5 ms·
Count distinct performance compared on top 4 SQL databases
- dccoolgai 13y agoCrazy to see those numbers on old MSSQL...there's still a whole bunch of stuff they're behind the curve on (json support / array support), but its tough to argue with those benchmarks...
- interactive_rep 13y agoOne of the reasons you are seeing that the subquery runs twice as fast on most of the databases ( like SQL server, oracle & MySQL), is that it is able to execute the query with one pass through of the data. When it is taking twice as long (without the subquery), it is doing a second pass for the count distinct.
- interactive_rep 13y agoAnother reason postgres could be running very slow, is that it may be spooling the intermediate result to a temp file on hard disk. Do you see a performance increase when using SSD Drives?
- hglaser 13y agoWe didn't try SSD drives. It's a good idea, worth a shot for a future post.
- stock_toaster 13y agoYou ran this on RDS? (mentioned in blog post) I would be worried about external factors, nonstandard/non-default configs, and other such things impacting your tests.
- AlterEgo20 13y agoDid you use default postgres configuration? By default postgres uses very small amount of RAM and is forced to store hash maps on disk -> very significant slowdown.
- toong 13y agoSee related discussion https://news.ycombinator.com/item?id=7114310 https://news.ycombinator.com/item?id=7114310