db-derby-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Gabriele Kahlout <gabri...@mysimpatico.com>
Subject Re: how to raise/throw an exception
Date Fri, 26 Feb 2010 18:50:31 GMT
Because of the trigger + procedure the update statement on the
affected table throws (not otherwise):

java.sql.SQLException: The exception 'java.sql.SQLException: A lock
could not be obtained within the time requested' was thrown while
evaluating an expression.

I tried to make it sleep for 250 nanos. But no avail.

2010/2/26, Rick Hillegas <Richard.Hillegas@sun.com>:
> Hi Gabriele,
>
> Some comments inline...
>
> Gabriele Kahlout wrote:
>> Perhaps last question: how to pass as parameter also the table?
>>
> Here's a revised version of the procedure which takes the table and
> column names as parameters:
>
>     public static void vetRow( String tableName, String columnName, int
> columnValue ) throws SQLException
>     {
>         Connection conn = DriverManager.getConnection(
> "jdbc:default:connection" );
>         String query = "select count(*) from " + tableName +" where " +
> columnName + " = ?";
>         System.out.println( query );
>         PreparedStatement ps = conn.prepareStatement( query );
>         ps.setInt( 1, columnValue );
>         ResultSet rs = ps.executeQuery();
>         rs.next();
>         int result = rs.getInt( 1 );
>
>         rs.close();
>         ps.close();
>
>         if ( result >= 4 ) { throw new SQLException( "Too many copies of
> some value." ); }
>     }
> }
>
> and the corresponding script to exercise this procedure:
>
> connect 'jdbc:derby:memory:dummy;create=true';
>
> create table t( a int );
>
> create procedure vetRow( in tableName varchar( 128 ), in columnName
> varchar( 128 ), in arg int )
> language java parameter style java reads sql data
> external name 'z.vetRow';
>
> create trigger t_trig
> no cascade before insert on t
> referencing new as newRow
> for each row call vetRow( 'T', 'A', newRow.a );
>
> insert into t( a ) values ( 1 );
> insert into t( a ) values ( 1 );
> insert into t( a ) values ( 1 );
> insert into t( a ) values ( 1 );
>
> -- fails
> insert into t( a ) values ( 1 );
>
>> Also, where can I look to understand why int a would be translated as
>> the column a?
>> This doesn't help:
>> http://developers.sun.com/docs/javadb/10.5.3.0/ref/index.html
>>
> You will want to read the syntax descriptions for the CREATE PROCEDURE
> and CREATE TRIGGER commands in that Reference Guide.
>
> Hope this helps,
> -Rick
>
>
>> 2010/2/26, Rick Hillegas <Richard.Hillegas@sun.com>:
>>
>>> Hi Gabriele,
>>>
>>> Here is a vetting procedure which the trigger could call:
>>>
>>>     public static void vetRow( int a ) throws SQLException
>>>     {
>>>         Connection conn = DriverManager.getConnection(
>>> "jdbc:default:connection" );
>>>         PreparedStatement ps = conn.prepareStatement( "select count(*)
>>> from t where a = ?" );
>>>         ps.setInt( 1, a );
>>>         ResultSet rs = ps.executeQuery();
>>>         rs.next();
>>>         int result = rs.getInt( 1 );
>>>
>>>         rs.close();
>>>         ps.close();
>>>
>>>         if ( result >= 4 ) { throw new SQLException( "Too many copies of
>>> value " + a + " in t." ); }
>>>     }
>>>
>>> And here again is the sql script which exercises the trigger. I had to
>>> modify the trigger declaration slightly: instead of saying that it
>>> "contains sql" this version says "reads sql data":
>>>
>>> connect 'jdbc:derby:memory:dummy;create=true';
>>>
>>> create table t( a int );
>>>
>>> create procedure vetRow( in arg int )
>>> language java parameter style java reads sql data
>>> external name 'z.vetRow';
>>>
>>> create trigger t_trig
>>> no cascade before insert on t
>>> referencing new as newRow
>>> for each row call vetRow( newRow.a );
>>>
>>> insert into t( a ) values ( 1 );
>>> insert into t( a ) values ( 1 );
>>> insert into t( a ) values ( 1 );
>>> insert into t( a ) values ( 1 );
>>>
>>> -- fails
>>> insert into t( a ) values ( 1 );
>>>
>>> Depending on your usage pattern, it might make more sense to declare an
>>> AFTER trigger which invokes a vetting procedure once per INSERT/UPDATE
>>> rather than per row.
>>>
>>> Hope this helps,
>>> -Rick
>>>
>>> Gabriele Kahlout wrote:
>>>
>>>> The procedure would run the sanity check query and raise an exception
>>>> if the sanity check failed.
