Lateral Joins vs. Subqueries: When Should I Use Which?

Lateral Joins vs. Subqueries: A Practical Comparison

This guide clarifies the distinctions between Lateral joins and subqueries in PostgreSQL, helping developers choose the optimal approach for query optimization.

Understanding Lateral Joins

Lateral joins (also called INSTEAD OF joins) uniquely allow subqueries to access columns from the preceding tables in the FROM clause. This characteristic mirrors the functionality of correlated subqueries, executing the subquery for each row of the left-hand table.

When to Use Lateral Joins

Lateral joins prove particularly useful in situations requiring:

• Access to preceding table columns within subqueries.
• Retrieval of multiple rows or columns from a subquery.
• Utilization of set-returning functions (like unnest()) with multiple parameters, which are typically restricted outside the FROM clause.

Key Differences: Lateral Joins vs. Subqueries

Feature

Lateral Join

Subquery

Feature	Lateral Join	Subquery
Scope	Correlated; evaluated per row of the left table	Evaluated once
Output	Multiple rows/columns possible	Typically single value; multiple rows possible with set-returning functions
Efficiency	Potentially more efficient due to query planner optimization	Can be less efficient, especially with correlated subqueries

Scope

Correlated; evaluated per row of the left table

Evaluated once

Output

Multiple rows/columns possible

Typically single value; multiple rows possible with set-returning functions

Efficiency

Potentially more efficient due to query planner optimization

Can be less efficient, especially with correlated subqueries

Limitations of Subqueries and Advantages of Lateral Joins
• Subqueries often fall short when dealing with:
• FROMMultiple Rows/Columns: Returning multiple rows or columns directly is cumbersome. Lateral joins seamlessly handle this using set-returning functions.FROM
• Set-Returning Functions in : Lateral joins readily integrate set-returning functions within the CROSS JOINs clause, expanding functional options.

CROSS JOINs: Lateral joins explicitly support , enabling joins without requiring join conditions.

• INNEREssential ConsiderationsOUTERNATURAL ON USINGFor
and
• joins, a join condition (CROSS JOINs,
, or
• clauses) is mandatory.

do not necessitate a join condition. Set-returning functions yielding no rows effectively remove the corresponding row from the result.

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