But in this case it doesn't even do equality comparison, it just returns the value.
That's the idea that I tried to express. The point is that we need to tell the user that there is no need to worry about it, rather than that they're wrong to ask about it. Though we should probably actually just throw an error.
Any operation over including columns is a deal for filter, not an index, so collation will not be used somewhere inside index.
2Alexander: ComputeIndexAttrs() may use whole vectors (for including columns too) just to simplify coding, in other places, seems, better to have exact size of vectors. Using full-sized vectors could mask a error, for exmaple, with setting opfamily to InvalidOid for key column. But if you insist on that, I'll modify attached patch to use full-sized vectors anywhere.
In attached path I did: 1) fix a bug in CompareIndexInfo() which checks opfamily for including column 2) correct size for all vectors 3) Add assertion in various places to be sure that we don't try to use including column as key column 4) per Peter Geoghegan idea add a error message if somebody adds options to include column instead silently ignore it.
I don't insist on using full-sized vectors everywhere. Attached patch looks OK for me. I haven't test it though.