Calvin Varney 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?
>
As there is a transaction covering the whole create call either the whole access
(selection+insert) is serialized or the second one should fail.
I have never experienced a problem with this scheme.
The main issues are if you need to insert from outside the application and if you need
that keys freed by objects removed are never reused.
>
> >>> 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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