TABLESAMPLE for random sampling in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select a random 10% sample of rows from the table named employees.

PostgreSQL

SELECT * FROM employees TABLESAMPLE [1] (10);

Drag options to blanks, or click blank then click option'

ASYSTEM

BRANDOM

CBERNOULLI

DHASH

Attempts:

3 left

💡 Hint

Common Mistakes

Using RANDOM which is not a valid TABLESAMPLE method in PostgreSQL.

Using BERNOULLI which is valid but less efficient than SYSTEM.

The SYSTEM method is a common TABLESAMPLE method in PostgreSQL to get a random sample of rows.

2fill in blank

medium

Complete the code to select a 5% random sample of rows using the BERNOULLI method.

PostgreSQL

SELECT * FROM sales TABLESAMPLE [1] (5);

Drag options to blanks, or click blank then click option'

AHASH

BRANDOM

CSYSTEM

DBERNOULLI

Attempts:

3 left

💡 Hint

Common Mistakes

Using SYSTEM which samples pages, not rows.

Using RANDOM which is not a valid method.

The BERNOULLI method samples rows individually with the given percentage.

3fill in blank

hard

Fix the error in the code to correctly sample 20% of rows from the products table.

PostgreSQL

SELECT * FROM products TABLESAMPLE [1] (20);

Drag options to blanks, or click blank then click option'

ARANDOM

BSYSTEM

CBERNOULLI

DHASH

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses around the percentage.

Using an invalid method name.

The sampling percentage must be inside parentheses. The correct syntax is TABLESAMPLE SYSTEM (20).

4fill in blank

hard

Fill both blanks to select a 15% random sample of rows from the orders table using the BERNOULLI method.

PostgreSQL

SELECT * FROM orders TABLESAMPLE [1] ([2]);

Drag options to blanks, or click blank then click option'

ABERNOULLI

B10

C15

DSYSTEM

Attempts:

3 left

💡 Hint

Common Mistakes

Using SYSTEM instead of BERNOULLI.

Using 10 instead of 15 for the percentage.

The BERNOULLI method samples rows with the given percentage, which is 15% here.

5fill in blank

hard

Fill all three blanks to select a 25% random sample of rows from the customers table using the SYSTEM method.

PostgreSQL

SELECT * FROM customers TABLESAMPLE [1] ([2]) REPEATABLE ([3]);

Drag options to blanks, or click blank then click option'

ABERNOULLI

BSYSTEM

C25

D12345

Attempts:

3 left

💡 Hint

Common Mistakes

Using BERNOULLI instead of SYSTEM.

Omitting the REPEATABLE clause or seed value.

The SYSTEM method samples pages randomly. The percentage is 25, and REPEATABLE uses a seed value like 12345 for repeatable results.

Practice

1.

What is the main purpose of using TABLESAMPLE in a PostgreSQL query?

easy

A. To retrieve a random subset of rows from a table

B. To sort the table rows in random order

C. To permanently delete random rows from a table

D. To create a backup of the table

Solution

1. Step 1: Understand the purpose of TABLESAMPLE

   TABLESAMPLE is used to get a random sample of rows from a table without scanning the entire table.

2. Step 2: Compare options with TABLESAMPLE usage

   Only To retrieve a random subset of rows from a table describes retrieving a random subset of rows, which matches TABLESAMPLE's purpose.

3. Final Answer:

   To retrieve a random subset of rows from a table -> Option A

4. Quick Check:

   TABLESAMPLE = random subset retrieval [OK]

Hint: TABLESAMPLE returns random rows, not sorting or deleting [OK]

Common Mistakes:

• Confusing TABLESAMPLE with ORDER BY RANDOM()
• Thinking TABLESAMPLE deletes rows
• Assuming TABLESAMPLE creates backups

2.

Which of the following is the correct syntax to sample approximately 10% of rows from a table named employees using the BERNOULLI method?

easy

A. SELECT * FROM employees TABLESAMPLE BERNOULLI(10);

B. SELECT * FROM employees TABLESAMPLE SYSTEM(10);

C. SELECT * FROM employees SAMPLE BERNOULLI(10);

D. SELECT * FROM employees TABLESAMPLE BERNOULLI 10;

Solution

1. Step 1: Recall correct TABLESAMPLE syntax

   The correct syntax uses TABLESAMPLE followed by the method name and percentage in parentheses.

