Paul Rogers created IMPALA-7781:
-----------------------------------
Summary: Clean up ad-hoc instance of, casts to use predicates in
expr rewriter
Key: IMPALA-7781
URL: https://issues.apache.org/jira/browse/IMPALA-7781
Project: IMPALA
Issue Type: Bug
Components: Frontend
Affects Versions: Impala 3.0
Reporter: Paul Rogers
The expression rewriter rules have evolved over time, it seems. When originally
written, it seem that the standard way to check if an expression is a null
literal is to do an {{instance of}} as in {{SimplifyConditionalsRule}}:
{code:java}
private Expr simplifyCaseExpr(CaseExpr expr, Analyzer analyzer)
throws AnalysisException {
...
if (child instanceof NullLiteral) continue;
{code}
Since this was written, we added {{Expr.isNullLiteral()}} which not only checks
if the expression is null, it also tests for the {{CAST(NULL AS <type>)}} form
created by the constant folding rule.
The result is that rewrites miss optimization cases for expressions such as
{{NULL + 1}} which are rewritten to {{CASE(NULL AS INT)}}. (IMPALA-7769).
Code also does manual casts to Boolean literals in the same function:
{code:java}
if (whenExpr instanceof BoolLiteral) {
if (((BoolLiteral) whenExpr).getValue()) {
{code}
Which can be replaced with the {{Expr.IS_TRUE_LITERAL}} predicate. Same is true
for the {{FALSE}} check.
Finally, there are places in the rewriter that check {{isLiteral()}} when it
wants to know a more generic "is near literal". Consider the {{CAST(NULL...)}}
issue above. The {{CAST}} is not a literal, but it acts like one in some cases
(such as in the constant folding rule itself, IMPALA-7769.)
In short, a number of minor, obscure errors could be avoided if we made
consistent use of the higher-level predicates already available.
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