Hello.
Indexes may be shared between constraints.
In the SQL Standard referential constraints may reference only columns with
already existing unique constraint. It isn't ensured by released versions
of H2, they allow referential constraints without unique constraints and
reuse their indexes.
Current H2 doesn't require it too, but creates a unique constraint
automatically if it doesn't exists. Attempt to drop the unique constraint
is disallowed by default and allowed with CASCADE clause and with such
clause the referential constraint is also dropped.
CREATE TABLE T1(ID INT, CONSTRAINT PK1 PRIMARY KEY(ID));
> ok
CREATE TABLE T2(ID INT, F INT,
CONSTRAINT PK2 PRIMARY KEY(ID),
CONSTRAINT FK2 FOREIGN KEY(F) REFERENCES T1);
> ok
ALTER TABLE T2 DROP CONSTRAINT PK1;
> Constraint "PUBLIC.PK1" is used by constraint "PUBLIC.FK2"
ALTER TABLE T2 DROP CONSTRAINT PK1 CASCADE;
> ok
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