Can you create an index to determine if a text column is null in mysql?

Question

I have a very large (> 100 million rows) table and a query that I need to write needs to check if a column of type TEXT is null. I don't care about the actual text, only if the column value is null or not.

I'm trying to write an index to make that query efficient, and I want the query to be satisfied by only looking at the index eg such that the explain output of that query shows Using index instead of Using index condition .

To be concrete, the table I'm dealing with has a structure like this:

CREATE TABLE foo (
    id INT NOT NULL AUTO_INCREMENT,
    overridden TEXT,
    rowState VARCHAR(32) NOT NULL,
    active TINYINT NOT NULL DEFAULT 0,
    PRIMARY KEY (id),
) ENGINE=InnoDB

And the query I want is this:

SELECT 
    IF(overridden IS NULL, "overridden", rowState) AS state, 
    COUNT(*)
FROM foo 
WHERE active=1
GROUP BY state

The overridden column is a description of why the column is overridden but, as I mentioned above, I don't care about its content, I only want to use it as a boolean flag.

I've tried creating an index on (active, rowState, overridden(1)) but the explain output still shows Using index condition .

Is there something better that I could do?

Answer 1

I suspect this would be a good application for a "prefix" index. (Most attempts at using such are a fiasco.)

The optimal order, I think, would be:

INDEX(active,         -- handle the entire WHERE first
      overridden(1),  -- IS NULL possibly works well
      rowState)       -- to make index "covering"

Using index condition means that the selection of the row(s) is handled in the Engine, not the "Handler".

EXPLAIN FORMAT=JSON SELECT ... may provide more insight into what is going on. So might the "Optimizer trace".

Or...

If the MySQL version is new enough, create a "generated column" that persists with the computed value of overridden IS NULL .