The Firebird 5.0 Language Reference | CREATE TRIGGER | Database Triggers | Execution Of Database Triggers And Exception Handling

Execution of Database Triggers and Exception Handling

CONNECT and DISCONNECT triggers are executed in a transaction created specifically for this purpose.This transaction uses the default isolation level, i.e. snapshot (concurrency), write and wait.If all goes well, the transaction is committed.Uncaught exceptions cause the transaction to roll back, and

for a CONNECT trigger, the connection is then broken and the exception is returned to the client
for a DISCONNECT trigger, exceptions are not reported.The connection is broken as intended

TRANSACTION triggers are executed within the transaction whose start, commit or rollback evokes them.The action taken after an uncaught exception depends on the event:

In a TRANSACTION START trigger, the exception is reported to the client and the transaction is rolled back
In a TRANSACTION COMMIT trigger, the exception is reported, the trigger’s actions so far are undone and the commit is cancelled
In a TRANSACTION ROLLBACK trigger, the exception is not reported and the transaction is rolled back as intended.

Traps

There is no direct way of knowing if a DISCONNECT or TRANSACTION ROLLBACK trigger caused an exception.It also follows that the connection to the database cannot happen if a CONNECT trigger causes an exception and a transaction cannot start if a TRANSACTION START trigger causes one, either.Both phenomena effectively lock you out of your database until you get in there with database triggers suppressed and fix the bad code.

Suppressing Database Triggers

Some Firebird command-line tools have been supplied with switches that an administrator can use to suppress the automatic firing of database triggers.So far, they are:

gbak -nodbtriggers
isql -nodbtriggers
nbackup -T

Two-phase Commit

In a two-phase commit scenario, TRANSACTION COMMIT triggers fire in the prepare phase, not at the commit.

Some Caveats

The use of the IN AUTONOMOUS TRANSACTION DO statement in the database event triggers related to transactions (TRANSACTION START, TRANSACTION ROLLBACK, TRANSACTION COMMIT) may cause the autonomous transaction to enter an infinite loop
The DISCONNECT and TRANSACTION ROLLBACK event triggers will not be executed when clients are disconnected via monitoring tables (DELETE FROM MON$ATTACHMENTS)

Examples of `CREATE TRIGGER` for “Database Triggers”

Creating a trigger for the event of connecting to the database that logs users logging into the system.The trigger is created as inactive.

CREATE TRIGGER tr_log_connect
INACTIVE ON CONNECT POSITION 0
AS
BEGIN
  INSERT INTO LOG_CONNECT (ID,
                           USERNAME,
                           ATIME)
  VALUES (NEXT VALUE FOR SEQ_LOG_CONNECT,
          CURRENT_USER,
          CURRENT_TIMESTAMP);
END

Creating a trigger for the event of connecting to the database that does not permit any users, except for SYSDBA, to log in during off hours.

CREATE EXCEPTION E_INCORRECT_WORKTIME 'The working day has not started yet.';

CREATE TRIGGER TR_LIMIT_WORKTIME ACTIVE
ON CONNECT POSITION 1
AS
BEGIN
  IF ((CURRENT_USER <> 'SYSDBA') AND
      NOT (CURRENT_TIME BETWEEN time '9:00' AND time '17:00')) THEN
    EXCEPTION E_INCORRECT_WORKTIME;
END

DDL Triggers

DDL triggers allow restrictions to be placed on users who attempt to create, alter or drop a DDL object.Their other purposes is to keep a metadata change log.

DDL triggers fire on specified metadata changes events in a specified phase.BEFORE triggers run before changes to system tables.AFTER triggers run after changes in system tables.

Important

The event type [BEFORE | AFTER] of a DDL trigger cannot be changed.

In a sense, DDL triggers are a sub-type of database triggers.

Who Can Create a DDL Trigger?

DDL triggers can be created by:

Administrators
Users with the ALTER DATABASE privilege

Suppressing DDL Triggers

A DDL trigger is a type of database trigger.See [fblangref50-ddl-trgr-dbtrigger-notrgr] how to suppress DDL — and database — triggers.

