Will agree with @Jasonnator - PostgreSQL has the same perf like a top most db system from IBM or MS.
But. The key advantages of "Big" databases - to provide flexible analytics and data mining features. Especially with big data. It is not enough just to make a fast system - all db has almost the same performance for primitive queries. The system should provide something more.
Good news - for trading it almost useless. No really. Most of all operations - select batch (for example, to do some backtesting) or updating toxic data. Or put the newest rowset. That is all!
In out app Hydra we started from DB (SQL Express was as underlying storage). But it shown us extremely poor performance in compare with file system. Maybe it will be faster with using some features like Clustering DB, but what the price for that?
Also one of the major questing - who will be use a trading app. If company - it is no any problem to maintain the db by special team. In case of one-man band" any human readable format like CSV can give odds to any DBMS - easy to read, supported by Excel, easy to load to any programming language, easy to modify (just open in Notepad and move forward).
Hope it will help.
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