Re: Transaction Snapshot Cloning

"Florian G. Pflug" <fgp@phlo.org> writes:
> I seem to remember there being some pushback to the idea of changing the 
> semantics of "set transaction isolation read only" from "soft" to "hard" 
> semantics though - on the basis that it might break existing 
> applications. If that has changed (or my memory tricks me ;-) ) I'd 
> volunteer to create a patch for 8.4 to make "set transaction read only" 
> a hard constraint.

AFAICT it would violate the SQL spec as well as breaking backward
compatibility.  SQL99 4.32 saith
        An SQL-transaction has an access mode that is either read-only        or read-write. The access mode may be
explicitlyset by a <set        transaction statement> before the start of an SQL-transaction or        by the use of a
<starttransaction statement> to start an SQL-        transaction; otherwise, it is implicitly set to the default access
      mode for the SQL-session before each SQL-transaction begins. If no        <set session characteristics statement>
hasset the default access        mode for the SQL-session, then the default access mode for the SQL-        session is
read-write.The term read-only applies only to viewed        tables and persistent base tables.
 

That last sentence is the basis for exempting temp tables from the
read-only restriction.

I'm not sure what the most convenient user API would be for an on-demand
hard-read-only mode, but we can't use SET TRANSACTION READ ONLY for it.
It'd have to be some other syntax.  Maybe just use a GUC variable
instead of bespoke syntax?  SET TRANSACTION is really just syntactic
sugar for GUC SET operations anyway ...
        regards, tom lane