last update time of a table

On Friday 05 December 2003 01:21, pg wrote:

Is there any simple way to query the most recent time of "changes" made to
a table?

I'm accessing my database with ODBC to a remote site thru internet. I want
to eliminate some DUPLICATE long queries by evaluating whether the data has
been
changed since last query. What should I do?

The canonical way is to add a last_changed column and a trigger to make sure
it gets updated whenever the rest of the row is.

Go over to http://techdocs.postgresql.org/ and check in the plpgsql cookbook
or my Postgresql notes, or the archives come to think of it.

--
Richard Huxton
Archonet Ltd

I read thru your info, thanks a lot.

In fact, I only need to decide whether a table (the whole) has been updated
since last query. I have some pulldown menus in a VB app which extract data
from a remote site with slow connection. And the data in those tables for
pulldowns changes rarely. So if the pulldown has to extract the data and
transmit it thru slow connection, the pulldown will take a few seconds to be
in action, which is a little bit annoying, especially if the data is the
same as in the array of client. So if I can query the table, knowing that no
data changed in the table since my last query, I can use the client side
array as pulldown data without waiting for long transmition time.

I wonder if there is some more direct method, or thru the pg system tables
to get this info. If there's not out there, I would use a trigger which will
update a seperate table containing the last update time of all tables (not
records) for pulldowns.

-Jason

----- Original Message -----
From: "Richard Huxton" <dev@archonet.com>
To: "pg" <pg@newhonest.com>; <pgsql-general@postgresql.org>
Sent: Friday, December 05, 2003 5:53 PM
Subject: Re: [GENERAL] last update time of a table

On Friday 05 December 2003 01:21, pg wrote:

Is there any simple way to query the most recent time of "changes" made

to

a table?

I'm accessing my database with ODBC to a remote site thru internet. I

want

to eliminate some DUPLICATE long queries by evaluating whether the data

has

been
changed since last query. What should I do?

The canonical way is to add a last_changed column and a trigger to make

sure

it gets updated whenever the rest of the row is.

Go over to http://techdocs.postgresql.org/ and check in the plpgsql

cookbook

pg wrote:

In fact, I only need to decide whether a table (the whole) has been updated
since last query. I have some pulldown menus in a VB app which extract data
from a remote site with slow connection. And the data in those tables for
pulldowns changes rarely. So if the pulldown has to extract the data and
transmit it thru slow connection, the pulldown will take a few seconds to be
in action, which is a little bit annoying, especially if the data is the
same as in the array of client. So if I can query the table, knowing that no
data changed in the table since my last query, I can use the client side
array as pulldown data without waiting for long transmition time.

I wonder if there is some more direct method, or thru the pg system tables
to get this info. If there's not out there, I would use a trigger which will
update a seperate table containing the last update time of all tables (not
records) for pulldowns.

You can use LISTEN/NOTIFY to do what you want:

http://www.postgresql.org/docs/current/static/sql-listen.html

-Jason

Mike Mascari
mascarm@mascari.com

In fact, I only need to decide whether a table (the whole) has been updated
since last query. I have some pulldown menus in a VB app which extract data
from a remote site with slow connection. And the data in those tables for
pulldowns changes rarely. So if the pulldown has to extract the data and
transmit it thru slow connection, the pulldown will take a few seconds to be
in action, which is a little bit annoying, especially if the data is the
same as in the array of client. So if I can query the table, knowing that no
data changed in the table since my last query, I can use the client side
array as pulldown data without waiting for long transmition time.

I wonder if there is some more direct method, or thru the pg system tables
to get this info. If there's not out there, I would use a trigger which will
update a seperate table containing the last update time of all tables (not
records) for pulldowns.

You can use NOTIFY/LISTEN with triggers on
update/delete/insert.

Karsten
--
GPG key ID E4071346 @ wwwkeys.pgp.net
E167 67FD A291 2BEA 73BD 4537 78B9 A9F9 E407 1346

pg wrote:

I have some pulldown menus in a VB app which extract data
from a remote site with slow connection. And the data in those tables for
pulldowns changes rarely. So if the pulldown has to extract the data and
transmit it thru slow connection, the pulldown will take a few seconds to

be

in action, which is a little bit annoying, especially if the data is the
same as in the array of client.

Probably you'll do it better storing a local cached copy of the pulldown
data (in the VB side) and having a background process to refresh it only
when the server notifies that it has changed.

hth
cl.

Thanks for your suggestion. I think this is exactly Karsten and Mike said :

create triggers to notify the client for any kind of updates.

I'm not sure whether VB can handle such events, so I have to further study
on this.

-Jason

----- Original Message -----
From: "Claudio Lapidus" <clapidus@hotmail.com>
To: "pg" <pg@newhonest.com>; <pgsql-general@postgresql.org>
Sent: Sunday, December 07, 2003 11:09 PM
Subject: Re: [GENERAL] last update time of a table

pg wrote:

I have some pulldown menus in a VB app which extract data
from a remote site with slow connection. And the data in those tables

for

pulldowns changes rarely. So if the pulldown has to extract the data and
transmit it thru slow connection, the pulldown will take a few seconds

to