Full table scans can be problematic for performance. Certainly if the scanned tables are large. The worst case is when full table scans are involved in joins and particularly when the scanned table is not the first one (this was dramatic before MySQL 8.0 as Block Nested Loop was used) !
A full table scans means that MySQL was not able to use an index (no index or no filters using it).
When Full Table Scans happen (depending of the size of course), a lot of data gets pulled into the Buffer Pool and maybe other important data from the working set is pulled out. Most of the time that new data in the Buffer Pool might even not be required by the application, what a waste of resources !
You then understand that another side effect of Full Table Scans is the increase of I/O operations.
The most noticeable symptoms of Full Table Scans are:
- increase of CPU usage
- increase of …