Re: UNION: SQL-tuning

From: Richard Foote <richard.foote_at_bigpond.com>
Date: Thu, 12 Sep 2002 17:30:53 +1000
Message-ID: <ofXf9.30168$g9.86535@newsfeeds.bigpond.com>

Hi Andrei,

You haven't given us much to go on.

No version, no sample code, no execution plans.

Best I can do is play pin the pony tail.

Note that UNION will perform sort operations to eliminate duplicate rows (whereas UNION ALL will not). Perhaps the cost of retrieving the combination of result sets is such that Oracle is saying, "nah, bugger it, may as well read it all".

More details, less generalistic guess work.

Cheers

Richard
"Andrei Romazanov" <romazanov.andrei_at_gfos.de> wrote in message news:alpddu$1rt0js$1_at_ID-70985.news.dfncis.de...
> Hello all,
>
> I have performance-problem in a UNION.
> Normally I can separated optimize the parts from UNION,
> in this case however show both part-SELECTS good performance,
> but in UNION suddenly appears FULL TABLE SCAN. Could someone explain to
me
> this
> behavior of the Optimizer?
>
> Thank you in advance
>
> Andrei
>
>
Received on Thu Sep 12 2002 - 02:30:53 CDT