This appears to be a question about SQL syntax rather than an Adobe  product.  See if this helps you. How to change PRIMARY KEY of an existing PostgreSQL table? · GitHub

Please tell us which Adobe software you are using so this question can be moved to a more appropriate forum. 

Moderator moved from The Lounge to Adobe Experience Manager​.

I think you are in the wrong forum.  Which Adobe product are you using?  I can branch and move this question the correct forum for you. Nancy

Run your site URL through Google PageSpeed Insights. https://developers.google.com/speed/pagespeed/insights/ Image optimization is the only thing we have direct control over.  The rest is maintained by Adobe Portfolio and thus out of our hands. Nancy

Also see this related discussion for tips on how to reduce image file size.DPI/PPI of my images?

I keep all my cover images well below 100 kb to promote a faster loading page.   With multiple projects, the combined filesize can make a big difference.