How does MySQL index not speed up update query? - mysql

Question

I have a table located in RAM and doing some performance tests.
Let's consider a sample query, adding explain sentences along with results
mysql> explain update users_ram set balance = balance + speed where sub = 1;
+----+-------------+-----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| 1 | UPDATE | users_ram | NULL | ALL | NULL | NULL | NULL | NULL | 2333333 | 100.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set (0.00 sec)
mysql> update users_ram set balance = balance + speed where sub = 1;
Query OK, 1166970 rows affected (0.37 sec)
Rows matched: 1166970 Changed: 1166970 Warnings: 0
As you can see, it takes 0.37 sec without index. Then I'm creating an index on the sub column, which is an int column with just two possible values of 0 and 1, and surprisingly nothing changes
mysql> create index sub on users_ram (sub);
Query OK, 2333333 rows affected (2.04 sec)
Records: 2333333 Duplicates: 0 Warnings: 0
mysql> show index from lords.users_ram;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| users_ram | 0 | user | 1 | user | NULL | 2333333 | NULL | NULL | YES | HASH | | |
| users_ram | 1 | sub | 1 | sub | NULL | 2 | NULL | NULL | | HASH | | |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> explain update users_ram set balance = balance + speed where sub = 1;
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
| 1 | UPDATE | users_ram | NULL | range | sub | sub | 5 | const | 1166666 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set (0.00 sec)
mysql> update users_ram set balance = balance + speed where sub = 1;
Query OK, 1166970 rows affected (0.37 sec)
Rows matched: 1166970 Changed: 1166970 Warnings: 0
If I remove the index and add it again, but now using btree, it gets even more weird
mysql> explain update users_ram set balance = balance + speed where sub = 1;
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
| 1 | UPDATE | users_ram | NULL | range | sub | sub | 5 | const | 1057987 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set (0.00 sec)
mysql> update users_ram set balance = balance + speed where sub = 1;
Query OK, 1166970 rows affected (0.62 sec)
Rows matched: 1166970 Changed: 1166970 Warnings: 0
How could adding an index could have no effect or even slow down the query?
Let's take into account that I'm not modifying the column which is indexed, so mysql doesn't have to do an extra write operation, so really I can't get what's really happening here.

Answer 1

"table located in RAM" -- I suspect that is technically incorrect. The possibilities (in MySQL):
The table lives on disk, but it is usually fully cached in the in-RAM "buffer_pool".
The table is ENGINE=MEMORY. But that is used only for temp stuff; it is completely lost if the server goes down.
update users_ram set balance = balance + speed where sub = 1;
The table users_ram needs some index starting with sub. With such, it can go directly to the row(s). But...
It seems that there are 1166970 such rows. That seems like half the table?? At which point, the index is pretty useless. But...
Updating 1M rows is terribly slow, regardless of indexing.
Plan A: Avoid the UPDATE. Perhaps this can be done by storing speed in some other table and doing the + whenever you read the data. (It is generally bad schema design to need huge updates like that.)
Plan B: Update in chunks: http://mysql.rjweb.org/doc.php/deletebig#deleting_in_chunks
How the heck did you get index-type to be HASH? Perhaps `ENGINE=MEMORY? What version of MySQL?
What is speed? Another column? A constant?
Please provide SHOW CREATE TABLE users_ram -- There are some other things we need to see, such as the PRIMARY KEY and ENGINE.
(I need some of the above info before tackling "How could adding an index could have no effect or even slow down the query?")