TABLESAMPLE for random sampling in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the TABLESAMPLE clause do in PostgreSQL?

It allows you to retrieve a random sample of rows from a table instead of the entire table, which is useful for quick analysis or testing.

beginner

How do you specify the size of the sample when using TABLESAMPLE?

You specify the sample size as a percentage depending on the sampling method, for example: TABLESAMPLE SYSTEM (10) samples approximately 10% of the table.

intermediate

What are the two main sampling methods supported by PostgreSQL's TABLESAMPLE?

The two main methods are SYSTEM and BERNOULLI. SYSTEM samples pages, while BERNOULLI samples rows individually.

intermediate

True or False: Using TABLESAMPLE SYSTEM guarantees the exact percentage of rows sampled.

False. SYSTEM sampling works at the page level, so the exact number of rows sampled can vary and is approximate.

beginner

Write a simple query to select approximately 5% random rows from a table named employees using TABLESAMPLE.

SELECT * FROM employees TABLESAMPLE SYSTEM (5);

Which TABLESAMPLE method samples rows individually in PostgreSQL?

A BERNOULLI

B SYSTEM

C RANDOM

D PAGE

BERNOULLI samples each row independently with the given probability, while SYSTEM samples pages.

What does TABLESAMPLE SYSTEM (10) do?

A Samples exactly 10 rows

B Samples approximately 10% of the table's rows

C Samples 10 random columns

D Samples 10 random tables

It samples approximately 10% of the table's rows by selecting pages randomly.

Can TABLESAMPLE guarantee the exact number of rows returned?

A Yes, always

B Only with BERNOULLI

C Only with SYSTEM

D No, it returns an approximate sample

Sampling methods return approximate results, not exact counts.

Which of the following is a valid TABLESAMPLE clause in PostgreSQL?

A TABLESAMPLE SYSTEM (20)

B TABLESAMPLE RANDOM (20)

C TABLESAMPLE ROWS (20)

D TABLESAMPLE PERCENT (20)

SYSTEM and BERNOULLI are valid methods; RANDOM, ROWS, and PERCENT are not.

Why might you use TABLESAMPLE in a query?

A To update all rows in a table

B To sort data alphabetically

C To speed up queries by working with a smaller random subset

D To create a new table

Sampling reduces data size for faster analysis or testing.

Explain how TABLESAMPLE works in PostgreSQL and the difference between SYSTEM and BERNOULLI methods.

Think about how data is stored in pages and how sampling can happen at different levels.

Describe a practical scenario where using TABLESAMPLE would be helpful.

Imagine you want to test a query or get a quick insight without scanning the whole table.

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