I was writing a helper method to check if a table is a view, for the merger of 'reserves' and 'old_reserves' into 'holds', and found that running things like [1]:

    my $sth = $dbh->prepare("
        SELECT COUNT( * ) AS count
        FROM information_schema.COLUMNS
        WHERE COLUMN_NAME =  'reserve_id'
        AND (
          TABLE_NAME LIKE  'reserves'
          OR
          TABLE_NAME LIKE  'old_reserves'
        )
    ");

Will give different results depending on the user permissions. So if (for some reason) the sysadmin sets two Koha instances on the same DB server, and the user has more privileges than expected, it will count things from databases other than those of the running instance.

In the case of the views this was easy to spot:

root@kohadevbox:misc4dev(master)$ mysql -hdb -ppassword
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 232
Server version: 10.3.27-MariaDB-1:10.3.27+maria~focal mariadb.org binary distribution

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SELECT TABLE_SCHEMA, TABLE_NAME FROM information_schema.tables WHERE TABLE_NAME='reserves';
+--------------+------------+
| TABLE_SCHEMA | TABLE_NAME |
+--------------+------------+
| koha_test    | reserves   |
| koha_kohadev | reserves   |
+--------------+------------+
2 rows in set (0.02 sec)


What we need to do, is adding the database name on the WHERE condition, like:

my $db_name = C4::Context->config('database');
...
   WHERE TABLE_SCHEMA=$db_name;


[1] taken from updatedatabase.pl:5573~5582