Paul Rogers created IMPALA-7753:
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Summary: Invalid logic when rewriting ORDER BY clause expressions
Key: IMPALA-7753
URL: https://issues.apache.org/jira/browse/IMPALA-7753
Project: IMPALA
Issue Type: Bug
Components: Frontend
Affects Versions: Impala 3.0
Reporter: Paul Rogers
Assignee: Paul Rogers
The select statement represents the ORDER BY clause in two distinct ways.
First, there is a list of the "original" ordering expressions,
{{orderByElements_}}. Second, there is an analyzed list in {{sortInfo_}}. The
explanation is:
{code:java}
// create copies, we don't want to modify the original parse node, in case
// we need to print it
{code}
Later, we apply rewrite rules to the {{ORDER BY}} expression, but we do so
using the original version, not the copy:
{code:java}
for (OrderByElement orderByElem: orderByElements_) {
orderByElem.setExpr(rewriteCheckOrdinalResult(rewriter,
orderByElem.getExpr()));
}
{code}
The result is that we apply rewrite rules to expressions which have not been
analyzed, triggering the assertion mentioned above. This assertion is telling
us something: we skipped a step. Here, it turns out we are rewriting the wrong
set of expressions. Modifying the code to rewrite those in {{sortInfo_}} solves
the problem. The current behavior is a bug as the rewrites currently do
nothing, and the expressions we thought we were rewriting are never touched.
The correct code would rewrite the expressions which are actually used when
analyzing the query:
{code} if (orderByElements_ != null) {
List<Expr> sortExprs = sortInfo_.getSortExprs();
for (int i = 0; i < sortExprs.size(); i++) {
sortExprs.set(i, rewriteCheckOrdinalResult(rewriter, sortExprs.get(i)));
}
{code}
We can, in addition, ask a more basic question: do we even need to do rewrites
for {{ORDER BY}} expressions? The only valid expressions are column references,
aren't they? Or, does Impala allow expressions in the {{ORDER BY}} clause?
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