[jira] [Created] (ARROW-11549) Expression::ToString() doesn't distinguish null and string literal 'null', causing issues with FilterCacheKey

["Sikan Chen (Jira)" <ji...@apache.org>]

Sikan Chen created ARROW-11549:
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             Summary: Expression::ToString() doesn't distinguish null and string literal 'null', causing issues with FilterCacheKey
                 Key: ARROW-11549
                 URL: https://issues.apache.org/jira/browse/ARROW-11549
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++, C++ - Gandiva
            Reporter: Sikan Chen


Gandiva's caching mechanism for filters relies on {{FilterCacheKey}} to return the correct cached filter. {{FilterCacheKey}}'s hash function factors in the string representation of a given expression, however {{Expression::ToString()}} doesn't really distinguish null and string literal 'null'. As a result, incorrect filters may be returned from the cache.

In our case, we are building a SQL parser on top of gandiva, but, for instance, both of
{code:java}
WHERE null = null
{code}
and
{code:java}
WHERE 'null' = 'null'
{code}
result in the same string representation of gandiva expression:
{code:java}
bool equal((const string) null, (const string) null)
{code}
A simple test to demonstrate the issue:
{code:java}
  auto f = field("foo", utf8());
  auto schema = arrow::schema({f});
  auto node_a = TreeExprBuilder::MakeStringLiteral("null");
  auto node_b = TreeExprBuilder::MakeStringLiteral("null");
  auto equal_func =
      TreeExprBuilder::MakeFunction("equal", {node_a, node_b}, arrow::boolean());
  auto condition = TreeExprBuilder::MakeCondition(equal_func);
  std::shared_ptr<Filter> filter1;
  auto status = Filter::Make(schema, condition, &filter1);
  EXPECT_TRUE(status.ok());

  auto string_type = std::make_shared<arrow::StringType>();
  node_a = TreeExprBuilder::MakeNull(string_type);
  node_b = TreeExprBuilder::MakeNull(string_type);
  equal_func =
      TreeExprBuilder::MakeFunction("equal", {node_a, node_b}, arrow::boolean());
  condition = TreeExprBuilder::MakeCondition(equal_func);
  std::shared_ptr<Filter> filter2;
  status = Filter::Make(schema, condition, &filter2);
  EXPECT_TRUE(status.ok());

  EXPECT_TRUE(filter1.get() != filter2.get());
{code}
 
 Making {{LiteralToStream}} adding quotes around the literal seems like a quick-and-dirty fix.



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