I think out_sql more likely causes errors. Because RDBMS requires strict schema while Fluentd uses dynamic typing.

Can we fallback to one-by-one INSERT mode if `import` caused an error? This mode runs INSERT for each rows instead of using `import`.

This is the ideal implementation of one-by-one INSERT mode:

* In one-by-one INSERT node, it distinguishes deterministic errors and non-deterministic errors.
  * deterministic errors: type errors, authentication errors, etc.
  * non-deterministic errors: network errors, lock timeout errors, etc.
* Users can configure exception handling behavior either 'ignore', 'log', or 'retry' for each deterministic errors and non-deterministic errors.
* Default behavior of deterministic errors is 'log', which logs to `$log` some messages and throws away the record.
* Default behavior of non-deterministic errors is "retry', which retries INSERTing later.

But I guess this ideal implementation is difficult to implement. So this is another reasonable idea:

* Here tries to detect deterministic errors as much as possible (best-effort). For example, ActiveRecord should have special exception classes for type mismatch.
* If it's a deterministic error, it throws away a record instead of retrying.