Constant-Folding Optimization in MySQL 8.0

May 7, 2019


In MySQL 8.0.16 the optimizer has improved again!
Comparisons of columns of numeric types with constant values are checked and folded or removed for invalid or out-of-rage values.
The goal is to speed up query execution.

The name of this article (Constant-Folding Optimization), named after this kind of optimization, is quite cryptic. Nevertheless the principle is simple and more important there is nothing to do from the user perspective.

What is “Constant-Folding Optimization” ?

From the MySQL Documentation :
Comparisons between constants and column values in which the constant value is out of range or of the wrong type with respect to the column type are now handled once during query optimization rather row-by-row than during execution.

From the MySQL Server Team Blog :
The goal is to speed up execution at the cost of a little more analysis at optimize time.
Always true and false comparisons are detected and eliminated.
In other cases, the type of the constant is adjusted to match that of the field if they are not the same, avoiding type conversion at execution time

Clear enough?

One example is worth a thousand words, so let’s have a deeper look comparing the old behavior in MySQL 8.0.15 to the new one beginning with MySQL 8.0.16.

I’m using the optimized MySQL Server Docker images, created, maintained and supported by the MySQL team at Oracle.

Deployment of MySQL 8.0.15 & MySQL 8.0.16:


Obviously using a password on the command line interface can be insecure.

Please read the best practices of deploying MySQL on Linux with Docker.

Copy the test table dump file on 8.0.15 & 8.0.16:

Load the test table into 8.0.15 instance:

Load the test table into 8.0.16 instance:

Let’s see what we have loaded:

What is important for us here is the non indexed column – num :

num int(10) unsigned DEFAULT NULL

It contains only positive numbers:

The old behavior

What happens if I looking for a negative number, let’s say -12345, on the column num ?
Remember that it contains only positive numbers and there is no index.

According to the EXPLAIN plan, we have a full table scan. In a way that makes sense because there is no index on num.
However we know that there is no negative value, so there is certainly some room for improvements 🙂

Running the query:

Indeed the full table scan could be costly.

The current behavior – 8.0.16+

The Constant-Folding Optimization improves the execution of this type of queries.

The EXPLAIN plan for MySQL 8.0.16 is completely different:

Did you notice the:

Extra: Impossible WHERE

Looking for the negative value in a strictly positive column was processed at the optimize time!
So they are obviously a positive impact on the query execution time:


In addition to the = operator, this optimization is currently possible for >, >=, <, <=, =, <>, != and <=> as well.

Indexed column

As a side note, if your column is indexed the optimizer already have the relevant information, so before 8.0.16, no need of Constant-Folding Optimization, to have a fast query :).


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