In response to Lefteris :
>
> airtraffic=# EXPLAIN ANALYZE SELECT "DayOfWeek", count(*) AS c FROM
> ontime WHERE "Year" BETWEEN 2000 AND 2008 GROUP BY "DayOfWeek" ORDER
> BY c DESC;
> QUERY
> PLAN
> ------------------------------------------------------------------------------------------------------------------------------------------
> Sort (cost=7407754.12..7407754.13 rows=4 width=2) (actual
> time=371188.821..371188.823 rows=7 loops=1)
> Sort Key: (count(*))
> Sort Method: quicksort Memory: 25kB
> -> HashAggregate (cost=7407754.04..7407754.08 rows=4 width=2)
> (actual time=371163.316..371163.320 rows=7 loops=1)
> -> Seq Scan on ontime (cost=0.00..7143875.40 rows=52775727
> width=2) (actual time=190938.959..346180.079 rows=52484047 loops=1)
> Filter: (("Year" >= 2000) AND ("Year" <= 2008))
> Total runtime: 371201.156 ms
> (7 rows)
>
>
> I understand that the problem here is the memory used by the sort
> method. *But*, I already changed the work_mem parameter to 6gigs:)
No.
The problem here is the Seq-Scan. It returns about 52.000.000 rows,
approximately roughly table, it needs 346 seconds.
The sort needs only 25 KByte and only 0.02ms.
Andreas
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Andreas Kretschmer
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