From: | Guido Neitzer <guido(dot)neitzer(at)pharmaline(dot)de> |
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To: | Carlos Benkendorf <carlosbenkendorf(at)yahoo(dot)com(dot)br> |
Cc: | postgres performance list <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Order by behaviour |
Date: | 2005-12-23 13:34:20 |
Message-ID: | 55DD6E49-BC80-443D-9F6E-573B6CEE961D@pharmaline.de |
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Thread: | |
Lists: | 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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