We created a benchmark to measure ExecuteScalarExpression performance in
ARROW-16014. We noticed significant thread contention (even though there shouldn't be much, if any, for this task) As part of ARROW-16138 we have been investigating possible causes.
One cause seems to be contention from copying shared_ptr<DataType> objects.
Two possible solutions jump to mind and I'm sure there are many more.
ExecBatch is an internal type and used inside of ExecuteScalarExpression as well as inside of the execution engine. In the former we can safely assume the data types will exist for the duration of the call. In the latter we can safely assume the data types will exist for the duration of the execution plan. Thus we can probably take a more targetted fix and migrate only ExecBatch to using DataType* (or const DataType&).
On the other hand, we might consider a more global approach. All of our "stock" data types are assumed to have static storage duration. However, we must use std::shared_ptr<DataType> because users could create their own extension types. We could invent an "extension type registration" system where extension types must first be registered with the C++ lib before being used. Then we could have long-lived DataType instances and we could replace std::shared_ptr<DataType> with DataType* (or const DataType&) throughout most of the entire code base.