[fauigerzigerk]

>The explain plan for a given sql query can change on you for no reason other than your table has grown

True, but that may well lead to more predictable performance. If you write a procedural program and then the dataset grows, your programm might slow down dramatically while the new execution plan generated by the SQL engine would adapt and keep performing well (...ideally).

You could argue that a procedural programmer could anticipate the growth of the dataset and choose a suitable algorithm from the start. But if you have not one but tens of datasets and value ranges whose size relative to each other determines what the best algorithm is then manual optimization becomes very difficult indeed.

A Haskell compiler doesn't use knowledge of input data to optimize the program though (at least not to the extent a database engine does). So I think a relational database engine is more useful in this regard.