FLATTEN for nested data in Snowflake - Time & Space Complexity

Time Complexity: FLATTEN for nested data

O(n)

Understanding Time Complexity

When working with nested data in Snowflake, we often use FLATTEN to turn arrays into rows. Understanding how the time to run FLATTEN grows as the nested data grows helps us plan and optimize queries.

We want to know: how does the work Snowflake does change as the nested array gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

SELECT
  t.id,
  f.value
FROM
  my_table t,
  LATERAL FLATTEN(input => t.nested_array) f;
    

This query takes each row from my_table and expands the nested array nested_array into multiple rows, one per element.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: The FLATTEN function iterates over each element in the nested array.
• How many times: Once for each element inside the nested array for every row in the table.

How Execution Grows With Input

As the number of elements in the nested array grows, the number of rows output by FLATTEN grows proportionally.

Input Size (n)	Approx. Api Calls/Operations
10	About 10 operations per row
100	About 100 operations per row
1000	About 1000 operations per row

Pattern observation: The work grows directly with the number of elements in the nested array.

Final Time Complexity

Time Complexity: O(n)

This means the time to run FLATTEN grows linearly with the size of the nested array.

Common Mistake

[X] Wrong: "FLATTEN runs in constant time no matter how big the array is."

[OK] Correct: FLATTEN must look at each element to output it, so more elements mean more work and more time.

Interview Connect

Understanding how FLATTEN scales helps you explain query performance and data processing costs clearly. This skill shows you can think about how data size affects work done, a key part of cloud data engineering.

Self-Check

"What if the nested array contains nested arrays itself? How would the time complexity of FLATTEN change if we flatten multiple levels?"