Re: Why MemoryContextSwitch in ExecRelCheck ?

On Mon, Jan 07, 2002 at 12:28:12PM -0500, Tom Lane wrote:
> Holger Krug <hkrug@rationalizer.com> writes:
> > Nevertheless in ExecRelCheck a context switch to per-query memory
> > context is made:
> > Is this a switch from per-query memory context to per-query memory
> > context, hence not necessary, or do I miss something ?
> 
> [ thinks ... ]  It might be unnecessary.  I'm not convinced that the
> per-query context would always be the active one when ExecRelCheck is
> called, however.  There are various per-tuple contexts that might be
> used as well.

I think, there aren't, but nevertheless, you're principles stated
below are convincing.
> MemoryContextSwitchTo() is cheap enough that I prefer to call it when
> there's any doubt, rather than build a routine that will fail silently
> if it's called in the wrong context.  There are two typical scenarios
> for routines that are building data structures that will outlive the
> routine's execution:
> 
> 1. Data structure is to be returned to the caller.  In this case the
> caller is responsible for identifying the context to allocate the data
> structure in, either explicitly or by passing it as the current context.
> 
> 2. Data structure is owned and managed by the routine, which must know
> which context it's supposed to live in. In these cases I think the
> routine ought always to explicitly switch to that context, not assume
> that it's being called in that context.

OK. I wondered, because this is not done for the trigger related
cache, but only for the check related cache. Now I understand, it's
work in progress. (I think, very good work, indeed, because the code
is astonishingly well readable.)


-- 
Holger Krug
hkrug@rationalizer.com