Thanks for trying the /var/tmp fix; sorry to hear it didn’t help your problem.
Would you be willing to upload your site’s metadata so I can see it? If so, please visit
Dashboard > Tools > Index MySQL > About
and click the Upload Metadata button. I’ll take a look. Also, take a look at the last hundred or so lines of MySQL’s log file. Maybe it explains the crash. On Ubuntu it’s at /var/log/mysql/error.log .
The companion plugin isn’t like this one. This one rekeys tables, and so has some straightforward SQL statements you can run just once. Your original question showed the entire once-and-done SQL statement that rekeys the wp_usermeta table.
That one (https://wordpress.org/plugins/index-wp-users-for-speed/) adds rows to wp_usermeta to get away from an inefficiency in WordPress Core’s way of looking up administrator, editor, and other users by role. That inefficiency doesn’t matter in sites with small numbers of users, but it does when you have more users. It’s necessary to add those rows in batches to keep the transaction sizes reasonable (a SQL statement that INSERTs tens of thousands of rows at once can blow out your logs and/or make your system thrash). Then, that plugin hooks WordPress’s user queries and replaces them with ones that use the added rows. Upshot: that plugin doesn’t have clean SQL you can run elsewhere.
