Only run 'SHOW TABLES' install check when an administrator is on the Dashboard

[boonebgorges]

The S2_Core::s2init() method, which runs at 'plugins_loaded' on every pageload, performs a SHOW TABLES LIKE ... query in order to see whether installation is required.

This frequent query is problematic. On installations with many database tables, such as a large Multisite installation, SHOW TABLES LIKE ... queries can take a long time to execute. For example, on the client site where I noticed this issue, with around 15,000 sites, SHOW TABLES LIKE... queries can take more than half a second to complete.

Moreover, this particular check is to see whether installation is necessary. In theory, installation should never be necessary, except the very first pageload after activation. So the query is unnecessary in the vast majority of cases.

There are a couple of ways to work around this:

• Depend on a version flag like you do for the upgrade routine.
• Run some other kind of query that isn't so costly. Ouch as $wpdb->get_var( "SELECT id FROM wp_subscribe2 ORDER BY ID DESC LIMIT 1" ), which would check for the first entry in the wp_subscribe table (perhaps you could have a placeholder entry so that it always returns an item if the table exists)
• Move the installation routine to an activation hook https://developer.wordpress.org/reference/functions/register_activation_hook/
• Limit the frequency with which the query is run.

The first three options seem like they're worth exploring. But I've opted to send a pull request that uses the last option, because it requires the least amount of code changes. I'm limiting the query to pageloads by someone with the manage_options capability, when viewing wp-admin. This doesn't solve the problem generally, but it does prevent the worst of the performance issues.

[boonebgorges]

Thanks for the quick attention.

[tareq1988]

Thanks @boonebgorges for the PR