Why performance tuning matters in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the employees table.

PostgreSQL

SELECT [1] FROM employees;

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Aeverything

BALL

C*

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ALL' instead of '*'

Using 'columns' which is not valid SQL

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to filter employees with salary greater than 50000.

PostgreSQL

SELECT * FROM employees WHERE salary [1] 50000;

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A>

B<

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which means less than

Using '=' which means equal

The '>' operator filters rows where salary is greater than 50000.

3fill in blank

hard

Fix the error in the query to count employees in each department.

PostgreSQL

SELECT department, COUNT([1]) FROM employees GROUP BY department;

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Asalary

B*

Cdepartment

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(department) which counts non-null departments

Using COUNT(employee_id) which may miss nulls

Using COUNT(*) counts all rows per department, which is the correct way to count employees.

4fill in blank

hard

Fill both blanks to create an index on the salary column to improve query speed.

PostgreSQL

CREATE INDEX idx_salary ON employees USING [1]([2]);

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ABTREE

BHASH

Csalary

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Using HASH which is less common and limited

Indexing the wrong column like department

BTREE is the default and most common index type in PostgreSQL. Indexing the salary column helps speed up queries filtering by salary.

5fill in blank

hard

Fill all three blanks to write a query that selects employee names and orders them by salary descending.

PostgreSQL

SELECT [1], salary FROM employees ORDER BY [2] [3];

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Aname

Bsalary

CDESC

DASC

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by name instead of salary

Using ASC instead of DESC for descending order

Select the 'name' column, order by 'salary' in descending order (DESC) to see highest salaries first.