Examples of DDL Triggers

Here is how you might use a DDL trigger to enforce a consistent naming scheme, in this case, stored procedure names should begin with the prefix “SP_”:

set auto on;
create exception e_invalid_sp_name 'Invalid SP name (should start with SP_)';

set term !;

create trigger trig_ddl_sp before CREATE PROCEDURE
as
begin
  if (rdb$get_context('DDL_TRIGGER', 'OBJECT_NAME') not starting 'SP_') then
    exception e_invalid_sp_name;
end!

Test

create procedure sp_test
as
begin
end!

create procedure test
as
begin
end!

-- The last command raises this exception and procedure TEST is not created
-- Statement failed, SQLSTATE = 42000
-- exception 1
-- -E_INVALID_SP_NAME
-- -Invalid SP name (should start with SP_)
-- -At trigger 'TRIG_DDL_SP' line: 4, col: 5

set term ;!

Implement custom DDL security, in this case restricting the running of DDL commands to certain users:

create exception e_access_denied 'Access denied';

set term !;

create trigger trig_ddl before any ddl statement
as
begin
  if (current_user <> 'SUPER_USER') then
    exception e_access_denied;
end!

Test

create procedure sp_test
as
begin
end!

-- The last command raises this exception and procedure SP_TEST is not created
-- Statement failed, SQLSTATE = 42000
-- exception 1
-- -E_ACCESS_DENIED
-- -Access denied
-- -At trigger 'TRIG_DDL' line: 4, col: 5

set term ;!

Note	Firebird has privileges for executing DDL statements, so writing a DDL trigger for this should be a last resort, if the same effect cannot be achieved using privileges.

Use a trigger to log DDL actions and attempts:

create sequence ddl_seq;

create table ddl_log (
  id bigint not null primary key,
  moment timestamp not null,
  user_name varchar(63) not null,
  event_type varchar(25) not null,
  object_type varchar(25) not null,
  ddl_event varchar(25) not null,
  object_name varchar(63) not null,
  sql_text blob sub_type text not null,
  ok char(1) not null
);

set term !;

create trigger trig_ddl_log_before before any ddl statement
as
  declare id type of column ddl_log.id;
begin
  -- We do the changes in an AUTONOMOUS TRANSACTION, so if an exception happens
  -- and the command didn't run, the log will survive.
  in autonomous transaction do
  begin
    insert into ddl_log (id, moment, user_name, event_type, object_type,
                         ddl_event, object_name, sql_text, ok)
      values (next value for ddl_seq, current_timestamp, current_user,
              rdb$get_context('DDL_TRIGGER', 'EVENT_TYPE'),
              rdb$get_context('DDL_TRIGGER', 'OBJECT_TYPE'),
              rdb$get_context('DDL_TRIGGER', 'DDL_EVENT'),
              rdb$get_context('DDL_TRIGGER', 'OBJECT_NAME'),
              rdb$get_context('DDL_TRIGGER', 'SQL_TEXT'),
              'N')
      returning id into id;
    rdb$set_context('USER_SESSION', 'trig_ddl_log_id', id);
  end
end!

The above trigger will fire for this DDL command.It’s a good idea to use -nodbtriggers when working with them!

create trigger trig_ddl_log_after after any ddl statement
as
begin
  -- Here we need an AUTONOMOUS TRANSACTION because the original transaction
  -- will not see the record inserted on the BEFORE trigger autonomous
  -- transaction if user transaction is not READ COMMITTED.
  in autonomous transaction do
     update ddl_log set ok = 'Y'
     where id = rdb$get_context('USER_SESSION', 'trig_ddl_log_id');
end!

commit!

set term ;!

