MySQL 8.0 – CHAR vs VARCHAR Performance

First, from looking at your existing indexes and comparing them to your example query, you are missing an index on just accountid. The way MySQL handles indexes is left-most, meaning you can have a composite index like this:

KEY `contactaccounts` (`contactid`,`accountid`,`journalcode`) USING BTREE,

and run a query that looks for contactid and the contactaccounts index would be a potential index*. However doing a query on accountid will not utilize the index, because accountid is not the left-most column.

If you never search for contactid without an accountid, I would create the index like this:

DROP INDEX `contactaccounts` ON `tbl_sessions_2012`;
CREATE INDEX `accountscontact` ON `tbl_sessions_2012` (`accountid`,`contactid`,`journalcode`);

Now, analyzing your other indexes, the first of each set is redundant using the left-most rule, and can be dropped in favor of the second:

KEY `contactaccounts` (`contactid`,`accountid`,`journalcode`) USING BTREE 
KEY `contactaccountloginmonths` (`contactid`,`accountid`,`journalcode`,`logintypeid`,`actionTime`) USING BTREE

KEY `contactaccountcollections` (`contactid`,`accountcollectionid`,`journalcode`) USING BTREE
KEY `contactaccountcollectionloginmonths` (`contactid`,`accountcollectionid`,`journalcode`,`logintypeid`,`actionTime`) USING BTREE

KEY `organisationaccounts` (`organisationid`,`accountid`) USING BTREE
KEY `organisationaccountloginmonths` (`organisationid`,`accountid`,`journalcode`,`logintypeid`,`actionTime`) USING BTREE

KEY `organisationaccountcollection` (`organisationid`,`accountcollectionid`,`journalcode`) USING BTREE
KEY `organisationaccountcollectionsloginmonths` (`organisationid`,`accountcollectionid`,`journalcode`,`logintypeid`,`actionTime`) USING BTREE

By having the duplicates you are using up a lot of space on indexes.

_{* I say potential because there is still the possibility that your index will not be used due to the amount of rows MySQL has to scan through. As your account_id IN () statement grows, MySQL will determine that it's faster just to do a full table scan regardless of the index.}

To elaborate on ypercube's correct comment, the problem is not that there is fragmentation on the VARCHAR, but in rows that contain variable length data.

If I were to have a table with these values:

+---+-------+
| i | v     |
+---+-------+
| 1 | some  |
| 2 | small |
| 3 | text  |
+---+-------+

And then issue:

UPDATE my_table SET v = 'dinosaur' WHERE i = 2

Then, depending on the storage engine, I might find that there's just no room for the word 'dinosaur' within the existing row location, since rows were written sequentially and compactly to disk. That might mean I would need to change the location of row #2, or place my 'dinosaur' text on an external location, or what have you.

On the other hand, if I were to issue:

UPDATE my_table SET v = 'a'

There would now be a lot of wasted space; I might be interested in reclaiming that space.

This is the fragmentation problem within rows.