Re: CLUSTER and indisclustered

Gavin Sherry wrote:
> > Gavin, is that a big win compared to just using the index and looping
> > through the entries, knowing that the index matches are on the same
> > page, and the heap matches are on the same page.
> 
> Bruce,
> 
> It would cut out the index over head. Besides at (1) (above) we would have
> determined that an index scan was too expensive and we would be using a
> SeqScan instead. This would just be faster, since a) we would locate the
> tuples more intelligently b) we wouldn't need to scan the whole heap once
> we'd found all tuples matching the scan key.

Yes, but in a clustered table, an index scan is _never_ (?) more
expensive than a sequential scan, at least if the optimizer is working
correctly.  Index scans are slower only because they assume random heap
access, but with a clustered table, there is no random heap access.  The
index takes to right to the spot to start.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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