Full table scan

Full table scan (also known as sequential scan) is a scan made on a database where each row of the table under scan is read in a sequential (serial) order and the columns encountered are checked for the validity of a condition.^[1] Full table scans are usually^[2] 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. Sequential scan takes place usually when the column or group of columns of a table (the table may be on disk or may be an intermediate table created by the join of two or more tables) needed for the scan do not contain an index which can be used for the purpose.

References

1. ↑ "Avoiding Table Scans". Oracle. 2011. 
2. ↑ "Which is Faster: Index Access or Table Scan?". Microsoft TechNet. 2002.