On 09/11/2013 06:36 AM, Maximilian Tyrtania wrote:
Hi there,
here is another one from the "why is my query so slow?" category. First post,
so please bare with me.
The query (which takes around 6 seconds) is this:
SET work_mem TO '256MB';//else sort spills to disk
SELECT
et.subject,
COALESCE (createperson.vorname || ' ', '') || createperson.nachname AS
"Sender/Empfänger",
to_char(es.sentonat, 'DD.MM.YY') AS "versendet am",
es.sentonat AS orderbydate,
COUNT (ct.*),
COALESCE (C . NAME, 'keine Angabe') :: TEXT AS "für Kunde",
COUNT (ct.datetimesentonat) :: TEXT || ' von ' || COUNT (ct.*) :: TEXT
|| ' versendet',
1 AS LEVEL,
TRUE AS hassubs,
FALSE AS opensubs,
'emailsendings:' || es. ID :: TEXT AS model_id,
NULL :: TEXT AS parent_model_id,
es. ID
FROM
emailtemplates et
JOIN emailsendings es ON et. ID = es.emailtemplate_id
LEFT JOIN companies C ON C . ID = es.customers_id
LEFT JOIN personen createperson ON createperson. ID = et.personen_create_id
LEFT JOIN contacts ct ON ct.emailsendings_id = es. ID WHERE
f_record_visible_to_currentuser(et.*::coretable) = true
GROUP BY
1,
2,
3,
4,
6,
8,
9,
10,
11,
12,
13
ORDER BY
es.sentonat desc
Explain analyze:
GroupAggregate (cost=35202.88..45530.77 rows=118033 width=142) (actual
time=5119.783..5810.680 rows=898 loops=1)
-> Sort (cost=35202.88..35497.96 rows=118033 width=142) (actual
time=5119.356..5200.457 rows=352744 loops=1)
Sort Key: es.sentonat, et.subject, ((COALESCE((createperson.vorname ||
' '::text), ''::text) || createperson.nachname)), (to_char(es.sentonat,
'DD.MM.YY'::text)), ((COALESCE(c.name, 'keine Angabe'::character
varying))::text), (1), (true), (false), (('emailsendings:'::text ||
(es.id)::text)), (NULL::text), es.id
Sort Method: quicksort Memory: 198999kB
-> Nested Loop Left Join (cost=0.00..25259.29 rows=118033 width=142)
(actual time=1.146..1896.382 rows=352744 loops=1)
-> Nested Loop Left Join (cost=0.00..2783.16 rows=302
width=102) (actual time=1.127..32.577 rows=898 loops=1)
-> Merge Join (cost=0.00..2120.06 rows=302 width=86)
(actual time=1.125..30.940 rows=898 loops=1)
Merge Cond: (et.id = es.emailtemplate_id)
-> Nested Loop Left Join (cost=0.00..2224.95
rows=277 width=74) (actual time=1.109..27.484 rows=830 loops=1)
-> Index Scan using emailtemplates_pkey on
emailtemplates et (cost=0.00..460.71 rows=277 width=63) (actual
time=1.097..20.541 rows=830 loops=1)
Filter:
f_record_visible_to_currentuser((et.*)::coretable)
-> Index Scan using personen_pkey on personen
createperson (cost=0.00..6.36 rows=1 width=19) (actual time=0.006..0.006 rows=1
loops=830)
Index Cond: (createperson.id =
et.personen_create_id)
-> Index Scan using
fki_emailsendings_emailtemplate_id_fkey on emailsendings es (cost=0.00..49.83
rows=905 width=20) (actual time=0.011..1.360 rows=898 loops=1)
-> Index Scan using firmen_pkey on companies c
(cost=0.00..2.18 rows=1 width=24) (actual time=0.001..0.001 rows=0 loops=898)
Index Cond: (c.id = es.customers_id)
-> Index Scan using fki_contacts_emailsendings_id_fkey on
contacts ct (cost=0.00..61.55 rows=561 width=44) (actual time=0.019..0.738
rows=393 loops=898)
Index Cond: (ct.emailsendings_id = es.id)
Total runtime: 5865.886 ms
I do have an index on es.sentonat. The sentonat-values are all unique, so I
don't think I need indexes on all the fields I sort by. But then again, my
understanding of this might be entirely wrong.
Depeszs' explain (http://explain.depesz.com/s/69O) tells me this:
node type count sum of times % of query
GroupAggregate 1 610.223 ms 10.5 %
Index Scan 5 690.503 ms 11.9 %
Merge Join 1 2.096 ms 0.0 %
Nested Loop Left Join 3 1203.783 ms 20.7 %
Sort 1 3304.075 ms 56.9 %
, so the sort appears to be the problem. Any pointers would be highly
appreciated.
I recently had to diagnose and remedy a case such as this.
The short answer is to rewrite your query so you don't have to group by
so many things. Collect your aggregates in a common table expression
query (or possibly more than one, depends what you need) using the
minimum non-aggregated columns to enable you to get correct results and
then later decorate that with all the extra things you need such as
constant columns and columns that are irrelevant to the aggregation.
This gets hard when queries are very complex, and harder still when the
query is written by a query generator. But a good generator should not
just say "grouo by everything that's not aggregated" and think it's
doing a good job. In your case it should be relatively straightforward.
cheers
andrew
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