Submitted by: @pavel-zotov

Assigned to: Claudio Valderrama C. (robocop)

Tested on WI-T3.0.0.31844, result in WI-V2.5.5.26871 is similar (just change row_number() to create sequence + gen_id).

SQL> recreate table s(id int, x int);
SQL> commit;
SQL> insert into s(id, x) select row_number()over(), 2 * row_number()over() from rdb$types rows 5;
SQL> commit;
SQL> select * from s;

      ID            X

============ ============
1 2
2 4
3 6
4 8
5 10

SQL> recreate table t(id int primary key, x int);
SQL> commit;

SQL> set count on;
SQL> merge into t using s on http://s.id = http://t.id when not matched then insert values(http://s.id, s.x);
Records affected: 5
SQL> select * from t;

      ID            X

============ ============
1 2
2 4
3 6
4 8
5 10

Records affected: 5
SQL> merge into t using s on http://s.id = http://t.id when matched then update set t.x = - 3 * s.x;

Records affected: 0 -- <<<<<<<<<<<<<<< ??? <<<<<<<<<<<<

-- Verify that last merge really changed target table:
SQL> select * from t;

      ID            X

============ ============
1 -6
2 -12
3 -18
4 -24
5 -30

Records affected: 5

Commits: 845120f b87075f FirebirdSQL/fbt-repository@4da3f9a