Is there any configuration not to use index merge in mariadb, if a cardinality of an index is below than that?

Question

There is a table have 2 indexes, called state and CATEGORY, which has a very low cardinality respectively 4 and 24 within 7,110,590 rows in the table.

When running query of select statements that includes above 2 indexes in where clause, an optimizer try to index merge, Using intersect(state,CATEGORY), that is less efficient than even table scan, takes about 20 sec.

Is there any way to except a index if the cardinality of the index is lower than a specific directed number when an optimizer is about to make a execution plan?

The database is MariaDB-1:10.6.11 Community version

But other databases which has lower version than previous mentioned database server, 10.3.28-MariaDB, handles the same query in 5ms. Servers of older version run a table scan using high cardinality index(date). All mentioned servers are same replicas of a source database with slightly different versions. Depends on difference of their version, their optimizers create query that have a lot of difference in performance.

What factors could bring that difference?

---

Full explain extended in newer version having low performance

id 1
select_type SIMPLE
table that_table
type index_merge
possible_keys CATEGORY,state
key state,CATEGORY
key_len 6,152
ref NULL
rows 1778388
filtered 100
Extra
Using intersect(state,CATEGORY); Using where; Using filesort

---

Full explain extended in old version having good performance

id 1
select_type SIMPLE
table that_table
type index
possible_keys CATEGORY,state
key DATE
key_len 62
ref NULL
rows 18
filtered 100
Extra Using where

---

Below is indexes on the table

# cardinality: 24, `CATEGORY` VARCHAR(50) DEFAULT '' NOT 
CREATE INDEX CATEGORY ON that_table (CATEGORY) ; 


# cardinality: 4, `state` VARCHAR(1) DEFAULT 'Y' NULL;
CREATE INDEX state ON that_table (state);

# cardinality: 7,110,590, `DATE` VARCHAR(20) DEFAULT '' NOT NULL;
CREATE INDEX DATE ON that_table (DATE);

---

The problematic query:

EXPLAIN EXTENDED
SELECT *
FROM that_table
WHERE category = 'blabla'
  AND state = 'Y'
ORDER BY date DESC
LIMIT 9;

---

A definition of table

CREATE TABLE `that_table` (
     `UID` int(10) NOT NULL AUTO_INCREMENT,
     `state` varchar(1) DEFAULT 'Y',
     `CATEGORY` varchar(50) NOT NULL DEFAULT '',
     `DATE` varchar(20) NOT NULL DEFAULT '',

     # .... other columns
     
     PRIMARY KEY (`UID`),
     KEY `DATE` (`DATE`),
     KEY `CATEGORY` (`CATEGORY`),
     KEY `state` (`state`),

     # ... other indexes
);

Answer 1

"Using intersect(state,CATEGORY)" almost always inplies that you have single-column INDEXes on each of those two columns.

Replace INDEX(state) with INDEX(state, category).

That will get rid of that Using intersect and speed up the query. (Using union is a different story.)

If you need further discussion, please provide SHOW CREATE TABLE and the full EXPLAIN SELECT ...

Meanwhile, don't bother to turn off "index merge" or use "ignore index"; they are usually better than nothing.

More

WHERE category = 'blabla'
  AND state = 'Y'
ORDER BY date DESC
LIMIT 9;

That would benefit even more from

INDEX(category, state,  -- either order is OK
      date)             -- last

More More

Different servers (even with the same MariaDB version) will sometimes happen to have different "statistics". This can lead to different query plans, hence the differences you are seeing.

My 3-column index, plus removing any index that is a prefix of that (namely `KEY(category)) is very likely to be picked and used effectively on all Replicas.

It is risky to try to explicitly direct the Optimizer (or use trickery) into using one plan over another. With a different state or category value or date range, and with different statistics, the attempt could backfire.