Subqueries And Performance

Grant Fritchey busts a myth:

I’ve written before about the concept of cargo cult data professionals. They see one issue, one time, and consequently extrapolate that to all issues, all the time. It’s the best explanation I have for why someone would suggest that a sub-query is flat out wrong and will hurt performance.

Let me put a caveat up front (which I will reiterate in the conclusion, just so we’re clear), there’s nothing magically good about sub-queries just like there is nothing magically evil about sub-queries. You can absolutely write a sub-query that performs horribly, does horrible things, runs badly, and therefore absolutely screws up your system. Just as you can with any kind of query. I am addressing the bad advice that a sub-query is to be avoided because they will inherently lead to poor performance.

There are times not to use subqueries, but this post is absolutely correct:  understand the reasons why things may or may not perform well, and don’t be afraid to try things out.

Related Posts

All Execution Plans Are Estimates

Grant Fritchey drops a bomb on us: All these resources, yet, for any given query, all the plans will be identical (assuming no recompile at work). Why? Because they’re all the same plan. Each and every one of them is an estimated plan. Only an estimated plan. This is why the estimated costs stay the […]

Read More

The Optimal Kafka Message Size

Guy Shilo wants to figure out the right chunk size for a Kafka message: I wrote a python program that runs a producer and a consumer for 30 minutes with different message sizes and measures how many messages per second it can deliver, or the Kafka cluster throughput. I did not care about the message […]

Read More


October 2016
« Sep Nov »