List partitioning by category in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is list partitioning in PostgreSQL?

List partitioning divides a table into smaller pieces called partitions, based on a list of values in a column. Each partition holds rows matching specific category values.

beginner

How do you create a list partitioned table by category in PostgreSQL?

You create a main table with PARTITION BY LIST on a column, then create partitions specifying VALUES IN (list of categories) for each partition.

intermediate

Why use list partitioning by category?

It helps organize data by specific categories, improves query speed by scanning only relevant partitions, and simplifies data management.

beginner

Can a row belong to more than one partition in list partitioning?

No, each row belongs to exactly one partition based on the category value in the partition key column.

intermediate

What happens if a row's category value does not match any partition in a list partitioned table?

If no matching partition exists, the insert will fail unless a default partition is defined to catch unmatched values.

Which clause is used to define list partitioning in PostgreSQL?

A PARTITION BY COLUMN

B PARTITION BY LIST

C PARTITION BY HASH

D PARTITION BY RANGE

List partitioning uses the PARTITION BY LIST clause to specify partitioning by specific category values.

In list partitioning, how do you specify which categories belong to a partition?

A VALUES HASH (category)

B VALUES FROM (start) TO (end)

C VALUES IN (category1, category2, ...)

D VALUES LIKE (pattern)

You specify categories for a partition using VALUES IN with a list of category values.

What is a benefit of list partitioning by category?

A Improves query speed by scanning only relevant partitions

B Automatically compresses data

C Allows rows to belong to multiple partitions

D Eliminates need for indexes

List partitioning improves query speed by limiting scans to partitions matching the category.

What happens if you insert a row with a category not listed in any partition and no default partition exists?

A Insert fails with an error

B Row is inserted into a random partition

C Row is discarded silently

D Row is inserted into the first partition

Without a matching or default partition, the insert will fail with an error.

Which of these is NOT a valid use case for list partitioning?

A Partitioning products by category

B Partitioning sales data by country

C Partitioning users by membership type

D Partitioning logs by date ranges

Partitioning logs by date ranges is better suited for range partitioning, not list partitioning.

Explain how to create a list partitioned table by category in PostgreSQL and why it is useful.

Think about how you split data into groups based on specific values.

Describe what happens when inserting data that does not match any partition in a list partitioned table.

Consider what happens if a category is missing from your partitions.

Practice

1. What is the main purpose of list partitioning by category in PostgreSQL?

easy

A. To split a table into parts based on specific category values

B. To combine multiple tables into one large table

C. To encrypt data in the table for security

D. To create temporary tables for faster queries

Solution

1. Step 1: Understand list partitioning concept

   List partitioning divides a table into smaller parts based on specific category values.

2. Step 2: Identify the main purpose

   This helps organize data and speeds up queries by focusing on relevant partitions.

3. Final Answer:

   To split a table into parts based on specific category values -> Option A

4. Quick Check:

   List partitioning = split by category [OK]

Hint: List partitioning splits tables by category values [OK]

Common Mistakes:

• Confusing list partitioning with table joins
• Thinking it encrypts data
• Assuming it merges tables

2. Which of the following is the correct syntax to create a list partitioned table by category in PostgreSQL?

easy

A. CREATE TABLE sales PARTITION BY GROUP (category);

B. CREATE TABLE sales PARTITION BY RANGE (category);

C. CREATE TABLE sales PARTITION BY HASH (category);

D. CREATE TABLE sales PARTITION BY LIST (category);

Solution

1. Step 1: Identify partition type syntax

   List partitioning uses PARTITION BY LIST in PostgreSQL.

2. Step 2: Match correct syntax

   Only CREATE TABLE sales PARTITION BY LIST (category); uses PARTITION BY LIST with the column name correctly.

3. Final Answer:

   CREATE TABLE sales PARTITION BY LIST (category); -> Option D

4. Quick Check:

   List partitioning syntax = PARTITION BY LIST [OK]

Hint: Use PARTITION BY LIST for list partitioning [OK]

Common Mistakes:

• Using PARTITION BY RANGE instead of LIST
• Using PARTITION BY HASH incorrectly
• Using non-existent PARTITION BY GROUP

3. Given the following setup:

CREATE TABLE products (
  id SERIAL PRIMARY KEY,
  name TEXT,
  category TEXT
) PARTITION BY LIST (category);

CREATE TABLE products_electronics PARTITION OF products FOR VALUES IN ('electronics');
CREATE TABLE products_clothing PARTITION OF products FOR VALUES IN ('clothing');

INSERT INTO products (name, category) VALUES ('Phone', 'electronics'), ('Shirt', 'clothing');

SELECT * FROM products WHERE category = 'electronics';

What will the SELECT query return?

medium

A. No rows because category is filtered incorrectly

B. All rows from both partitions

C. Only rows where category is 'electronics', here the 'Phone' row

D. An error because partitions are not queried directly

Solution

1. Step 1: Understand partition filtering

   Query filters category = 'electronics', so only that partition is scanned.

