Server version:
WI-V2.5.1.26353 Firebird 2.5
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SQL> create database "testz.fdb" pagesize 2048;
SQL> commit;
SQL> connect testz.fdb;
Database: testz.fdb
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-- SESSION #1:
recreate table t(id int not null, f01 int, constraint t_pk primary key(id), constraint t_unq unique(f01));
commit;
insert into t values(1, 1 );
insert into t values(2,null);
insert into t values(3,null);
commit;
update t set f01=null where id=1; -- <<<<<<<<<<<<< SET UNIQUE FIELD TO NULL <<<<<<<

-- SESSION #2:
commit; set transaction read committed record_version no wait; update t set f01=1 where id=3;

-- SESSION #1:
ROLLBACK;

-- SESSION #2:
COMMIT; -- PASSED! NO MESSAGES ABOUT VIOLATION OF UNIQUE INDEX!

-- SESSION #1:
SQL> select * from t;

          ID F01
============ ============
           1 1
           2 <null>
           3 1

-- SESSION #2:
SQL> select * from t;

          ID F01
============ ============
           1 1
           2 <null>
           3 1