1. The city temps table is a toy example, not meant to be realistic :-)
2. Yes, my (Java) algorithm is deterministic ... it will return
exactly one row per city, and that will be the row (or strictly, *a*
row) containing the highest temp. Temp value ties will break in favour
of earlier rows in Guinness Book of Records tradition :-) It's
equivalent to a HashAggregate implementation.
The following two query plans (from my real schema) illustrate the
itch I am trying to scratch .... I want the functionality of the 2nd
one, but with the execution plan structure of the first:
# explain analyse select a, max(b) from perf_raw_2010_02_23 group by a;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=117953.09..117961.07 rows=639 width=8) (actual
time=10861.845..10863.008 rows=1023 loops=1)
-> Seq Scan on perf_raw_2010_02_23 (cost=0.00..91572.39
rows=5276139 width=8) (actual time=0.038..4459.222 rows=5276139
loops=1)
Total runtime: 10863.856 ms
(3 rows)
Time: 10864.817 ms
# explain analyse select distinct on (a) * from perf_raw_2010_02_23
order by a, b desc ;
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------------------
Unique (cost=1059395.04..1085775.73 rows=639 width=28) (actual
time=46011.204..58428.210 rows=1023 loops=1)
-> Sort (cost=1059395.04..1072585.39 rows=5276139 width=28)
(actual time=46011.200..53561.112 rows=5276139 loops=1)
Sort Key: a, b
Sort Method: external merge Disk: 247584kB
-- actually OS RAM buffers
-> Seq Scan on perf_raw_2010_02_23 (cost=0.00..91572.39
rows=5276139 width=28) (actual time=0.047..6491.036 rows=5276139
loops=1)
Total runtime: 58516.185 ms
(6 rows)
Time: 58517.233 ms
The only difference between these two is that the second query returns
the whole row. The *ratio* in cost between these two plans increases
in proportion to log(n) of the table size ... at 5.5m rows its
livable, at 500m it's probably not :-!
Cheers
Dave
On Wed, Feb 24, 2010 at 5:12 PM, Richard Huxton <[email protected]> wrote:
> On 24/02/10 22:47, Dave Crooke wrote:
>>
>> I'd imagine it would be possible to have a query planner optimization
>> that would convert Garrett's DISTINCT ON syntax to do what I was
>> trying to, by realizing that DISTINCT ON X ... ORDER BY Y DESC is
>> going to return the the one row for each X which has the highest value
>> of Y, and so use a MAX-structured accumulation instead of a sort.
>
> Why is there only one row? For city temperatures, that seems unlikely.
>
> In the event of more than one row does your algorithm give repeatable
> results?
>
> --
> Richard Huxton
> Archonet Ltd
>
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