Commented by: @pavel-zotov

Consider script from attach (" core-0188_sql_and_logs_in_FB_25_vs_30.7z " ): we have a trouble in WI-V2.5.6.26980.

Following is the source of trigger defined for updatable view, plus debug RDB$SET_CONTEXT-statements for displaying values after statement will finish:

create trigger v\_test\_bu for v\_test active before update position 0 as
  declare o int;
begin
  rdb$set\_context\('USER\_SESSION','trigger\_sees\_old\_id', <http://old.id>\);
  rdb$set\_context\('USER\_SESSION','trigger\_sees\_old\_of\_b1', old\.row\_from\_table\_b\);
  rdb$set\_context\('USER\_SESSION','trigger\_sees\_new\_id', <http://new.id>\);
  rdb$set\_context\('USER\_SESSION','trigger\_sees\_new\_of\_b', new\.row\_from\_table\_b\);
  if \(new\.row\_from\_table\_b = 1\) then
      begin
          rdb$set\_context\('USER\_SESSION','trigger\_sees\_old\_of\_b2', old\.row\_from\_table\_b\);
          if \(old\.row\_from\_table\_b = 0\) then
          begin
              rdb$set\_context\('USER\_SESSION','trigger\_sees\_old\_of\_b3', old\.row\_from\_table\_b\);
              execute statement \( 'insert into test\_b\(id\) values\(?\)'\) \(<http://new.id>\);
              execute statement \( 'delete from test\_a where id = ?' \) \(<http://new.id>\);
              rdb$set\_context\('USER\_SESSION','trigger\_DID\_its\_job', <http://new.id>\);
          end
      end
  else
      delete from test\_b
      where id = <http://old.id>;
end

===

If query to view returns:
ID ROW_FROM_TABLE_B
=== ================
1 0

-- then it means that tables test_a and test_b are:

select \* from test\_a;

Records affected: 0

select \* from test\_b;

      ID 

============
1

Records affected: 1

Further, when trigger "v_test_bu" fires before statement:

update v_test set row_from_table_b = 1 where id = 1;

-- it must get old.row_from_table_b = 0.

But one may see that in 2.5 value of old.row_from_table_b:
1) is WRONG when control fall inside this trigger but not yet processes old.row_from_table_b in conditional statement ("IF ...")
2) is changed inside this trigger in this statement: "if (old.row_from_table_b = 0) then" (!!)

This is debug info that will be produced when run script on 2.5:

CTX_NAME CTX_VAL
============================== ==========
trigger_DID_its_job 1
trigger_sees_new_id 1
trigger_sees_new_of_b 1
trigger_sees_old_id 1
trigger_sees_old_of_b1 1 <<< ???
trigger_sees_old_of_b2 1 <<< ???
trigger_sees_old_of_b3 0 <<< ?!?!?!

Compare with result in 3.0:

CTX_NAME CTX_VAL
============================== ==========
trigger_DID_its_job 1
trigger_sees_new_id 1
trigger_sees_new_of_b 1
trigger_sees_old_id 1
trigger_sees_old_of_b1 0 -- OK, here and after
trigger_sees_old_of_b2 0
trigger_sees_old_of_b3 0