Pavan Deolasee wrote:
> On Wed, Jan 25, 2017 at 10:06 PM, Alvaro Herrera <alvherre@2ndquadrant.com>
> wrote:
> > > +( \
> > > + ((tup)->t_infomask2 & HEAP_LATEST_TUPLE) != 0 \
> > > +)
> > >
> > > +#define HeapTupleHeaderGetRootOffset(tup) \
> > > +( \
> > > + AssertMacro(((tup)->t_infomask2 & HEAP_LATEST_TUPLE) != 0), \
> > > + ItemPointerGetOffsetNumber(&(tup)->t_ctid) \
> > > +)
> >
> > Interesting stuff; it took me a bit to see why these macros are this
> > way. I propose the following wording which I think is clearer:
> >
> > Return whether the tuple has a cached root offset. We don't use
> > HeapTupleHeaderIsHeapLatest because that one also considers the slow
> > case of scanning the whole block.
>
> Umm, not scanning the whole block, but HeapTupleHeaderIsHeapLatest compares
> t_ctid with the passed in TID and returns true if those matches. To know if
> root lp is cached, we only rely on the HEAP_LATEST_TUPLE flag. Though if
> the flag is set, then it implies latest tuple too.
Well, I'm just trying to fix the problem that when I saw that macro, I
thought "why is this checking the bitmask directly instead of using the
existing IsHeapLatest macro?" when I saw the code. It turned out that
IsHeapLatest is not just simply comparing the bitmask, but it also does
more expensive processing which is unwanted in this case. I think the
comment to this macro should explain why the other macro cannot be used.
> > Please flag the macros that have multiple evaluation hazards -- there
> > are a few of them.
>
> Can you please tell me an example? I must be missing something.
Any macro that uses an argument more than once is subject to multiple
evaluations of that argument; for example, if you pass a function call to
the macro as one of the parameters, the function is called multiple
times. In many cases this is not a problem because the argument is
always a constant, but sometimes it does become a problem.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
