Allow memory size exhausted problem!

Hi all

I have this dump message in apache error log as:

PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 85 bytes) in /var/www/mysite.com/system/database/drivers/mysqli/mysqli_result.php on line 183


And this seems to happen only in CI 3.1.5.  , i have tried to increase it up to 1G and it doesnt fix the problem

Please help if you have been through this .

Thanks