Re: Order by behaviour

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От Guido Neitzer
Тема Re: Order by behaviour
Дата
Msg-id 55DD6E49-BC80-443D-9F6E-573B6CEE961D@pharmaline.de
обсуждение исходный текст
Ответ на Order by behaviour  (Carlos Benkendorf <carlosbenkendorf@yahoo.com.br>)
Ответы Re: Order by behaviour  (Carlos Benkendorf <carlosbenkendorf@yahoo.com.br>)
Список pgsql-performance
On 23.12.2005, at 13:34 Uhr, Carlos Benkendorf wrote:

> For some implementation reason in 8.0.3 the query is returning the
> rows in the correct order even without the order by but in 8.1.1
> probably the implementation changed and the rows are not returning
> in the correct order.

You will never be sure to get rows in a specific order without an
"order by".

I don't know why PG is faster without ordering, perhaps others can
help with that so you don't need a workaround like this:

If you can't force PostgreSQL to perform better on the ordered query,
what about retrieving only the primary keys for the rows you want
unordered in a subquery and using an "where primaryKey in (...) order
by ..." statement with ordering the five rows?

Like this:

select * from mytable where pk in (select pk from mytable where ...)
order by ...;

I don't know whether the query optimizer will flatten this query, but
you can try it.

cug


--
PharmaLine Essen, GERMANY and
Big Nerd Ranch Europe - PostgreSQL Training, Feb. 2006, Rome, Italy
http://www.bignerdranch.com/classes/postgresql.shtml



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