Here is a rebased version of this patch.
On 27.08.21 14:38, Peter Eisentraut wrote:
> The SQL standard has been ambiguous about whether null values in
> unique constraints should be considered equal or not. Different
> implementations have different behaviors. In the SQL:202x draft, this
> has been formalized by making this implementation-defined and adding
> an option on unique constraint definitions UNIQUE [ NULLS [NOT]
> DISTINCT ] to choose a behavior explicitly.
>
> This patch adds this option to PostgreSQL. The default behavior
> remains UNIQUE NULLS DISTINCT. Making this happen in the btree code
> is apparently pretty easy; most of the patch is just to carry the flag
> around to all the places that need it.
>
> The CREATE UNIQUE INDEX syntax extension is not from the standard,
> it's my own invention.
>
> (I named all the internal flags, catalog columns, etc. in the
> negative ("nulls not distinct") so that the default PostgreSQL
> behavior is the default if the flag is false. But perhaps the double
> negatives make some code harder to read.)