Decode the Join: A Spark Data Engineer’s Visual Handbook
Understand when and why to use Broadcast, Shuffle, or Sort-Merge Joins in Spark— with clear visuals, real-world use cases, and strategy tips tailored for data engineers.
Ever stared at a Spark job and wondered which join strategy it picked—and why your cluster suddenly feels like it’s running through molasses? This visual handbook is here to help. Whether you're optimizing joins in production or just trying to wrap your head around what happens under the hood, this guide breaks down Broadcast, Shuffle, and Sort-Merge Joins using clear diagrams, code snippets, and real-world scenarios. Decode the logic, spot the trade-offs, and make smarter join decisions in your next big data pipeline.
A big thank you to
for the opportunity to contribute to this space. It’s always a pleasure to share insights with fellow data enthusiasts. If this visual guide helped demystify Spark joins for you, feel free to share your thoughts or questions in the comments—I’d love to hear from you! | A guest post by
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Thank you very much for the article. Very lucid explanation with detailed illustrations on such an advanced topic. Without those detailed illustrations, I would have found it difficult to visualize. Now, my understanding of it is crystal clear.
The timing of this article was perfect, as I was going to be doing some research of my own on the internet to have a clear understanding of the Spark Join strategies. Now I don't need. Your post saved me time and hassle in this regard. More importantly the clarity in my understanding.
However, I have a question and a clarification to ask about:
Question: For Broadcast Hash Join and Shuffle Hash Join, as I understand from this article, only INNER join is supported. Does that mean, OUTER join isn't supported? If that's the case, just curious to know as to why?
Also, I would like to seek clarification on the below:
Based on the "How it works" section of the article, I understand, Sort Merge join uses Sort phase followed by Merge phase, which is the main difference when compared to Shuffle Hash Join. However on Sort Merge join diagram itself, towards the middle of the diagram, under the "strategy" bullet point, I see a mention of the hash join? This is where I have a little bit of confusion. Are you suggesting that Sort Merge join still uses hash join too? Or, was it simply a case of copy-and-paste error from the Shuffle Hash Join diagram?
Could you please clarify?
Regards,
Jayasurya Pilli
Data Engineer