Along with Alexandre and Markus who posted earlier, we have found Jonas will only let
us access one instance of any particular entity bean at a time. I haven't looked into
it yet, so I don't know what the issues are behind it.. But if this behavior is
normal, the select max() would be thread safe.
-Erik
At 04:15 PM 6/28/00 +1200, you wrote:
>
>Performance will depend on how the dbms has been implemented but generally speaking a
>select max would be very efficient as the primary key should be indexed (possibly
>clustered) so the max value can be obtained directly from the index without even
>referring to the table.
>
>However, I'm still not convinced this is a safe thing to do. What happens if you have
>two eb's being created at the same time, won't they both select the same max value,
>add one leaving us with two identical primary keys?
>
>>>> Ronald Perrella <[EMAIL PROTECTED]> 28/06/00 14:28:05 >>>
>Isn't that going to have lousy performance? after all, the Max() function has
>to do a table-scan to get that value?
>
>-Ron
>--- Santiago Gala <[EMAIL PROTECTED]> wrote:
>>
>>
>> Phil Windley wrote:
>>
>> > Its faster to write, to be sure, but seems like its going to establish a
>> > new connection for every creation event. If those events are a small
>> > percentage of overall operations, it wouldn't be a problem, but I'm
>> > envisioning a system that has large numbers of creations, so this would be
>> > pretty expensive.
>> >
>> > Anyone else have ideas on getting a serial type (preferably from
>> > Postgresql) to work with in CMP to automatically create the PK for the
>> > bean?
>> >
>>
>> What I'm doing is:
>>
>> Create a findGreatestId() method for each CMP bean. the where clause would be
>> something like:
>>
>> WHERE id = (SELECT max(id) FROM table)
>>
>> Call it inside the ejbCreate method to get the biggest used id, increment it
>> and
>> assign it before returning. Warning: it will raise and exception, that must
>> be
>> caught, if the table is empty. In this case, set id to 1.
>>
>> It should work well, it is completely portable and it uses all the connection
>> pooling and similar characteristics in Jonas.
>>
>> It is not very elegant, but we have seen no problems using it. If you require
>> permanent unique ids, and you are removing objects, it is not for you, as the
>> same
>> id can be reused if the last object is created and then removed.
>>
>> >
>> > --phil--
>> >
>> > On Fri, 23 Jun 2000 11:16:16 -0600 [EMAIL PROTECTED] writes
>> > +--------------------
>> > | I couldn't get the serial type to work with CMP either. We added a
>> function
>> > like the following in each of our entity beans to use automatically
>> generated i
>> > d's from postgres (mytable_id_seq is a SEQUENCE):
>> > |
>> > | private Integer _getNewID()
>> > | throws SQLException
>> > | {
>> > | Connection conn=_dsource_.getConnection();
>> > | String sql="select nextval('mytable_id_seq')";
>> > | Statement stmt=conn.createStatement();
>> > | ResultSet rs=stmt.executeQuery(sql);
>> > | if(!rs.next())
>> > | throw new SQLException("Unable to get next
>> sequence.");
>> > | Integer newid=new Integer(rs.getInt(1));
>> > | stmt.close();
>> > | conn.close();
>> > | return newid;
>> > | }
>> > |
>> > | I know this isn't an ideal solution, but it is faster than two beans..
>> > |
>> > | -Erik
>> > |
>> > |
>> > | At 09:55 AM 6/23/00 -0600, you wrote:
>> > | >
>> > | >I use Postgresql as well, but I couldn't figure out how to get container
>> > | >managed entity beans to automatically set the primary key from the
>> SEQUENCE
>> > | >data type. I ended up creating a session bean to grab sequence numbers
>> > | >from the database each time I created an entity bean.
>> > | >
>> > | >Is there a better way? I'm all for it---creating two beans every time I
>> nee
>> > d
>> > | >one is not my idea of a good way to spend time.
>> > | >
>> > | >--phil--
>> > | >
>> > | >On Thu, 15 Jun 2000 08:33:58 -0400 "John M. Jones" writes
>> > | >+--------------------
>> > | >| Does your database support serial fields? These fields generate a
>> counter
>> > | >| for each record added. There's a trick to getting the next id when
>> calling
>> > | >| create(), but it eliviates coding a unique id generator. I use
>> PostgreSQL,
>> > | >| and it has a serial data type.
>> > | >|
>> > | >| Hope this helps,
>> > | >| John
>> > | >|
>> > | >| -----Original Message-----
>> > | >| From: [EMAIL PROTECTED]
>> > | >| [mailto:[EMAIL PROTECTED]]On Behalf Of Blasius Lofi
>> > | >| Dewanto
>> > | >| Sent: Thursday, June 15, 2000 5:13 AM
>> > | >| To: [EMAIL PROTECTED]
>> > | >| Subject: Automatic counter
>> > | >|
>> > | >|
>> > | >| Hi all,
>> > | >|
>> > | >| I just wonder, whether someone can help me...
>> > | >|
>> > | >| How can I implement a counter or a unique id
>> > | >| for my PrimaryKey in EJB? Because I don't want
>> > | >| to take care about it in my EJB.
>> > | >|
>> > | >| Another question:
>> > | >| When I quit from the EJBServer through the JonasAdmin,
>> > | >| I always get this exception:
>> > | >| Cannot unbind Datasources: java.lang.ClassCastException
>> > | >| Is this normal?
>> > | >|
>> > | >| Thank's a lot!
>> > | >| --
>> > | >| ---------------------------------------------------
>> > | >| Blasius Lofi Dewanto
>> > | >| ---------------------------------------------------
>> > | >| OpenUSS - Open Source University Support System
>> > | >| http://openuss.sourceforge.net
>> > | >| ---------------------------------------------------
>> > | >|
>> > | >|
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