>>>> Can you show me an example of how this exception could be thrown?
>>>>
>>>> About the user-coded function, how may I code what I asked for? A loop
>>>> that counts for each distinct value it's occurences, and returns
>>>> true/false if some count is greater than 5.
>>>>
>>>> 2010/2/26, Rick Hillegas <Richard.Hillegas@sun.com>:
>>>>
>>>>
>>>>> Hi Gabriele,
>>>>>
>>>>> Here's an example of how Derby triggers invoke database procedures. If
>>>>> this is still not clear, please keep asking questions. We will get to
>>>>> the bottom of this:
>>>>>
>>>>> connect 'jdbc:derby:memory:dummy;create=true';
>>>>>
>>>>> create table t( a int );
>>>>>
>>>>> create procedure vetRow( in arg int )
>>>>> language java parameter style java contains sql
>>>>> external name 'z.vetRow';
>>>>>
>>>>> create trigger t_trig
>>>>> no cascade before insert on t
>>>>> referencing new as newRow
>>>>> for each row call vetRow( newRow.a );
>>>>>
>>>>> insert into t( a ) values ( 1 );
>>>>>
>>>>> Another comment inline...
>>>>>
>>>>> Gabriele Kahlout wrote:
>>>>>
>>>>>
>>>>>> That's what i do in SQLite. The question is how to do it in java
DB
>>>>>> sql
>>>>>> syntax.
>>>>>> In sqlite it is SELECT( RAISE, 'error) WHERE ...
>>>>>> in java db??
>>>>>>
>>>>>> Also, I was wondering if I could find a way to implement the
>>>>>> constraint as a column constraint. Unfortunately the details , are
far
>>>>>> from complete.
>>>>>> http://developers.sun.com/docs/javadb/10.5.3.0/ref/index.html
>>>>>>
>>>>>>
>>>>>>
>>>>> I don't think so. The only kind of constraint you could use here would
>>>>> be a CHECK constraint which invokes a user-coded function. The function
>>>>> would not be allowed to issue SQL.
>>>>>
>>>>> Hope this helps,
>>>>> -Rick
>>>>>
>>>>>
>>>>>> Basically I'm trying to check that no more than 4 rows have the same
>>>>>> value for the column. In SQL that would be the count -group by query,
>>>>>> as mentioned below, but since that counts as a not allowed subquery,
>>>>>> is there some other way in java db to achieve exactly that?
>>>>>>
>>>>>> 2010/2/26, Rick Hillegas <Richard.Hillegas@sun.com>:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Hi Gabriele,
>>>>>>>
>>>>>>> One way to solve this problem would be to have your trigger call
a
>>>>>>> database procedure, passing in whatever parameters you need from
the
>>>>>>> triggering row. The procedure would run the sanity check query
and
>>>>>>> raise
>>>>>>> an exception if the sanity check failed.
>>>>>>>
>>>>>>> Hope this helps,
>>>>>>> -Rick
>>>>>>>
>>>>>>> Gabriele Kahlout wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>> Hello,
>>>>>>>>
>>>>>>>> I'm porting my application from SQLite to Java DB, but don't
know
>>>>>>>> how
>>>>>>>> to enforce an assertion, achieved in SQLite by:
>>>>>>>>
>>>>>>>> st.execute("CREATE TRIGGER _fk_meanings_update_1 BEFORE UPDATE
ON "
>>>>>>>> +
>>>>>>>> batTable.getName()  + " FOR EACH ROW BEGIN SELECT RAISE(FAIL,
>>>>>>>> 'error')
>>>>>>>> WHERE (EXISTS (SELECT COUNT(*) FROM " + batTable.getName()
+ " GROUP
>>>>>>>> BY " + lastPubColumn.getName() + " HAVING COUNT(*)>"+maxPub+"));
>>>>>>>> END;");
>>>>>>>>
>>>>>>>> However the constraint couldn't be added as a table constraint,
and
>>>>>>>> I'm not finding how to raise an exception with Java DB, although
>>>>>>>> it's
>>>>>>>> described here:
>>>>>>>>
>>>>>>>> http://developers.sun.com/docs/javadb/10.5.3.0/devguide/index.html
>>>>>>>>
>>>>>>>> One way was to set the identity column to null, but that
was not
>>>>>>>> allowed at compile time too. I guess I'd have to use some
dirty way.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>
>>>>
>>>>
>>>
>>
>>
>>
>
>


-- 
Regards,
K. Gabriele

--- unchanged since 25/1/10 ---
P.S. Unless a notification (LON), please reply either with an answer
OR with " ACK" appended to this subject within 48 hours. Otherwise, I
might resend.
In(LON, this) ∨ In(48h, TimeNow) ∨ ∃x. In(x, MyInbox) ∧ IsAnswerTo(x,
this) ∨ (In(subject(this), subject(x)) ∧ In(ACK, subject(x)) ∧
¬IsAnswerTo(x,this)) ⇒ ¬IResend(this).

Also note that correspondence may be received only from specified a
priori senders, or if the subject of this email ends with a code, eg.
-LICHT01X, then also from senders whose reply contains it.
∀x. In(x, MyInbox) ⇒ In(senderAddress(x), MySafeSenderList) ∨ (∃y.
In(y, subject(this) ) ∧ In(y,x) ∧ isCodeLike(y, -LICHT01X) ).

Mime
View raw message