It also doesn't matter if the app is cross-platform. Most people use only a single platform, and it turns out that Macs are very popular among my target audience (web developers & data analysts).
It also doesn't need to support a lot of features. I focussed on features that are important for web developers and data analysts, and I made sure they work really well. Yeah, pgAdmin has more admin features, but my users don't care about that.
I discovered most of what I know of the market as I went along, by talking to customers. Nobody bought the early versions of my app; it took three years of iterating on customer feedback to get to 10k a month.
Talking to (potential) customers is really important. In the beginning, I thought that DB admins would be my target audience. But from talking to people at pgConf, I discovered that most PostgreSQL admins don't care about GUIs, but web developers do. So that's where I put my focus.
I think the most important choice I made in the beginning was to focus on PostgreSQL rather than MySQL. The latter might be 10x as popular, but PostgrSQL is gaining popularity and growing, and there's lots of interest in PostgreSQL -- perfect conditions for starting a business.