Re: Question Regarding Locks

Tom,

thanks ! You are even helping lurkers like me that haven't
asked anything :-)

...
> A better design is to fetch the data without locking it, allow the
> user to edit as he sees fit, and then when he clicks "save" you do
> something like
>
> begin;
> select row for update;
> if [ row has not changed since you originally pulled it ] then
> update row with changed values;
> commit;
> else
> abort;
> notify user of conflicts
> let user edit new data to resolve conflicts and try again
> fi
>
> In this design the row lock is only held for milliseconds.
>
> You need to provide some code to let the user merge what he did with the
> prior changes, so that he doesn't have to start over from scratch in the
> failure case.

> BTW, a handy proxy for "row has not changed" is to see if its XMIN
> system column is still the same as before. If so, no transaction has
> committed an update to it. (This may or may not help much, since you're
> probably going to end up groveling over all the fields anyway in the
> "notify user" part, but it's a cool hack if you can use it.)

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