Question

I have several threads executing some SQL select queries with serializable isolation level. I am not sure which implementation to choose. This:

_repository.Select(...)

or this

lock (_lockObject)
{
   _repository.Select(...);
}

In other words, is it possible several transactions will start executing at the same time and partially block records inside Select operation range.

P. S. I am using MySQL but I guess it is a more general question.

Answer 1

Transactions performing SELECT queries place a shared lock on the rows, permitting other transactions to read those rows, but preventing them from making changes to the rows (including inserting new records into the gaps)

Locking in the application is doing something else, it will not allow other threads to enter the code block which fetches the data from the repository, This approach can lead to very bad performance for a few reasons:

1. If any of the rows are locked by another transaction (outside the application) via a exclusive lock, the lock in the application will not help.
2. Multiple transactions will not be able to perform reads even on rows that are not locked in exclusive mode (not being updated).
3. The lock will not be released until all the data is fetched and returned to the client. This includes the network latency and any other overhead that it takes converting the MySql result set to a code object.
4. Most importantly, Enforcing data integrity & atomicity is the databases job, it knows how to handle it very well, how to detect potential deadlocks. When to perform record locks, and when to add Index gap locks. It is what databases are for, and MySql is ACID complaint and is proven to handle these situations

I suggest you read through Section 13.2.8. The InnoDB Transaction Model and Locking of the MySql docs, it will give you a great insight how locking in InnoDB is performed.