Explain how sequential scan can execute query
Sequential scan is a method used by a database system to execute a query on a table. It involves scanning every row in the table, one by one, until a match to the query condition is found or all rows have been checked.
Here is a step-by-step explanation of how sequential scan executes a query:
1. The database system receives a query from the user, containing conditions like a search term or a set of filters, along with the table name on which the query should be executed.
2. The database system starts at the first row of the table and examines each row sequentially.
3. For each row, the database system checks if the conditions specified in the query are satisfied.
4. If the conditions are satisfied, the row is added to the result set.
5. The system continues to scan through each row of the table until the end is reached, or until the maximum number of rows to return (if specified in the query) is reached.
6. Once all rows have been checked, the result set, containing the rows that match the query conditions, is returned to the user.
It is important to note that sequential scan can be slower than other query execution methods, such as index-based scans, because it involves examining every row in the table. However, it is a necessary method when there is no appropriate index or when the result set is expected to be large and would require scanning most, if not all, rows anyway.