Thank you: that's exactly what I needed.
> I think you want to use a full outer join with slightly unusual
> bracketing:
>
> SELECT t.id, COALESCE(a.t,b.t) AS t, a.a, b.b
> FROM tabid t LEFT JOIN (
> taba a FULL OUTER JOIN tabb b ON (a.id,a.t) = (b.id,b.t))
> ON t.id = COALESCE(a.id,b.id);
>
> This will start by doing the inner most thing first, join taba to tabb
> matching rows where the id and timestamp is the same, then go on to
> match these to tabid. The COALESCEs are needed because when either side
> is missing their values will be NULL and hence we need to look at both.
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