use null or 0 in foreign key column, to mean "no value"?

Hi,

(Maybe my subject line should be: `is not distinct from` and indexes.)

In Postgres 9.4, I’ve got a table of ‘items’ that references a table ‘colors’. Not all items have colors, so I created a nullable column in items like:

color_id bigint references colors

There is also an index on color_id:

create index on items (color_id);

I thought this was the right way to do it, but now I’m not so sure... In application code, prepared statements want to say: `select * from items where color_id = ?` and that `?` might be a int or null, so that doesn’t work. I used `is not distinct from` instead of =, which has the right meaning, but now I notice it doesn’t use the index for queries that replace `=` with `is not distinct from`, and queries run much slower. Using `explain` confirms: it’s doing sequential scans where `=` was using index.

So… is this bad DB design to use null to mean that an item has no color? Should I instead put a special row in `colors`, maybe with id = 0, to represent the “no color” value? Or is there some way to make an index work with nulls and `is not distinct from`?

thank you,
Rob