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From "Hunsberger, Peter" <Peter.Hunsber...@stjude.org>
Subject RE: [ESQL] Improvement....
Date Mon, 27 Jan 2003 16:35:13 GMT
>>> WHAT IS WRONG?
>>>
>>> In example A all is OK.
>>>
>>> In example B we are not using the power of the Database Manager.
>>>
>>> WHY?
>>>
>>> The LIMIT clause was designed to tell the database engine:
>>>
>>> "Let find just X rows", then the database engine when it got the X 
>>> rows stop searching and return the X rows. It improves the response 
>>> time, since does not to continue searching!
>>>
>>> Now think in a 10 million row table and YOU KNOW you need only 5 
>>> rows!
>>
>> whether 10 million rows or not - the current sollution will only ask 
>> for  6 - one additional row. Is that the time penalty you are talking 
>> about?
>
> Yes, because you already know that there are only 5 rows for every
register. Then the database will > search the last 6th row that no exist.
Forcing to searh in the ENTIRE table.
> 
> Maybe your 5 rows are at the beginning of the table but the non existent
6th row will force the 
> Database Engine to search for this.

Antonio, 

There is only one search for which your 5 rows will be at the start of the
table.  In general, you need an index on the table in order to find your
data with any efficiency.  If you have an index that matches your search
pattern then the search will stop after looking at 6 rows if it determines
that the 6th row does not follow sequentially in the index after the other 5
rows.  There is no difference whether there are 10 or 10 million rows in the
database.

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