[tsimionescu]

It's not that easy to analyze a binary logic predicate to extract a set of possible values (especially given the three-value logic that SQL uses, where `a=x or a≠x` is not true). It's easy in the easy cases of course, but doing the analysis for a more complex expression quickly gets ugly. And I'm not sure it's easy to tell ahead of time if an expression will be easy or hard to analyze. They could hard-code a few simple patterns, but at that point it may get very unpredictable whether the optimization will be applied or not, defeating the purpose.

[mike_hock]

Every optimizing compiler does this kind of analysis and then some.

It's already completely unpredictable which kind of trivial optimization Postgres fails to apply, it could hardly get any worse.