Re: A bad behavior under autocommit off mode

Tom Lane wrote:
> Barry Lind <blind@xythos.com> writes:
> 
>>In the previous email I discusses why autocommit perhaps shouldn't be a 
>>GUC parameter.  From a jdbc perspective what was said in that email 
>>generally applies to client_encoding and datestyle as well.
> 
> 
> Hmm.  I'm not sure that it makes sense for clients to want to change
> client_encoding on the fly, but it's harder to make that claim for
> datestyle.  And datestyle has been a SET variable for so long that we
> would get a *lot* of compatibility flak if we took it away.
> 

The issues I was raising are more theoretical than practical.  The only 
reason I was raising them, is that while we are looking at changes to 
the FE/BE protocol we should look at all the issues.  Good general 
purpose mechanisms (like what Tom is suggesting below) can be created 
when a better understanding of all the issues is known.

--Barry

> I had originally been thinking of reporting client_encoding as a field
> of some message sent only at backend startup.  However, what if we send
> such a message whenever one of the variables it includes changes?
> (This is basically Bruce's idea from last night, but refined to only
> send the data when it changes, rather than on every query; that should
> eliminate the bandwidth-wastage objection.)
> 
> I'm not convinced that it's worth the trouble to make the set of
> reported variables be configurable on-the-fly as Bruce was suggesting.
> client_encoding and datestyle are candidates to send this way, but are
> there really others?
> 
> A point that would have to be considered is what to do when a SET
> operation is rolled back by transaction abort.  I think what we'd need
> to do is retransmit each time the effective value changes; so it'd
> work like this:
> 
>     -- initial DateStyle is foo
>     BEGIN;
>     SET DateStyle TO bar;
>     -- backend sends status message showing DateStyle = bar
>     SELECT ...
>     -- reported dates will use DateStyle bar
>     ROLLBACK;
>     -- backend sends status message showing DateStyle = foo
> 
>             regards, tom lane
> 
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