I have a question about the following. The table has an index on (clicked at
time zone 'PST'). I am using postgres 8.1.3
Actually, I think I answered my own question already. But I want to confirm -
Is the GROUP BY faster because it doesn't have to sort results, whereas
DISTINCT must produce sorted results? This wasn't clear to me from the
documentation. If it's true, then I could save considerable time by using
GROUP BY where I have been using DISTINCT in the past. Usually I simply want a
count of the distinct values, and there is no need to sort for that.
I'm also interested in the count(distinct) case at the bottom. The cost
estimate seems similar to the GROUP BY, but the actual cost is much higher.
The table is insert-only and was analyzed before running these queries. The
domain column being aggregated has around 16k distinct values, and there are
780k rows in total (for the entire table, not the slice being selected in these
queries).
Thanks,
Brian
live:parking=> explain analyze SELECT domain
FROM parked_redirects
WHERE (clicked at time zone 'PST') >= '2006-12-17'
AND (clicked at time zone 'PST')
< '2006-12-18'::timestamp without time zone + '1 day'::interval
GROUP BY domain;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=19078.50..19085.29 rows=679 width=18) (actual
time=709.309..717.096 rows=14526 loops=1)
-> Index Scan using parked_redirects_pst on parked_redirects
(cost=0.01..17846.82 rows=492672 width=18) (actual time=0.073..406.510
rows=504972 loops=1)
Index Cond: ((timezone('PST'::text, clicked) >= '2006-12-17
00:00:00'::timestamp without time zone) AND (timezone('PST'::text, clicked) <
'2006-12-19 00:00:00'::timestamp without time zone))
Total runtime: 719.810 ms
(4 rows)
live:parking=> explain analyze SELECT DISTINCT domain
FROM parked_redirects
WHERE (clicked at time zone 'PST') >= '2006-12-17'
AND (clicked at time zone 'PST')
< '2006-12-18'::timestamp without time zone + '1 day'::interval;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Unique (cost=64433.98..66897.52 rows=679 width=18) (actual
time=15329.904..15647.849 rows=14526 loops=1)
-> Sort (cost=64433.98..65665.75 rows=492709 width=18) (actual
time=15329.901..15511.479 rows=504972 loops=1)
Sort Key: "domain"
-> Index Scan using parked_redirects_pst on parked_redirects
(cost=0.01..17847.41 rows=492709 width=18) (actual time=0.068..519.696
rows=504972 loops=1)
Index Cond: ((timezone('PST'::text, clicked) >= '2006-12-17
00:00:00'::timestamp without time zone) AND (timezone('PST'::text, clicked) <
'2006-12-19 00:00:00'::timestamp without time zone))
Total runtime: 15666.863 ms
(6 rows)
live:parking=> explain analyze SELECT count(DISTINCT domain)
FROM parked_redirects
WHERE (clicked at time zone 'PST') >= '2006-12-17'
AND (clicked at time zone 'PST')
< '2006-12-18'::timestamp without time zone + '1 day'::interval;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=19107.20..19107.21 rows=1 width=18) (actual
time=11380.530..11380.531 rows=1 loops=1)
-> Index Scan using parked_redirects_pst on parked_redirects
(cost=0.01..17873.67 rows=493412 width=18) (actual time=0.022..347.473
rows=504972 loops=1)
Index Cond: ((timezone('PST'::text, clicked) >= '2006-12-17
00:00:00'::timestamp without time zone) AND (timezone('PST'::text, clicked) <
'2006-12-19 00:00:00'::timestamp without time zone))
Total runtime: 11384.923 ms
(4 rows)
---------------------------(end of broadcast)---------------------------
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