Re: Query optimization - Mailing list pgsql-general
| From | Jorge Arevalo |
|---|---|
| Subject | Re: Query optimization |
| Date | |
| Msg-id | CAMhtMNOoGMB3fSMdjjYf2ukq=DmhDp4eHsnM8H_cdfATCjTRyQ@mail.gmail.com Whole thread Raw |
| In response to | Re: Query optimization (David G Johnston <david.g.johnston@gmail.com>) |
| Responses |
Re: Query optimization
Re: Query optimization |
| List | pgsql-general |
Hello David, many thanks for your responses,
Sorry for not providing the content of the fill_table3_function, but it just executes 3 insert queries in 3 different tables. And I've checked the time consuming operation is in this query (by the way, there was a little mistake in the name of the fields of the inner select, I've corrected it)SELECT value1,value2,value3,value4,
value5, hstore(ARRAY['field9', 'field10', 'field11', 'field12', 'field13', 'field14'], ARRAY[field9, field10, field11, field12, field13, field14) as metadata, value7, (select array((select row(f1, f2) from table2 p where p.f3 = field7))) as values_array FROM table1
This is the result of EXPLAIN ANALYZE
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using table1_pkey on table1 (cost=67846.38..395773.45 rows=8419127 width=88) (actual time=7122.704..22670.680 rows=8419127 loops=1)
InitPlan 2 (returns $1)
-> Result (cost=67846.29..67846.29 rows=1 width=0) (actual time=7009.063..7009.065 rows=1 loops=1)
InitPlan 1 (returns $0)
-> Seq Scan on table2 p (cost=0.00..67846.29 rows=12689 width=20) (actual time=14.971..5069.840 rows=2537787 loops=1)
Filter: (f3 = field7)
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using table1_pkey on table1 (cost=67846.38..395773.45 rows=8419127 width=88) (actual time=7122.704..22670.680 rows=8419127 loops=1)
InitPlan 2 (returns $1)
-> Result (cost=67846.29..67846.29 rows=1 width=0) (actual time=7009.063..7009.065 rows=1 loops=1)
InitPlan 1 (returns $0)
-> Seq Scan on table2 p (cost=0.00..67846.29 rows=12689 width=20) (actual time=14.971..5069.840 rows=2537787 loops=1)
Filter: (f3 = field7)
So, there's a sequential scan over table2 (in the query to get values_array), instead of a index scan. Could it be because the SELECT returns more than approximately 5-10% of all rows in the table? (I've heard that, under those conditions, a sequential scan is faster than index scan, because the amount of I/O operations required for each row)
Anyway, if I understood well, I should try:
- Avoiding that inner query by using a JOIN instead
- Return a composite type instead of an array
Am I right? What kind of additional context information would you need?
Many thanks!!
--
Jorge
On Wed, Oct 29, 2014 at 2:54 AM, David G Johnston <david.g.johnston@gmail.com> wrote:
Jorge Arévalo-2 wrote
> (SELECT value1,value2,value3,value4,value5, hstore(ARRAY['field9',
> 'field10', 'field11', 'field12', 'field13', 'field14'], ARRAY[field9,
> field10, field11, field12, field13, field14) as metadata, value7, (select
> array((select row(f1, f2) from table2 p where p.field7 = field7))) as
> values_array FROM table1)
You might try seeing whether:
FROM table1 JOIN (
SELECT field7, array_agg(row(f1, f2)) AS values_array FROM table2 GROUP BY
field7
) tbl2_agg USING (field7)
helps...
I'm also dubious (though this isn't necessarily a performance issue) of:
array[...] AS metadata
Without context I would say this would be better as a composite type instead
of an array. You may find it useful to use named composite types elsewhere
too...
David J.
--
View this message in context: http://postgresql.1045698.n5.nabble.com/Query-optimization-tp5824739p5824746.htmlSent from the PostgreSQL - general mailing list archive at Nabble.com.
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Jorge Arevalo
Freelance developer
http://about.me/jorgeas80
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