fitzjarrell_at_cox.net wrote:
> Comments embedded.
> On Nov 5, 5:06 am, tamsun <tam..._at_gmail.com> wrote:
>> hi,
>>
>> Database version: Oracle Database 10.2.0.2.0
>> OS: RedHat Enterprise 4
>>
>> I find select statement with sample clause return random count records,
>
> As it should.
>
>> e.g.
>>
>> select count(*) from ADS_DENDOC;
>> select count(*) from (select * from ADS_DENDOC sample(1)) a;
>>
>> 1st run:
>> -----------------
>> 1302; -- the record count of table ADS_DENDOC
>> 9; -- the record count of sample recordset
>>
>> 2nd run:
>> -----------------
>> 1302;
>> 12;
>>
>> 3rd run:
>> -----------------
>> 1302;
>> 14;
>>
>> 4th run:
>> -----------------
>> 1302;
>> 10;
>>
>> As Oracle sql reference written,sample_percent is a number specifying
>> the percentage of the total row or block count to be included in the
>> sample. The value must be in the range .000001 to (but not including)
>> 100. Why the record count of sample recordset is variable
>
> You didn't read far enough into that paragraph, the rest of which
> reads:
>
> "This percentage indicates the probability of each row, or each
> cluster of rows in the case of block sampling, being selected as part
> of the sample. It does not mean that the database will retrieve
> exactly sample_percent of the rows of table."
>
> This is an ESTIMATE, not an absolute. Your 'understanding' is
> actually misunderstanding, apparently based up on reading only what
> you want to read, rather than what you should be reading. If you
> think you understand a function, but it's behaviour isn't what you
> expect, then it's highly likely you don't fully understand all that
> you should about that function. Selective reading of the
> documentation is a prime source for such confusion.
>
>> Best regards
>> tamsun
>
>
> David Fitzjarrell
>
thank you， David.
Received on Mon Nov 05 2007 - 19:53:46 CST
