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
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