How to expand site_content system table (add couple of columns)?

Question

MODX

The task is to add a certain number of columns to the site_content table in order to be able to filter and sort later when outputting through getResources.
I know that in addition to adding to the database, you still need to make changes to the XML schema, but attempts to find intelligible information have not been successful.
Please tell me where you can read about this in detail, which XML file needs to be edited in order to make changes specifically to site_content. New tables are not interested (yet :)).
By the way, we are talking about MODx Revo 2.3.5
Thank you.

Answer 1

The approach is strange. At the first update, you will say goodbye to your entire structure. I would suggest...
1) Use faster and more flexible pdoResources
2) Use TV filtering
3) Analogue: use TV and use LEFT JOIN filtering directly in the query