[grandinj]

You could add a TableEngine extension to H2 (h2database.com), pretty easily which would give you full SQL query functionality over such a file

[throwaway84742]

Nope. Protos have repeated fields and can be hierarchical (that is, can contain other protos) and even recursive (that is, contain themselves, possibly as repeated fields). H2 is not going to work.

[grandinj]

Yeah, a normal SQL model is not perfect because of the requirement that it look like a flat table i.e. fixed number of columns.

But H2 has ARRAY for repeated fields, and with some custom functions for decomposing other functions, you could get pretty far.

Just saying, not perfect, but could be useful without too much effort.