think that both sides of indexqual look like index keys.  An example is
create table inside (f1 float8 primary key);
create table outside (g1 float8, g2 float8);
select * from inside,outside where f1 = atan2(g1+1, g2);
ERROR:  ExecInitIndexScan: both left and right ops are rel-vars
(note that failure is potentially platform-dependent).  Solution is a
cleanup I had had in mind to make anyway: functional index keys should
be represented as Var nodes in the fixed indexqual, just like regular
index keys.