Full table scan is a scan made on the database

Database

A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

 where each row

Row (database)

In the context of a relational database, a row—also called a record or tuple—represents a single, implicitly structured data item in a table. In simple terms, a database table can be thought of as consisting of rows and columns or fields...

 of the table

Table (database)

In relational databases and flat file databases, a table is a set of data elements that is organized using a model of vertical columns and horizontal rows. A table has a specified number of columns, but can have any number of rows...

 under scan is read in a sequential (serial) order and the columns encountered are checked for the validity of a condition. Full table scans are usually the slowest method of scanning a table due to the heavy amount of I/O reads and writes 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

Column (database)

In the context of a relational database table, a column is a set of data values of a particular simple type, one for each row of the table. The columns provide the structure according to which the rows are composed....

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.