Avoid Full Table Scans

Full table scans are where the database will read the entire table without an index.
Some of the reasons why full table scans are performed:

1. no WHERE clause
2. no index on any field in WHERE clause
3. poor selectivity on an indexed field
4. too many records meet WHERE conditions
5. using SELECT * FROM
6. function used on the indexed column in the query

Tip - Generate Explain plans to understand how database optimizer treats the query