Paul Hill <> wrote:
> On 2/1/07, Malcolm Greene <[EMAIL PROTECTED]> wrote:
>> Are there any ways to optimize UNION performance - in general or with
>> an Oracle 8.17 backend?
>>
>> I have 5 data sets that I need to merge via a UNION and the time to
>> its to build each set individually is 9 secs but the time it takes to
>> build the same set of data and union together into a single set is
>> 33 seconds.
>>
>> The total set size is 1M of records with the largest set being
>> unioned
>> being about 750K.
>>
>> I've tried rearranging the position of the big data set in the union
>> process, but this does not affect performance.
>>
>> Any tips or am I just seeing what's pretty much standard overhead for
>> performing unions.
>
> IIRC a UNION will only include unique records from each query.
> Probably that's where the slowdown is happening. Do you need this?
> If not UNION ALL should be faster.
>
> Another approach would be to use a temporary table.
A union is used for two different reasons. To gain data from multiple input
tables as well as in place of the "OR" in a where clause
Stephen Russell
DBA / .Net Developer
Memphis TN 38115
901.246-0159
"A good way to judge people is by observing how they treat those who
can do them absolutely no good." ---Unknown
http://spaces.msn.com/members/srussell/
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