In SQL server table When there is no index to use for
searching, the result is similar to the reader who looks at every page in a
book to find a word. The SQL engine needs to visit every row in a table. In
database terminology we call this behavior a table scan, or just scan. A full
table scan of a very large table can adversely affect the performance. Creating
proper indexes will allow the database to quickly narrow in on the rows to
satisfy the query, and avoid scanning every row in the table.