How repair mysql database (missing performance_schema table) after host crash in nextcloud.export?

I have a Nextcloud Snappy installation (from channel=17/stable) running on Ubuntu 18.04.03 LTS on a Lenovo Ideapad L340. A nightly cron job executes the “nextcloud.export” CLI utility. All has been working fine.

Last night the cron job aborted in the middle of exporting the database. The following morning the filesystem on the L340 had become read-only and I had to run fsck to repair it - lots of repairs (around 50) were done. After that the L340 boots up fine into Ubuntu and the Nextcloud server comes up and seems still to be working fine today. However now anytime when I try to export the database (e.g. by running nextcloud.export, or nextcloud.mysqldump) I get this message:

mysqldump: Couldn’t execute ‘SHOW VARIABLES LIKE ‘gtid_mode’’: Table ‘performance_schema.session_variables’ doesn’t exist (1146)

I.e. it appears that I’ve lost at least the ‘performance_schema.session_variables’ table out of my database. Conceivably there is other damage to it as well.

I ran ‘nextcloud.occ maintenance:repair’ and it ran ok, but I still have the same error about my missing database table.

Is there a way I can repair or rebuild my mysql database? Thanks.