How Simultaneous Queries Are Handled in a MySQL Database

How simultaneous queries are handled in a MySQL database?

Queries are always handled in parallel between multiple sessions (i.e. client connections). All queries on a single connections are run one-after-another. The level of parallelism between multiple connections can be configured depending on your available server resources.

Generally, some operations are guarded between individual query sessions (called transactions). These are supported by InnoDB backends, but not MyISAM tables (but it supports a concept called atomic operations). There are various level of isolation which differ in which operations are guarded from each other (and thus how operations in one parallel transactions affect another) and in their performance impact.

For more information read about transactions in general and the implementation in MySQL.

Handling simultaneous MySQL queries from different users

I was able to solve this problem by using PDO's beginTransaction(), rollBack(), and commit() methods to prevent overbooking.

Simplified, here's what I did:

<?php

$db->beginTransaction();

//add to list
$sth = $db->prepare("INSERT INTO...");
$sth->execute();

//select count from list
$sth = $db->prepare("SELECT ... FROM ...");
$sth->execute();

//check to see whether the list is overbooked -- in this case, 5 is the list limit
if($sth->rowCount() > 5) {
    $db->rollBack();
}
else {
    $db->commit();
}

?>

Multiple queries in mysql to the information schema

Since your are using the myISAM storage engine and are worrying about concurrent SELECT statements:

READ (SELECT) can happen concurrently as long as there is no WRITE (INSERT, UPDATE, DELETE or ALTER TABLE). Ie. you can have either one writer or several readers.

Otherwise the operations are queued and executed as soon as possible.
There is a special case : concurrent inserts.

Note : if you are wondering about the choice between the two main mySQL storage engines myISAM and InnoDB, InnoDB is usually a good choice, please read this SO question.

MySQL slow concurrent/parallel queries in Python

We found, that it has nothing to do with MySQL but with the Global Interpreter Lock (GIL) of Python.

Due to the fact that the function read_db() /pd.read_sql(query, con) is CPU-bound and Python has the GIL, the query results are received and processed sequentially.

One solution is to use multiprocessing instead of multithreading. One can easily exchange the ThreadPoolExecutor with the ProcessPoolExecutor from concurrent.futures.

How are simultaneous client connections quantified in mysql

When a visitor goes to your website and the server-side script connects to the database it is 1 connection - you can make as many queries as necessary during that connection to any number of tables/databases - and on termination of the script the connection ends. If 31 people request a page (and hence a db connection) and your limit is 30, then the 31st person will get an error.

You can upgrade server hardware so MySQL can efficiently handle loads of connections or spread the load across multiple database servers. It is possible to have your server-side scripting environment maintain a persistent connection to MySQL in which case all scripts make queries through that single connection. This will probably have adverse effects on the correct queuing of queries and their order to maintain usable speeds under high load, and ultimately doesn't solve the CPU/memory/disk bottlenecks with handling large numbers of queries.

In the case of a webmail application, the query to check for new messages runs so fast (in the milliseconds) that hitting server limits isn't likely unless it's on a large scale.

Google's applications scale on a level previously unheard of. Check out the docs on MapReduce, GoogleFS, etc. It's awesome.

In answer to your edit - anything that connects directly to MySQL is considered a client in this case. Each PHP script that connects to MySQL is a client, as is the MySQL console on the command line, or anything else.

Hope that helps

How does MySQL handle concurrent inserts?

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 for MyISAM on the MySQL's documentation page says something like this:

If MyISAM storage is used and table has no holes, multiple INSERT statements 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.

How many MySql queries/second can be handled by a server?

Yoshinori Matsunobu in one of his articles claims 105,000 queries per second using SQL, and 750,000 queries per second using native InnoDB API.

All queries are simple PK lookups.

On a shared hosting these numbers will of course be much lower. How much exactly of course depends on the shared hosting.

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