Re: Slow query with backwards index scan

От: Jeremy Harris
Тема: Re: Slow query with backwards index scan
Дата: ,
Msg-id: 46AB9B9C.8070009@wizmail.org
(см: обсуждение, исходный текст)
Ответ на: Re: Slow query with backwards index scan  (Tilmann Singer)
Список: pgsql-performance

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Slow query with backwards index scan  (Tilmann Singer, )
 Re: Slow query with backwards index scan  (Nis Jørgensen, )
  Re: Slow query with backwards index scan  (Tilmann Singer, )
 Re: Slow query with backwards index scan  (, )
  Re: Slow query with backwards index scan  (Tilmann Singer, )
   Re: Slow query with backwards index scan  (Craig James, )
    Re: Slow query with backwards index scan  (Tilmann Singer, )
     Re: Slow query with backwards index scan  (Nis Jørgensen, )
      Re: Slow query with backwards index scan  (Tilmann Singer, )
       Re: Slow query with backwards index scan  (Nis Jørgensen, )
   Re: Slow query with backwards index scan  (Jeremy Harris, )
 Re: Slow query with backwards index scan  (, )

Tilmann Singer wrote:
> *  <> [20070728 21:05]:
>> Let's try putting the sort/limit in each piece of the UNION to speed them up separately.
>>
>> SELECT * FROM (
>>  (SELECT * FROM large_table lt
>>  WHERE lt.user_id = 12345
>>  ORDER BY created_at DESC LIMIT 10) AS q1
>>  UNION
>>  (SELECT * FROM large_table lt
>>  WHERE user_id IN (SELECT contact_id FROM relationships WHERE user_id=12345)
>>  ORDER BY created_at DESC LIMIT 10) AS q2
>> ORDER BY created_at DESC LIMIT 10;
>
> It's not possible to use ORDER BY or LIMIT within unioned queries.
>
> http://www.postgresql.org/docs/8.2/static/sql-select.html#SQL-UNION

"ORDER BY and LIMIT can be attached to a subexpression if it is enclosed in parentheses"


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