How nested loop, hash, and merge joins work.
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- čas přidán 25. 04. 2024
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In this video, I delved into the essential concept of joins in databases, exploring the algorithms used for table joins. I explained the nested loop join, where each row from one table is matched with every row from the other. The merge join involves sorting tables on join attributes for efficient merging. Hash join uses hash functions to prepare and match rows based on the join attribute. SQL engines choose these algorithms based on table statistics. Understanding these join algorithms is key to optimizing database performance.
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Database internals are just amazing. Thanks for explaining.
The biggest take away is, this gives some better algorithms to use when you need to join data coming from two different services and then augument it with your own DB's data and present it. Manual joins as we call them.
Hash join is something that we also do in application level code when large data is to be processed. Good to know various others ways as well✌️
Bro I was thinking about it a few days back as how it works internally, thanks a lot
Thanks for explaining arpit 👍🏻
Excellent explanation, thanks so much.
This is good, thank you!
Summary of this Video:
Database joins are essential for relational databases, enabling transactional and analytical queries. Databases employ various algorithms to perform joins, including:
Nested Loop Join: Iterates through each row in the left table and matches it with every row in the right table, checking the join condition.
Merge Join: Sorts both tables on the join attribute and merges them, efficiently joining rows with matching join attributes.
Hash Join: Creates a hash table from one table using the join attribute as the key, then performs a hash lookup for each row in the other table to find matching rows.
The choice of algorithm depends on factors such as data size, distribution, and join type. Database query optimizers analyze statistics to determine the most efficient algorithm for each join operation.
This is so far best way to explain things everything by example and the beauty is making it extremely simple to understand is your your USP
Thanks!
I just discovered your channel today, the way you explain things is unique and i love it ❤
Thank you so much :)
Arpit, you are one of the most intelligent software engineer I have ever come accross in my life. Lots of respect.
Thank you Rushi!
I didn't know, the implementation of joins could be so simple.
amazing. Does anyone know if we can get these notes anywhere. Helps in recalling post watching
First of all, thanks for the information. So, i wanted to know, can we tell mysql or postgress to do this type of merge on these two tables evrytime or when i merge this on this specific column only, or i can force it using some kywords in my sql query. If yes, then please upload next part of this video as practical explaination with query running on mysql, as this was mostly theoretical.
next on subQuery vs joins please
One question: In merge join, does the SQL engine choose the type of sorting algorithm to implement as well or it is usually fixed?
Average Time complexity for HashJoin algorithm is O(n + k), where n=no. of users & k=no. of ids stored in one hash index
Please correct me if I’m wrong
Is the intial query that was shown in the start of the video correct? I believe we need to group by to get the count of all belongs from a user
something like
select u.id, count(*) b_count from blogs b inner join users u on b.user_id = u.id grouby u.id order by b_count desc
Agree. Makes author lose huge credibility when i saw that
yes it should be like this
What are the best resources to the internals of databases?
The book Database Internals by Alex Petrov is a great starting point. amzn.to/3V36Btw
Ji, y blog id is not sorted how to do that too while joining
because the join clause uses the user id, not really necessary in this context
but if you want the result sorted bu blog id you can just add an order by clause to your query
Does merge join also go through a nested loop? If so, how is it better than nested loop join?
Because of sorting, the join becomes an O(n) operation. Exactly how we merge two sorted arrays into one is O(n).
Shouldn't the query have group by
I'm a an average programmer and I want to develop my own programming language but I didn't know anything about programming languages other than the high level.
Could you please list down the things that I would need to know in order to develop my own programming language.
The best thing to do is to ask ChatGPT, it lists you everything needed.
you start a DBMS series
There are 25 database engineering videos on my channel. You can find the playlist on my channel
This looks like a leetcode question
No fluff as always.
Rancho of Computer Science😅😅
me first
hehehe
what happened to your eyes?