Re: Constant "JTable" update

Is there anything in the jdbc spec which supports this?

I have had a quick look, and haven't found anything.

I suppose this could be a postgres specific extension if it doesn't
exist in the database

Alternatively instead of getting all the rows to the database you could
do this another way


Create the table with a sequential id

Ie create table foo (id serial, ....)

Get the largest id  of the rows

while(1){
    newmax = 0;

    select max (id) from foo;

    if max > newmax, then you have more data

    select * from table where id > max

    newmax = max
}

This is just pseudo code, but it should be close

Dave
-----Original Message-----
From: pgsql-jdbc-owner@postgresql.org
[mailto:pgsql-jdbc-owner@postgresql.org] On Behalf Of Per-Olof Norén
Sent: November 9, 2001 8:05 AM
To: Jean-Christophe ARNU; pgsql-jdbc@postgresql.org
Subject: Re: [JDBC] Constant "JTable" update



----- Original Message -----
From: "Jean-Christophe ARNU" <arnu@paratronic.fr>
To: <pgsql-jdbc@postgresql.org>
Sent: Thursday, November 08, 2001 10:38 PM
Subject: [JDBC] Constant "JTable" update


> The straightforward solution seems to have database listeners on the
> table that wakes-up a notifier in the java program. But such kind of
> mechanism seems not to be implemented in the JDBC API (up to my small
> knowledge).

I haven´t seen such a mechanism, either :-)


> The second tortuous solutions (the one I use) is to query the database
> relatively often to get the freshest results. This is quite bandwidth
> consumming (assuming that some users should use a quite small
bandwidth
> connection). Using this kind of solution makes the Java application
> slow...
First of all, this is how I interpreted your config.
You do a executeQuery once the rendering of the chart is done for one
execution?
And you process the entire ResultSet everytime, even though no changes
are
made?

If this is the case, I would suggest a change in the following
direction:
1. Create a little status table containing just one column: create table
last_change (lastchange datetime). Also add one row to the table
2. Create a trigger on the measurer table, that updates the date of the
status table.
3. Design your algorithm something like this


check status by executing a select on status table.

if changed {
    store the date from status query
    execute data query
    render chart
}

This would reduce the bandwith by not sending the resultset when no
changes
are made.

By measuring the average change in time between , say the last five
updates
to the status table, you
could even put the rendering of the chart in its on thread and let it
sleep
a little shorter than the average time

Regards,
Per-Olof Norén




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