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How are index hints used in MySQL optimizer?
One of the ways we can influence the way a query is going to be executed is by using index hints. The optimizer makes decisions about the best index for a query, and this is based on index statistics provided to it by the InnoDB engine. Let’s first see how InnoDB statistics work and how we can change it.
When does MySQL optimizer decide which table to join?
When you are executing any query with JOINs, the MySQL optimizer has to decide the order in which those tables should be joined. You might not be happy with the order it comes up with. Let’s look at this query.
Why does MySQL use a cost-based optimizer?
MySQL uses a cost-based optimizer to determine the best way to execute a query. It usually does a great job, but not all the time. There are different reasons for that. In some cases, it might not have enough information about the data and plan queries in a non-optimal way.
What happens when MySQL execution plan is not optimal?
It may happen that a query execution plan is not optimal or (even worse) it’s flapping between several versions. This is serious as it causes unstable performance. Luckily, MySQL gives us the ability to amend query execution plans when we find them not suitable for our queries.
How to control the optimizer strategy in MySQL?
One means of control over optimizer strategies is to set the optimizer_switch system variable (see Section 8.9.2, “Switchable Optimizations” ). Changes to this variable affect execution of all subsequent queries; to affect one query differently from another, it is necessary to change optimizer_switch before each one.
Which is an example of a hint in MySQL?
For tbl_name @ query_block_name syntax, the hint applies to the named table in the named query block. To assign a name to a query block, see Optimizer Hints for Naming Query Blocks . Example: Hints control the behavior of semijoin tables that are merged to the outer query block.