Paul Rogers created IMPALA-7965:
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Summary: FE should use "permissive" constant folding evaluation
Key: IMPALA-7965
URL: https://issues.apache.org/jira/browse/IMPALA-7965
Project: IMPALA
Issue Type: Bug
Components: Frontend
Affects Versions: Impala 3.1.0
Reporter: Paul Rogers
Assignee: Paul Rogers
The FE implements "constant folding" the conversion of constant expressions
into constant literals. For example, {{1 + 1}} is constant-folded to {{2}}.
Subtle cases occur when the BE cannot evaluate the expression. For example:
* DECIMAL-value overflow or underflow e.g. {{CAST(10 AS DECIMAL(1,0))}}
* Illegal casts e.g. {{CAST('foo' AS TIMESTAMP)}}
* Result strings that contain non-ASCII characters e.g. {{hex('D3')}}
In this case, the FE does not fail the query, it just leaves the original SQL
and let's the BE fail the query at runtime. (Doing so seems more of a bug than
a feature, but let's set that issue aside at present.)
There are cases in which the error is benign. Consider this case from
{{ExprRewriteRulesTest.TestFoldConstantsRule()}}:
Note that a side rule is that the result of constant folding should be the
result of the BE executing the entire expression. From
{{ExprRewriteRulesTest.TestFoldConstantsRule()}}:
{code:java}
// Tests correct handling of strings with chars > 127. Should not be folded.
verifyRewrite("hex(unhex(hex(unhex('D3'))))", null);
{code}
This test says that the following SQL expression cannot be evaluated on the BE:
{code:sql}
hex(unhex(hex(unhex('D3'))))
{code}
But, the BE can happily evaluate the whole thing, and produce a result of
{{'D3'}}. What fails is:
{code:sql}
unhex('D3')
{code}
But not:
{code:sql}
hex(unhex('D3'))
{code}
Similar reasoning applies to:
{code:sql}
FALSE AND CAST(20 AS DECIMAL(1,0))
{code}
In the present code, the entire test expression fails. But, under the, "let the
BE decide" rule, the expression:
{code:sql}
hex(unhex(hex(unhex('D3'))))
{code}
Can be rewritten to:
{code:sql}
hex('D3')
{code}
The inner three functions are evaluated properly on the BE. Only the outer
function fails and is preserved in the AST.
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