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I use a PHP script which modifies a mysql database with multiple PDO queries. I was wondering, if two users load a page exactly the same time, will mysql first handle all the queries from user one in one go, or is there a risk that some queries from user one are handled, after which some queries from user two and then some more from user one?
I hope my question is clear. Many thanks in advance ;-).
Unless you use locks or transactions, you can’t guarantee the order of execution of queries. If you have multiple PHP scripts that are executed simultaneously, They will interact with the database unaware of each other.
For example, if a script is executed twice simultaneously and has two queries with a few seconds of space between them, script one’s query 1 could be executed by the database before script two’s query one, but script two’s query 2 could be executed before script one’s query 2.
From a higher level perspective, all of the queries are executed at the same time, but if there is something in the code that depends on the queries being executed in a specific order, you’d need to use a lock or transaction to make sure everything goes well.