[1] Full table scans [2] are usually the slowest method of scanning a table due to the heavy amount of I/O reads required from the disk which consists of multiple seeks as well as costly disk to memory transfers.
In a database, a query that is not indexed results in a full table scan, where the database processes each record of the table to find all records meeting the given requirements.
This usually results in suboptimal performance but may be acceptable with very small tables or when the overhead of keeping indexes up to date is high.
This means that a full table scan should be used when it is the fastest and cannot use a different access path.
In some extreme cases - e.g. the statistics maintained by the database engine indicate that the table contains a very small number of rows - the optimizer may still decide to use a table scan for this type of query: Pros: Cons: