How to Fix WordPress Critical Error: “Allowed memory size…exhausted in wp-includes/class-wpdb.php”

Recently, a client of mine reported the dreaded “white screen of death” critical error on one of my sites. On the front end, the error message was simply “There has been a critical error on this website.”

In the error log, I saw this:

[03-Mar-2025 16:51:31 UTC] PHP Fatal error:  Allowed memory size of 805306368 bytes exhausted (tried to allocate 122880 bytes) in /home/customer/www/[URL].org/public_html/wp-includes/class-wpdb.php on line 2316

Unfortunately, this line was in WP core, not in my custom theme, so I was a bit baffled on how to debug this.

When I copied it to local, I was unable to reproduce the problem, even when I performed the exact same actions. So, I copied it to a staging site on the same hosting service and still wasn’t able to reproduce the problem.

I was only able to see the problem when I did the actions on the live site, which had identical code. Huh!

Debugging the Problem

So I had the situation where the exact same code was generating an error on one environment and not another!

The area of WP core where the site crashed looked like this:

if ( $this->result instanceof mysqli_result ) {
  while ( $row = mysqli_fetch_object( $this->result ) ) {
    $this->last_result[ $num_rows ] = $row;
      ++$num_rows;
    }
  }

So the clue here is it had something to do with the database (MySQL).

I had encountered situations where a corrupted WP database caused fatal crashes.

And, the act of copying a database (i.e., to my local install) can sometimes clean up these errors. That would explain why I didn’t see the problems on local or staging.

The Solution

This was my best theory so I went ahead and installed the free version of the WP Optimize plugin on the live site. After making a backup, I ran the database optimizations and voila!! The problem was fixed!!!

So the problem in this case was a corrupted database!

I don’t know how my database got corrupted, but I do know that this site is heavy in database operations. I’ll keep monitoring and add whatever insights I find here!

Now, this won’t solve the problem in every situation where there’s an exhausted memory error, but it’s something to try, especially if the site is crashing in a database-related area of the code.

Did this fix your problem? Let me know below in the comments.

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