How to Calculate Average Landed Cost in Postgres Using PL/pgSQL

Описание: Discover how to efficiently calculate the average landed cost for SKUs in Postgres. Learn about using recursive CTE and PL/pgSQL functions for precise data analysis.
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Understanding the Challenge in Calculating Average Landed Cost in Postgres

When working with inventory management, one of the key metrics is the average landed cost of products. This cost includes various factors beyond just the purchase price, making it critical for accurate financial reporting and budgeting. If you're using PostgreSQL and have faced issues retrieving this information due to complex SQL queries, you're not alone.

In this guide, we'll explore how to derive the average landed cost from your PostgreSQL database, particularly utilizing a method in PL/pgSQL.

The Problem

You have a table containing historical records of inventory, showing data such as incoming unit quantity, incoming unit cost, and landed unit cost for different SKUs over time. Your goal is to generate a view or table where each SKU’s average landed cost is calculated dynamically using prior records.

The initial approach attempted involved a recursive Common Table Expression (CTE). However, this method resulted in errors related to recursive queries. Understanding the limitation of the SQL approach led to an alternative solution using PL/pgSQL.

The Solution: Utilizing PL/pgSQL for Average Landed Cost Calculation

1. Defining the Data Structure

To begin, we need to define a custom data type that will hold all relevant fields:

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2. Creating a PL/pgSQL Function

Next, we create a function that will iterate through the data, calculating the average landed cost for each SKU:

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3. Calling the Function to Retrieve Results

After creating the function, you can call it easily using:

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Explanation of the Function

Temporary Table: A temporary table landed_costs is created to store the results. This ensures that we can calculate and store ongoing results without affecting the main database.

For Loop: The loop iterates over each row from lc_history, ensuring they are processed in chronological order.

Calculating Landed Unit Cost: For each SKU, it retrieves the most recent average cost prior to the date of the current record.

Inserting Values: Then it calculates the new average cost using the weighted average formula and inserts the data into the temporary table.

Final Insight

Adopting PL/pgSQL for this calculation might not seem as elegant as a pure SQL query, but it can effectively achieve what we need without running into recursive query errors. This method is beneficial for those dealing with complex data structures in PostgreSQL.

By using this approach, you can ensure more consistent inventory cost reporting and a deeper understanding of your financial standings over time.



By addressing the challenges inherent in calculating average landed costs and implementing a clear solution in PL/pgSQL, you can enhance how you manage and report your inventory costs. Happy querying!