Hi, thanks for reaching out. I think our web x-match service can help: https://datalab.noao.edu/xmatch.php
Be logged in, upload your CSV table (it must have one header line and then your comma-separated rows, and the columns must include ra and dec). Then select your table (from mydb), tell it what are the ra and dec columns, and which of your columns you want to keep in the x-matched result table. Then do the same for one of the DL-hosted tables. Finally, toggle your x-matching and output preferences, and hit Submit. 300k rows should match fairly quickly.
Hope this helps!
Robert for the Data Lab team