ref() function for model dependencies in dbt - Time & Space Complexity

Time Complexity: ref() function for model dependencies

O(n)

Understanding Time Complexity

When using dbt, the ref() function helps models depend on each other. Understanding how this affects execution time is important.

We want to know how the time to build models grows as the number of dependencies increases.

Scenario Under Consideration

Analyze the time complexity of the following dbt model code using ref().

select *
from {{ ref('base_model') }}
where id in (
  select id from {{ ref('dependent_model') }}
)
    

This code selects data from one model that depends on two other models using ref().

Identify Repeating Operations

Look for repeated work in model building and data fetching.

• Primary operation: Running SQL queries for each referenced model.
• How many times: Once per model dependency, plus the current model.

How Execution Grows With Input

As you add more models that depend on others, the total work grows.

Number of Models (n)	Approx. Operations
3	3 queries
10	10 queries
100	100 queries

Pattern observation: Each model adds one query to run, so work grows linearly.

Final Time Complexity

Time Complexity: O(n)

This means the time to build models grows directly with the number of models referenced.

Common Mistake

[X] Wrong: "Using ref() does not add any extra time because it just points to models."

[OK] Correct: Each ref() causes dbt to run that model's SQL, so more refs mean more queries and more time.

Interview Connect

Knowing how dependencies affect build time helps you design efficient data pipelines and explain your choices clearly.

Self-Check

What if we changed from many small models with ref() to one big model without dependencies? How would the time complexity change?

Practice

1. What is the main purpose of the ref() function in dbt?

easy

A. To create new database users

B. To write raw SQL queries inside dbt models

C. To link models and define dependencies between them

D. To schedule dbt runs automatically

Solution

1. Step 1: Understand the role of ref()

   The ref() function is used to link one model to another in dbt, so dbt knows the order to run models and their dependencies.

2. Step 2: Identify what ref() does not do

   It does not write raw SQL, create users, or schedule runs. Its main role is linking models.

3. Final Answer:

   To link models and define dependencies between them -> Option C

4. Quick Check:

   ref() links models = A [OK]

Hint: Remember: ref() connects models, not SQL or users [OK]

Common Mistakes:

• Thinking ref() writes SQL code
• Confusing ref() with scheduling tools
• Assuming ref() manages database users

2. Which of the following is the correct syntax to reference a model named customers inside another model using ref()?

easy

A. select * from {{ ref('customers') }}

B. select * from ref('customers')

C. select * from ref(customers)

D. select * from {{ ref(customers) }}

Solution

1. Step 1: Recall dbt Jinja syntax for ref()

   In dbt, ref() must be wrapped in double curly braces and the model name must be a string in quotes.

2. Step 2: Check each option

   select * from {{ ref('customers') }} uses {{ ref('customers') }} which is correct. Options B and C miss the curly braces or quotes. select * from {{ ref(customers) }} misses quotes around the model name.

3. Final Answer:

   select * from {{ ref('customers') }} -> Option A

4. Quick Check:

   Use {{ ref('model_name') }} syntax = A [OK]

Hint: Always use {{ ref('model_name') }} with quotes and braces [OK]

Common Mistakes:

• Omitting curly braces {{ }}
• Not putting model name in quotes
• Using ref() without Jinja syntax

3. Given the following dbt model code, what will be the output SQL after compilation if the orders model exists?

select order_id, customer_id
from {{ ref('orders') }}

medium

A. select order_id, customer_id from orders

B. select order_id, customer_id from {{ ref('orders') }}

C. select order_id, customer_id from dbt.orders

D. select order_id, customer_id from ref('orders')

Solution

1. Step 1: Understand what ref() compiles to

   The ref() function compiles to the actual table name of the referenced model, usually just the model name like 'orders'.

2. Step 2: Check the compiled SQL output

   The compiled SQL replaces {{ ref('orders') }} with orders, so the output is select order_id, customer_id from orders.

3. Final Answer:

   select order_id, customer_id from orders -> Option A

4. Quick Check:

   ref('orders') compiles to orders = C [OK]

Hint: ref() compiles to the model's table name without braces [OK]

Common Mistakes:

• Leaving ref() uncompiled in SQL
• Adding extra schema prefix without config
• Using ref() as a string literal

4. You wrote this dbt model code:

select * from ref('sales')

When you run dbt, you get an error. What is the problem?

medium

A. Quotes around 'sales' should be removed

B. Model name 'sales' does not exist

C. ref() cannot be used inside select statements

D. Missing double curly braces around ref()

Solution

1. Step 1: Check the syntax of ref() usage

   In dbt, ref() must be wrapped in double curly braces to be interpreted as Jinja code.

2. Step 2: Identify the error cause

   The code uses ref('sales') without {{ }}, so dbt treats it as plain text, causing an error.

3. Final Answer:

   Missing double curly braces around ref() -> Option D

4. Quick Check:

   Use {{ ref('model') }} not ref('model') alone = D [OK]

Hint: Always wrap ref() in {{ }} to avoid errors [OK]

Common Mistakes:

• Forgetting {{ }} around ref()
• Assuming ref() works without Jinja
• Removing quotes from model name

5. You have two models: customers and orders. You want to create a new model customer_orders that joins these two. Which is the best way to use ref() to ensure correct dependencies and flexible naming?

hard

A. select c.customer_id, o.order_id from customers c join orders o on c.customer_id = o.customer_id

B. select c.customer_id, o.order_id from {{ ref('customers') }} c join {{ ref('orders') }} o on c.customer_id = o.customer_id

C. select c.customer_id, o.order_id from 'customers' c join 'orders' o on c.customer_id = o.customer_id

D. select c.customer_id, o.order_id from ref('customers') c join ref('orders') o on c.customer_id = o.customer_id

Solution

1. Step 1: Use ref() with correct Jinja syntax for both models

   To link models and ensure dbt knows dependencies, use {{ ref('model_name') }} for both customers and orders.

2. Step 2: Avoid hardcoding table names or missing Jinja syntax

   Options A and C hardcode names or use quotes incorrectly. select c.customer_id, o.order_id from ref('customers') c join ref('orders') o on c.customer_id = o.customer_id misses curly braces, so it won't compile.

3. Final Answer:

   select c.customer_id, o.order_id from {{ ref('customers') }} c join {{ ref('orders') }} o on c.customer_id = o.customer_id -> Option B

4. Quick Check:

   Use {{ ref('model') }} for all dependencies = B [OK]

Hint: Use {{ ref('model') }} for all model references [OK]

Common Mistakes:

• Hardcoding table names instead of using ref()
• Forgetting curly braces around ref()
• Using quotes incorrectly around model names