Mview changelog tables are gone when they shouldn't be

[peterjaap]

Preconditions

1. Magento 2.1.9 (maybe other versions as well)

Steps to reproduce

1. Run php bin/magento ind:show
2. If mode is update on save ('realtime'), run php bin/magento ind:set-mode schedule
3. Run echo "show tables like '%_cl'" | mysql magentodatabasename
4. _cl tables should exist for every indexer
5. Run php bin/magento ind:set-mode realtime
6. _cl tables should still exist
7. Run php bin/magento ind:set-mode schedule (you should see XXX was changed from 'Update on Save' to 'Update by Schedule')
8. Check _cl tables; they should still exist
9. Run php bin/magento ind:set-mode schedule again (you should see 'XXX has not been changed')

Expected result

1. The _cl tables should never be removed once they exist; not even when you switch to realtime. Because when you switch to realtime and back to schedule, mview_state still has a version number which will be higher than the value in the _cl table (since that would be 0 for a new empty table).

Actual result

1. Check _cl tables; they are now gone
2. Panic! My _cl tables are gone while indexer mode is on schedule!

[magento-engcom-team]

Hello @peterjaap. Thanks for reporting. We can not reproduce Your issue. No matter how tried the _cl tables permanently remained there after they had been created. Your steps to reproduce are really clear. Seems there are some other crucial factors causing this issue. Could You please provide us some additional information You deem as some that could influence the issue.

[peterjaap]

@pascaladriaansen found a similar issue and found the culprit to be a Mirasvit extension, see here; #5836 (comment)

[peterjaap]

See also #10204