2. Step 2: Check each option

   SELECT * FROM employees TABLESAMPLE BERNOULLI(10); matches the correct syntax exactly. SELECT * FROM employees TABLESAMPLE SYSTEM(10); uses SYSTEM instead of BERNOULLI. SELECT * FROM employees SAMPLE BERNOULLI(10); misses TABLESAMPLE keyword. SELECT * FROM employees TABLESAMPLE BERNOULLI 10; misses parentheses around 10.

3. Final Answer:

   SELECT * FROM employees TABLESAMPLE BERNOULLI(10); -> Option A

4. Quick Check:

   Correct TABLESAMPLE syntax = SELECT * FROM employees TABLESAMPLE BERNOULLI(10); [OK]

Hint: Use TABLESAMPLE METHOD(percentage) with parentheses [OK]

Common Mistakes:

• Omitting TABLESAMPLE keyword
• Using wrong method name
• Missing parentheses around percentage

3.

Given a table products with 1000 rows, what is the expected approximate number of rows returned by this query?

SELECT * FROM products TABLESAMPLE SYSTEM(5);

medium

A. Exactly 5 rows

B. About 50 rows

C. About 500 rows

D. All 1000 rows

Solution

1. Step 1: Understand SYSTEM sampling percentage

   SYSTEM(5) means approximately 5% of rows are sampled.

2. Step 2: Calculate expected rows from 1000 total

   5% of 1000 is 0.05 * 1000 = 50 rows approximately.

3. Final Answer:

   About 50 rows -> Option B

4. Quick Check:

   5% of 1000 = 50 [OK]

Hint: Percentage in TABLESAMPLE is approximate row count percent [OK]

Common Mistakes:

• Confusing percentage with exact row count
• Assuming SYSTEM returns fewer rows than BERNOULLI always
• Thinking TABLESAMPLE returns all rows

4.

Identify the error in this query that tries to sample 20% of rows using BERNOULLI:

SELECT * FROM orders TABLESAMPLE BERNOULLI 20;

medium

A. TABLESAMPLE keyword is misspelled

B. BERNOULLI method cannot be used with TABLESAMPLE

C. Percentage value must be between 0 and 1

D. Missing parentheses around the percentage value

Solution

1. Step 1: Check TABLESAMPLE BERNOULLI syntax

   The percentage must be inside parentheses, like BERNOULLI(20).

2. Step 2: Identify the error in the query

   The query misses parentheses around 20, causing syntax error.

3. Final Answer:

   Missing parentheses around the percentage value -> Option D

4. Quick Check:

   Percentage needs parentheses in TABLESAMPLE [OK]

Hint: Always put percentage in parentheses after method name [OK]

Common Mistakes:

• Omitting parentheses around percentage
• Using invalid percentage range
• Misspelling TABLESAMPLE keyword

5.

You want to quickly test a query on a large table sales but need a random sample that is more uniformly random than SYSTEM sampling. Which query is best?

Options:
A) SELECT * FROM sales TABLESAMPLE SYSTEM(10);
B) SELECT * FROM sales TABLESAMPLE BERNOULLI(10);
C) SELECT * FROM sales ORDER BY RANDOM() LIMIT 100;
D) SELECT * FROM sales TABLESAMPLE SYSTEM(100);

hard

A. Use SYSTEM sampling for speed but less randomness

B. Use ORDER BY RANDOM() for perfect randomness but slower

C. Use BERNOULLI sampling for better randomness

D. Use SYSTEM sampling with 100% to get all rows

Solution

1. Step 1: Compare randomness and speed of methods

   SYSTEM is faster but less random; BERNOULLI is slower but more uniformly random.

2. Step 2: Choose best method for uniform randomness and speed

   BERNOULLI(10) gives a good balance of randomness and reasonable speed for sampling.

3. Final Answer:

   Use BERNOULLI sampling for better randomness -> Option C

4. Quick Check:

   BERNOULLI = better randomness than SYSTEM [OK]

Hint: BERNOULLI is more random; SYSTEM is faster [OK]

Common Mistakes:

• Choosing SYSTEM when better randomness is needed
• Using ORDER BY RANDOM() on large tables (slow)
• Misunderstanding 100% SYSTEM returns all rows