indexes_equal() fails if the column comments do not match #45

As suggested by the code comment in indexes_equal(), the string comparison isn't quite enough. If one column being compared has a comment and the other one doesn't (or the comments are different), the comparison will fail:

This might be quite hard to fix properly. You would need to compare the list of expressions on each index. Or you could argue that the indexes should not be considered equal if they have different comments.

See ucam-department-of-psychiatry/crate#243

The workaround is to ensure that the columns referenced by the indexes have identical comments.