disallowing multiple NULLs in a unique constraint - Mailing list pgsql-general

SQL Server only allow one NULL in a unique constraint column (it's the
unique index that does that, so the unique constraint behaves like that
too). The question is, what is the best way to simulate that behaviour
in Postgres? Can a CHECK constraint does that? Will a trigger with
SELECT count(*) ... WHERE f IS NULL be too slow if the table is large?

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dave