2. Step 2: Check inserted data

   Only 'Phone' has category 'electronics', so only that row is returned.

3. Final Answer:

   Only rows where category is 'electronics', here the 'Phone' row -> Option C

4. Quick Check:

   Partition filter returns matching rows [OK]

Hint: Query filters partition by category value [OK]

Common Mistakes:

• Expecting all rows without filter
• Thinking query causes error
• Assuming partitions must be queried separately

4. Consider this incorrect partition creation:

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  product TEXT,
  category TEXT
) PARTITION BY LIST (category);

CREATE TABLE orders_electronics PARTITION OF orders FOR VALUES IN ('electronics', 'gadgets');

What is the error in this partition definition?

medium

A. FOR VALUES IN must list values as separate strings, not combined

B. Partitions cannot have multiple values in FOR VALUES IN clause

C. The partition table name is invalid

D. The parent table cannot be partitioned by LIST

Solution

1. Step 1: Check FOR VALUES IN syntax

   FOR VALUES IN expects a list of values as separate strings, e.g. ('electronics', 'gadgets').

2. Step 2: Identify error in given code

   The code uses multiple values in one partition which is not allowed in PostgreSQL list partitioning.

3. Final Answer:

   Partitions cannot have multiple values in FOR VALUES IN clause -> Option B

4. Quick Check:

   Each partition must have exactly one value [OK]

Hint: Each partition must have a single value in FOR VALUES IN clause [OK]

Common Mistakes:

• Using one string with commas inside
• Misnaming partition tables
• Thinking LIST partitioning is not allowed

5. You want to create a list partitioned table events by event_type with partitions for 'login', 'logout', and 'purchase'. Which of the following is the correct way to create the partitions and insert a new 'purchase' event?

hard

A. CREATE TABLE events PARTITION BY LIST (event_type);
CREATE TABLE events_login PARTITION OF events FOR VALUES IN ('login');
CREATE TABLE events_logout PARTITION OF events FOR VALUES IN ('logout');
CREATE TABLE events_purchase PARTITION OF events FOR VALUES IN ('purchase');
INSERT INTO events (event_type) VALUES ('purchase');

B. CREATE TABLE events PARTITION BY RANGE (event_type);
CREATE TABLE events_login PARTITION OF events FOR VALUES FROM ('login') TO ('logout');
INSERT INTO events (event_type) VALUES ('purchase');

C. CREATE TABLE events PARTITION BY LIST (event_type);
CREATE TABLE events_all PARTITION OF events FOR VALUES IN ('login', 'logout', 'purchase');
INSERT INTO events (event_type) VALUES ('purchase');

D. CREATE TABLE events PARTITION BY HASH (event_type);
CREATE TABLE events_hash PARTITION OF events FOR VALUES IN (1);
INSERT INTO events (event_type) VALUES ('purchase');

Solution

1. Step 1: Choose correct partition type and syntax

   List partitioning by event_type requires PARTITION BY LIST and partitions with FOR VALUES IN for each category.

2. Step 2: Verify partitions and insert

   CREATE TABLE events PARTITION BY LIST (event_type); CREATE TABLE events_login PARTITION OF events FOR VALUES IN ('login'); CREATE TABLE events_logout PARTITION OF events FOR VALUES IN ('logout'); CREATE TABLE events_purchase PARTITION OF events FOR VALUES IN ('purchase'); INSERT INTO events (event_type) VALUES ('purchase'); correctly creates partitions for 'login', 'logout', and 'purchase' and inserts a 'purchase' event into the parent table.

3. Final Answer:

   Option A with correct list partitions and insert -> Option A

4. Quick Check:

   List partitions per category + insert into parent [OK]

Hint: Create partitions per category, insert into parent table [OK]

Common Mistakes:

• Using RANGE or HASH instead of LIST
• Creating one partition for all values
• Inserting into partitions directly instead of parent