
> > > > > > > Can someone tell me what we use indislossy for?
> > > >
> > > > Ok, so the interpretation of this field is:
> > > > 	A match in the index needs to be reevaluated in the heap tuple data,
> > > > 	since a match in the index does not necessarily mean, that the heap tuple
> > > > 	matches.
> > > > 	If the heap tuple data matches, the index must always match.
> > 
> > AFAIK, this is true for all indexes in PostgreSQL, because index rows
> > don't store the transactions status.  Of course those are two different
> > underlying reasons why a heap lookup is always necessary, but there
> > shouldn't be any functional difference in the current implementation.
> 
> Seems it is something they added for the index abstraction and not for
> practical use by PostgreSQL.

Why, you do not need to call the comparison function on the heap data
if the index is not lossy, saves some CPU cycles.

Andreas

Tue, 10 Jul 2001 17:06:12 +0200
Zeugswetter Andreas SB  <ZeugswetterA@wien.spardat.at>
AW: AW: pg_index.indislossy
> 
> > > > > > > > Can someone tell me what we use indislossy for?
> > > > >
> > > > > Ok, so the interpretation of this field is:
> > > > > 	A match in the index needs to be reevaluated in the heap tuple data,
> > > > > 	since a match in the index does not necessarily mean, that the heap tuple
> > > > > 	matches.
> > > > > 	If the heap tuple data matches, the index must always match.
> > > 
> > > AFAIK, this is true for all indexes in PostgreSQL, because index rows
> > > don't store the transactions status.  Of course those are two different
> > > underlying reasons why a heap lookup is always necessary, but there
> > > shouldn't be any functional difference in the current implementation.
> > 
> > Seems it is something they added for the index abstraction and not for
> > practical use by PostgreSQL.
> 
> Why, you do not need to call the comparison function on the heap data
> if the index is not lossy, saves some CPU cycles.

Because we don't know of the tuples expired status until we check the
heap.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
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  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

Tue, 10 Jul 2001 13:04:16 -0400 (EDT)
Bruce Momjian <pgman@candle.pha.pa.us>
Re: AW: AW: pg_index.indislossy