteral()}} method won’t match the cast and the simplification won’t
> fire.
> 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.
> In addition to the {{CASE}} issue above, the {{FoldConstantsRule}} itself is
> tripped up. It is supposed to simplify the {{... IS NULL}} expression above,
> but does not.
> 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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