How does MySQL handle concurrent inserts?

Question

I know there is one issue in MySQL with concurrent SELECT and INSERT. However, my question is if I open up two connections with MySQL and keep loading data using both of them, does MySQL takes data concurrently or waits for one to finish before loading another?

I'd like to know how MySQL behaves in both cases. Like when I am trying to load data in the same table or different tables concurrently when opening separate connections.

Answer 1

If you will create a new connection to the database and perform inserts from both the links, then from the database's perspective, it will still be sequential .

The documentation of Concurrent Inserts on the MySQL's documentation page says:

If there are multiple INSERT statements, they are queued and performed in sequence, concurrently with the SELECT statements.

Mind that there is no control over the order in which two concurrent inserts will take place. The order in this concurrency is at the mercy of a lot of different factors. To ensure order, by default you will have to sacrifice concurrency.

Answer 2

MySQL does support parallel data inserts into the same table.

But approaches for concurrent read/write depends upon storage engine you use.

InnoDB

MySQL uses row-level locking for InnoDB tables to support simultaneous write access by multiple sessions, making them suitable for multi-user, highly concurrent, and OLTP applications.

MyISAM

MySQL uses table-level locking for MyISAM, MEMORY, and MERGE tables, allowing only one session to update those tables at a time, making them more suitable for read-only, read-mostly, or single-user applications

But, the above mentioned behavior of MyISAM tables can be altered by concurrent_insert system variable in order to achieve concurrent write. Kindly refer to this link for details.

Hence, as a matter of fact, MySQL does support concurrent insert for InnoDB and MyISAM storage engine.

Answer 3

You ask about Deadlock detection, ACID and particulary MVCC, locking and transactions:

Deadlock Detection and Rollback

InnoDB automatically detects transaction deadlocks and rolls back a transaction or transactions to break the deadlock. InnoDB tries to pick small transactions to roll back, where the size of a transaction is determined by the number of rows inserted, updated, or deleted. When InnoDB performs a complete rollback of a transaction, all locks set by the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the statement may be preserved. This happens because InnoDB stores row locks in a format such that it cannot know afterward which lock was set by which statement.

https://dev.mysql.com/doc/refman/5.6/en/innodb-deadlock-detection.html

Locking

The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for good concurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.

http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_locking

ACID

An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable in a database system, and are all closely tied to the notion of a transaction. The transactional features of InnoDB adhere to the ACID principles. Transactions are atomic units of work that can be committed or rolled back. When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back. The database remains in a consistent state at all times -- after each commit or rollback, and while transactions are in progress. If related data is being updated across multiple tables, queries see either all old values or all new values, not a mix of old and new values. Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with each other or see each other's uncommitted data. This isolation is achieved through the locking mechanism. Experienced users can adjust the isolation level, trading off less protection in favor of increased performance and concurrency, when they can be sure that the transactions really do not interfere with each other.

http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_acid

MVCC

InnoDB is a multiversion concurrency control (MVCC) storage engine which means many versions of the single row can exist at the same time. In fact there can be a huge amount of such row versions. Depending on the isolation mode you have chosen, InnoDB might have to keep all row versions going back to the earliest active read view, but at the very least it will have to keep all versions going back to the start of SELECT query which is currently running

https://www.percona.com/blog/2014/12/17/innodbs-multi-versioning-handling-can-be-achilles-heel/

Answer 4

It depends.

It depends on the client -- some clients allow concurrent access; some will serialize access, thereby losing the expected gain. You have not even specified PHP vs Java vs ... or Apache vs ... or Windows vs ... Many combinations simply do not provide any parallelism.

If different tables, there is only general contention for I/O, CPU, Mutexes on the buffer_pool, etc. A reasonable amount of parallelism is possible.

If same table, it depends on the indexes and access patterns. In some cases the threads will block each other. In some cases it will even "deadlock" and rollback one of the transactions. Deadlocks not only slow you down, but make you retry the inserts.

If you looking for high speed ingestion of a lot of rows, see my blog . It lays out techniques, and points out sever of the ramifications, such as replication, Engine choice, multi-threading.

Multiple threads inserting into the same tables -- It depend a lot on the values you are providing for any PRIMARY or UNIQUE keys. It depends on whether other actions are taken in the same transaction. It depends on how much I/O is involved. It depends on whether you are doing single-row inserts, or batching. It depends on ... (Sorry to be vague, but your question is not very specific.)

If you would like to present specifics on two or three designs, we can discuss the specifics.