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.
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.
regards, tom lane