-- Delete the record about trig_ddl_log_after creation.
delete from ddl_log;
commit;

Test

-- This will be logged one time
-- (as T1 did not exist, RECREATE acts as CREATE) with OK = Y.
recreate table t1 (
  n1 integer,
  n2 integer
);

-- This will fail as T1 already exists, so OK will be N.
create table t1 (
  n1 integer,
  n2 integer
);

-- T2 does not exist. There will be no log.
drop table t2;

-- This will be logged twice
-- (as T1 exists, RECREATE acts as DROP and CREATE) with OK = Y.
recreate table t1 (
  n integer
);

commit;

select id, ddl_event, object_name, sql_text, ok
  from ddl_log order by id;

 ID DDL_EVENT                 OBJECT_NAME                      SQL_TEXT OK
=== ========================= ======================= ================= ======
  2 CREATE TABLE              T1                                   80:3 Y
====================================================
SQL_TEXT:
recreate table t1 (
    n1 integer,
    n2 integer
)
====================================================
  3 CREATE TABLE              T1                                   80:2 N
====================================================
SQL_TEXT:
create table t1 (
    n1 integer,
    n2 integer
)
====================================================
  4 DROP TABLE                T1                                   80:6 Y
====================================================
SQL_TEXT:
recreate table t1 (
    n integer
)
====================================================
  5 CREATE TABLE              T1                                   80:9 Y
====================================================
SQL_TEXT:
recreate table t1 (
    n integer
)
====================================================

`ALTER TRIGGER`

Alters a trigger

Available in

DSQL, ESQL

Syntax

ALTER TRIGGER trigname
  [ACTIVE | INACTIVE]
  [{BEFORE | AFTER} <mutation_list>]
  [POSITION number]
  [{<psql_trigger> | <external-module-body>}]

<psql_trigger> ::=
  [<sql_security>]
  [<psql-module-body>]

<sql_security> ::=
    SQL SECURITY {INVOKER | DEFINER}
  | DROP SQL SECURITY

!! See syntax of CREATE TRIGGER for further rules !!

The ALTER TRIGGER statement only allows certain changes to the header and body of a trigger.

Permitted Changes to Triggers

Status (ACTIVE | INACTIVE)
Phase (BEFORE | AFTER) (of DML triggers)
Events (of DML triggers)
Position in the firing order
Modifications to code in the trigger body

If an element is not specified, it remains unchanged.

Note	A DML trigger cannot be changed to a database or DDL trigger. It is not possible to change the event(s) or phase of a database or DDL trigger.

Note

Reminders

The BEFORE keyword directs that the trigger be executed before the associated event occurs;the AFTER keyword directs that it be executed after the event.

More than one DML event — INSERT, UPDATE, DELETE — can be covered in a single trigger.The events should be separated with the keyword OR.No event should be mentioned more than once.

The keyword POSITION allows an optional execution order (“firing order”) to be specified for a series of triggers that have the same phase and event as their target.The default position is 0.If no positions are specified, or if several triggers have a single position number, the triggers will be executed in the alphabetical order of their names.

Who Can Alter a Trigger?

DML triggers can be altered by:

Administrators
The owner of the table (or view)
Users with — for a table — the ALTER ANY TABLE, or — for a view — ALTER ANY VIEW privilege

Database and DDL triggers can be altered by:

Administrators
Users with the ALTER DATABASE privilege

Examples using ALTER TRIGGER

Deactivating the set_cust_no trigger (switching it to the inactive status).
```
ALTER TRIGGER set_cust_no INACTIVE;
```
Changing the firing order position of the set_cust_no trigger.
```
ALTER TRIGGER set_cust_no POSITION 14;
```
Switching the TR_CUST_LOG trigger to the inactive status and modifying the list of events.
```
ALTER TRIGGER TR_CUST_LOG
INACTIVE AFTER INSERT OR UPDATE;
```

Switching the tr_log_connect trigger to the active status, changing its position and body.

