Invalid Field
You can ignore them.
I have the same messages on my server when I scan a library.
Many warning messages are just noise. The Plex developers may have changed something and not cleaned up the messages in the log files.
Slow Query
The Slow Query is a bit different.
It can show up when Plex is busy scanning media, updating metadata, etc. and a client browses a library. The system will be slow to answer and will log the message.
If it happens a lot, and the server seems slow, then it can mean the database needs optimized.
In Settings → Scheduled Tasks, make sure Optimize database every week is checked. It will run during the scheduled task window.
If you are comfortable with the Linux command line, you can also run DBRepair for Plex Media Server. In addition to optimizing the database it will look for structural errors and try to repair them.
I run it on my server about once a month. It takes 15 - 20 minutes on my DS918+.
See this post: Updated Plex Manually, then Synology Package Center, won't run manually stops - #24 by FordGuy61. It shows downloading and running DB Repair.
