Re: [HACKERS] TODO list updated

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > We currently do not use indexes to handle ORDER BY because it is slower,
> 
> Er, actually, we *do* use indexes for ORDER BY currently:
> 
> regression=# explain select * from tenk1 order by unique1;
> NOTICE:  QUERY PLAN:
> Index Scan using tenk1_unique1 on tenk1  (cost=760.00 rows=10000 width=148)
> 
> If you start psql with PGOPTIONS="-fi" you can see that the optimizer
> believes an explicit sort would be much slower:
> 
> regression=# explain select * from tenk1 order by unique1;
> NOTICE:  QUERY PLAN:
> Sort  (cost=3233.91 rows=10000 width=148)
>   ->  Seq Scan on tenk1  (cost=563.00 rows=10000 width=148)
> 
> but (at least on my machine) the explicit sort is marginally faster.
> Evidently, the cost estimate for an explicit sort is *way* too high.

But it shouldn't be using the ORDER BY, except when the number of rows
processed is less than the full table, right?

> 
> I have been poking at this and am currently thinking that the CPU-vs-
> disk scaling constants (_cpu_page_weight_ and cpu_index_page_weight_)
> may be drastically off for modern hardware.  This is one of the
> optimizer issues that I'm hoping to resolve for 7.0.

Makes sense.  CPU's have gotten much faster than disk.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
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