ALTER TRIGGER tr_log_connect
ACTIVE POSITION 1
AS
BEGIN
  INSERT INTO LOG_CONNECT (ID,
                           USERNAME,
                           ROLENAME,
                           ATIME)
  VALUES (NEXT VALUE FOR SEQ_LOG_CONNECT,
          CURRENT_USER,
          CURRENT_ROLE,
          CURRENT_TIMESTAMP);
END

`CREATE OR ALTER TRIGGER`

Creates a trigger if it doesn’t exist, or alters a trigger

Available in

DSQL

Syntax

CREATE OR ALTER TRIGGER trigname
  { <relation_trigger_legacy>
  | <relation_trigger_sql>
  | <database_trigger>
  | <ddl_trigger> }
  {<psql_trigger> | <external-module-body>}

!! See syntax of CREATE TRIGGER for further rules !!

The CREATE OR ALTER TRIGGER statement creates a new trigger if it does not exist;otherwise it alters and recompiles it with the privileges intact and dependencies unaffected.

Example of `CREATE OR ALTER TRIGGER`

Creating a new trigger if it does not exist or altering it if it does exist

CREATE OR ALTER TRIGGER set_cust_no
ACTIVE BEFORE INSERT ON customer POSITION 0
AS
BEGIN
  IF (NEW.cust_no IS NULL) THEN
    NEW.cust_no = GEN_ID(cust_no_gen, 1);
END

`DROP TRIGGER`

Drops a trigger

Available in

DSQL, ESQL

Syntax

DROP TRIGGER trigname

Table 1. `DROP TRIGGER` Statement Parameter
Parameter	Description
trigname	Trigger name

The DROP TRIGGER statement drops (deletes) an existing trigger.

Who Can Drop a Trigger?

DML triggers can be dropped by:

Administrators
The owner of the table (or view)
Users with — for a table — the ALTER ANY TABLE, or — for a view — ALTER ANY VIEW privilege

Database and DDL triggers can be dropped by:

Administrators
Users with the ALTER DATABASE privilege

Example of `DROP TRIGGER`

Deleting the set_cust_no trigger

DROP TRIGGER set_cust_no;

`RECREATE TRIGGER`

Drops a trigger if it exists, and creates a trigger

Available in

DSQL

Syntax

RECREATE TRIGGER trigname
  { <relation_trigger_legacy>
  | <relation_trigger_sql>
  | <database_trigger>
  | <ddl_trigger> }
  {<psql_trigger> | <external-module-body>}

!! See syntax of CREATE TRIGGER for further rules !!

The RECREATE TRIGGER statement creates a new trigger if no trigger with the specified name exists;otherwise the RECREATE TRIGGER statement tries to drop the existing trigger and create a new one.The operation will fail on COMMIT if the trigger is in use.

Warning

Be aware that dependency errors are not detected until the COMMIT phase of this operation.

Example of `RECREATE TRIGGER`

Creating or recreating the set_cust_no trigger.

RECREATE TRIGGER set_cust_no
ACTIVE BEFORE INSERT ON customer POSITION 0
AS
BEGIN
  IF (NEW.cust_no IS NULL) THEN
    NEW.cust_no = GEN_ID(cust_no_gen, 1);
END

Execution Of Database Triggers And Exception Handling

Execution of Database Triggers and Exception Handling

Traps

docnext count = 19

Suppressing Database Triggers

Two-phase Commit

Some Caveats

Examples of CREATE TRIGGER for “Database Triggers”

DDL Triggers

Who Can Create a DDL Trigger?

Suppressing DDL Triggers

Examples of DDL Triggers

ALTER TRIGGER

Permitted Changes to Triggers

Who Can Alter a Trigger?

Examples using ALTER TRIGGER

CREATE OR ALTER TRIGGER

Example of CREATE OR ALTER TRIGGER

DROP TRIGGER

Who Can Drop a Trigger?

Example of DROP TRIGGER

RECREATE TRIGGER

Example of RECREATE TRIGGER

Examples of `CREATE TRIGGER` for “Database Triggers”

`ALTER TRIGGER`

`CREATE OR ALTER TRIGGER`

Example of `CREATE OR ALTER TRIGGER`

`DROP TRIGGER`

Example of `DROP TRIGGER`

`RECREATE TRIGGER`

Example of `RECREATE TRIGGER`