That's why I'd second the Interbase recommendation
I don't think you need to split your research time any further. IMO your free options should be Postgres vs Interbase. MySQL has too many issues like this; cf. wharris' mention of accuracy (below) and the locking issues (although your description of essentially read-only ameliorates that one somewhat). But these are symptomatic of the many design trade-offs that were made all in the name of speed and small footprint. Comparing Oracle and MySQL is, IMO, an apples-and-oranges comparison. IB, on the other hand, is much closer to Oracle functionality and design.
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A stupid despot may constrain his slaves with iron chains; but a true politician binds them even more strongly by the chain of their own ideas;...despair and time eat away the bonds of iron and steel, but they are powerless against the habitual union of ideas, they can only tighten it still more; and on the soft fibres of the brain is founded the unshakable base of the soundest of Empires."
Jacques Servan, 1767