Disable creating sort index SELECT post_modified_gmt

Searching on Google for the mentioned query, I believe your assumption that it is coming from WordPress is correct. This ticket can be found at https://core.trac.wordpress.org/ticket/31171.

In my opinion, you should check what requests are coming to your website and block bad bots/users who are making multiple requests (based on that ticket, probably you have multiple RSS feed requests).

Thank you,

tech