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What is ELT & How Does It Work?

What is ELT? Nucleusbox Blog post

ELT stands for Extract, Load, Transform. It’s a data integration methodology that prioritizes rapid data ingestion over upfront transformation.


In the ever-evolving world of data, organizations are constantly collecting and storing information from various sources. This data, however, often resides in disparate systems with different formats and structures. This creates a challenge: how to analyze and utilize this data effectively for informed decision-making? Enter ELT, a data integration methodology that offers an alternative approach to ETL (Extract, Transform, Load).

What Does ELT Stand For?

ELT, short for Extract, Load, Transform, prioritizes quickly bringing data into a central location for later transformation. Let’s break down the three key stages involved:

1 Extract:

This stage involves retrieving data from various source systems. These sources can be diverse, encompassing:
ELT Extract part is the same as ETL so you can refer to ETL BLOG.

  • Relational databases: Structured data repositories like MySQL, Oracle, or SQL Server, often storing customer information, transaction details, or product data.
  • Flat files: Text-based files like CSV or TXT, commonly used for exporting data from applications or systems.
  • Cloud applications: SaaS platforms like Salesforce, Marketo, or Google Analytics, offer valuable data through APIs.
  • Social media platforms: Platforms like Twitter, Facebook, or Instagram, generate vast amounts of user activity and sentiment data.

2 Load:

This is the key difference between ELT and ETL. Instead of meticulously transforming data before loading, ELT prioritizes rapid data ingestion. The extracted data is directly loaded into a target system, which can be either a:

Data Lake:

  • Definition: A large, scalable repository designed to store massive volumes of both structured and unstructured data. Data lakes offer flexibility with schema, making them suitable for ELT’s diverse data formats.
  • Example:
    • Apache Atlas (Open source): A platform for data governance and metadata management within a data lake.
    • Apache Iceberg, Delta Lake, Apache Hudi: Open-source data lake storage formats that provide ACID transactions and data versioning capabilities.
    • S3 (Amazon Simple Storage Service): A scalable object storage service offered by AWS, commonly used as a data lake target.
    • Databricks Data Lake: A cloud-based data lake platform offering storage, computing, and analytics capabilities.

Data Warehouse:

Definition: A central repository for storing and analyzing structured data. Data warehouses typically require a predefined schema, which might necessitate some upfront transformation in ELT.

  • Example:
    • Redshift: A cloud-based data warehouse service offered by AWS, optimized for fast query performance.
    • Databricks: A cloud-based platform offering data warehousing capabilities alongside data lake functionality.
    • Snowflake: A cloud-based data warehouse service known for its scalability and ease of use.
    • MySQL/Postgres: Open-source relational databases that can be used as data warehouses for smaller datasets.
    • Google BigQuery: A cloud-based data warehouse service offered by Google, known for its serverless architecture and fast analytics.

The choice of a target system (Data Lake or Data Warehouse) depends on factors like the type of data, desired level of flexibility, and analytical needs.


Imagine a university with the following source systems:

  • Student Information System (SIS): Stores student data like names, IDs, majors, and grades.
  • Learning Management System (LMS): Tracks student activity and performance in online courses.
  • Library Management System: Captures book borrowing and return information.

In the ELT approach, the data from these sources would be directly loaded into a data lake. This data lake would then serve as the target system for further processing and analysis.

3. Transform:

Once the data resides within the target system (data lake or data warehouse), the transformation stage commences. This stage involves cleaning, standardizing, and transforming the data into a consistent structure suitable for analysis.


Continuing with the university scenario, the data lake might undergo the following transformations:

  • Data Cleaning: Identifying and correcting errors or inconsistencies in the data, such as missing grades in the SIS, duplicate entries in the LMS, or inaccurate book titles in the library system.
  • Data Standardization: Converting data into a consistent format, such as standardizing date and time formats across different source systems or ensuring consistent course names and codes.
  • Data Enrichment: Adding additional information to the data, such as enriching student profiles with demographic data based on zip codes or calculating student performance metrics based on course grades and LMS activity.

By understanding the specific source and target systems involved in each stage, we gain a clearer picture of how ELT operates and its advantages in handling diverse data for rapid analysis.

Key Differences Between ELT and ETL:

While both ETL and ELT aim to integrate data from various sources, they differ in their approach:

Transformation StageUpfront, before loading data into the target system.After loading data into the target system.
Target SystemTypically a data warehouse with a predefined schema.Often a data lake with a flexible schema.
Processing Power RequirementsPotentially higher processing power is required for in-target system transformation.It may require additional data quality checks after transformation.
Data Quality FocusFocuses on ensuring data quality before loading.May require additional data quality checks after transformation.

Why is ELT Gaining Popularity?

ELT is gaining traction for several reasons:

  • Faster Data Ingestion: By prioritizing initial data loading, ELT enables quicker data availability for analysis, particularly for large datasets.
  • Flexibility: Data lakes, the common target system for ELT, offer a flexible schema that can accommodate diverse data formats and structures.
  • Scalability: Data lakes are highly scalable, making them suitable for handling massive data volumes.
  • Cost-Effectiveness: ELT can potentially be more cost-effective than ETL due to reduced upfront processing requirements.

ELT Use Cases

  • Real-time analytics: When immediate access to data is crucial, ELT allows for faster data ingestion and near real-time analysis.
  • Big data analytics: For massive datasets with diverse formats, ELT’s flexible data lake approach is advantageous.
  • Data warehousing with flexible schema: When the data schema is not clearly defined upfront, ELT’s adaptable nature can be beneficial.

Example: ELT in Action

Imagine a social media platform with user data scattered across various databases:

  • User profiles (names, locations, interests)
  • Activity logs (posts, comments, interactions)
  • Sentiment analysis data

Using ELT, the platform could:

  1. Extract: Utilize APIs or data connectors to retrieve data from each database.
  2. Load: Dump the extracted data directly into a data lake, a massive storage repository.
  3. Transform: Once in the data lake, the data is cleaned, standardized, and transformed based on specific analysis needs. This might involve filtering user data based on location, analyzing sentiment trends from activity logs, or combining user profiles with sentiment data for deeper insights.

By adopting ELT, the social media platform can quickly ingest vast amounts of data and perform analysis on a flexible and scalable platform, enabling it to understand user behavior, optimize content strategies, and personalize user experiences.

Benefits of Using ELT

Implementing ELT offers several advantages:

  • Faster Time to Insights: Data is readily available for analysis sooner due to the prioritized loading stage.
  • Scalability and Flexibility: Data lakes can handle massive data volumes and diverse formats with ease.
  • Cost-Effectiveness: Reduced upfront processing requirements can potentially lead to lower costs.
  • Agile Data Management: The flexible nature of ELT allows for adapting to evolving data needs.


ELT has emerged as a valuable data integration approach, particularly for organizations dealing with large, diverse datasets and requiring rapid data analysis. By prioritizing data loading and leveraging the flexibility of data lakes, ELT empowers organizations to unlock valuable insights from their data and make informed decisions. As data volumes and analytics needs continue to evolve, ELT is likely to play an increasingly significant role in the data management landscape.


Additional Reading

OK, that’s it, we are done now. If you have any questions or suggestions, please feel free to comment. I’ll come up with more Machine Learning and Data Engineering topics soon. Please also comment and subs if you like my work any suggestions are welcome and appreciated.

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