The range access method and why you should use EXPLAIN JSON

I got an interesting question about EXPLAIN and the range access method recently. The person had a query that could be written either with a BETWEEN predicate or an IN predicate, something similar to this:

mysql> EXPLAIN SELECT * 
    -> FROM orders WHERE customer_id BETWEEN 7 AND 10 AND value > 500;
+----+-------------+--------+-------+----------+----------+------+------
| id | select_type | table  | type  | key      | key_len  | rows | Extra
+----+-------------+--------+-------+----------+----------+------+------
|  1 | SIMPLE      | orders | range | cust_val | 10       |   91 |  ...
+----+-------------+--------+-------+----------+----------+------+------

mysql> EXPLAIN SELECT * 
    -> FROM orders WHERE customer_id IN (7,8,9,10) AND value > 500;
+----+-------------+--------+-------+----------+----------+------+------
| id | select_type | table  | type  | key      | key_len  | rows | Extra
+----+-------------+--------+-------+----------+----------+------+------
|  1 | SIMPLE      | orders | range | cust_val | 10       |   44 |  ...
+----+-------------+--------+-------+----------+----------+------+------

The table was:

CREATE TABLE orders (

   order_id INT NOT NULL PRIMARY KEY,

   customer_id INT,

   value INT,

   order_date DATE,

   KEY custid_value (customer_id, value)

)

Given that customer_id is an integer value, these queries should be equivalent. And that's what EXPLAIN seems to tell us too: same access method, same key, same key_length etc.

You may have guessed the question by now: If the queries are equivalent, why isn't the rows estimate identical? Is one of the numbers wrong or do these queries have different execution plans after all?

FAQ: InnoDB extended secondary keys

MySQL 5.6 introduced a new feature called extended secondary keys. We get a lot of questions about it and find that most of them come from a few incorrect assumption. In this post I'll try to get rid of the confusion once and for all. Famous last words... here goes:

Q1: Do I need to do anything to enable extended secondary keys?

No, nothing at all. It's on by default and I can't see any sensible reason why you would want to disable it. However, it is possible to disable it by tuning the optimizer_switch: SET optimizer_switch='use_index_extensions={on|off}'.

Q2: Does extended secondary keys only work with InnoDB?

No, it should work with any storage engine that uses the primary key columns as reference to the row, which means most storage engines with clustered primary keys. I say "should" because it requires a minimum of work from the storage engine provider; it must announce to MySQL that it is supported.