Tag: reddit

Who logged to system from multiple countries in 2 hours?

Yesterday someone posted a set of queries for interviews, all centered on answering business-like questions from database.

Today this post is hidden behind some “subscribe to read more" thing, so I will not even link it, but one question there caught my eye.

Since I can't copy paste the text, I'll try to write what I remember:

Given table sessions, with columns: user_id, login_time, and country_id, list all cases where single account logged to the system from more than one country within 2 hour time frame.

The idea behind is that it would be a tool to find hacked account, based on idea that you generally can't change country within 2 hours. Which is somewhat true.

Solution in the blogpost suggested joining sessions table with itself, using some inequality condition. I think we can do better…

Continue reading Who logged to system from multiple countries in 2 hours?

Getting count of distinct elements, per group, in PostgreSQL.

So, couple of days ago, some guy, from Periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries.

This was then submitted to Hacker News and r/Programming on Reddit.

Then, the original authors submitted second blogpost comparing speed between four different DB engines. Which, in turn, was also commented on Reddit.

I found the numbers presented by Periscope (as their improvement) as not that great.

Unfortunately – their blog doesn't allow for comments, so I decided to test it, and write on my own blog, what I can find about it.

Continue reading Getting count of distinct elements, per group, in PostgreSQL.