topic Re: Database optimiser in Application Development and Automation Discussions

Database optimiser

Former Member — Sun, 20 Dec 2009 12:45:30 GMT

Hi all

What is the difference between select * and select upto 1 row ?

Please explain in simple language.

Sandeep

Re: Database optimiser

Former Member — Sun, 20 Dec 2009 12:52:57 GMT

This is the very basic question , please search SDN or press F1 on SELECT and Upto 1 row to understand the concept.

Re: Database optimiser

Former Member — Sun, 20 Dec 2009 12:59:42 GMT

please do find this

According to SAP Performance course the SELECT UP TO 1 ROWS is faster than SELECT SINGLE because you are not using all the primary key fields.

select single is a construct designed to read database records with primary key. In the absence of the primary key, it might end up doing a sequential search, whereas the select up to 1 rows may assume that there is no primary key supplied and will try to find most suitable index.

The best way to find out is through sql trace or runtime analysis.

Use "select up to 1 rows" only if you are sure that all the records returned will have the same value for the field(s) you are interested in. If not, you will be reading only the first record which matches the criteria, but may be the second or the third record has the value you are looking for.

The System test result showed that the variant Single * takes less time than Up to 1 rows as there is an additional level for COUNT STOP KEY for SELECT ENDSELECT UP TO 1 ROWS.

The 'SELECT SINGLE' statement selects the first row in the database that it finds that fulfils the 'WHERE' clause. If this results in multiple records then only the first one will be returned and therefore may not be unique.

Mainly: to read data from the 'SELECT .... UP TO 1 ROWS' statement is subtly different. The database selects all of the relevant records that are defined by the WHERE clause, applies any aggregate, ordering or grouping functions to them and then returns the first record of the result set.

Hope this will be helpful to you

Re: Database optimiser

Former Member — Sun, 20 Dec 2009 15:55:42 GMT

Thank you Harsh - I moved the ponits to you.

Not only could Sandeep and Subin have easily searched first, but they are the same person "playing" a dishonest ponits-game.

Cheers,

Julius

Re: Database optimiser

Former Member — Sun, 20 Dec 2009 23:41:12 GMT


> Not only could Sandeep and Subin have easily searched first...

But he did search - search the forum for"subtly". But use a date range of all. We have managed to stop this until now.

Rob

Re: Database optimiser

Former Member — Mon, 21 Dec 2009 11:40:14 GMT

> the same person "playing" a dishonest ponits-game.

nice to know ... because the answer is a bad as the question

question: select * versus select up to n rows

answer: select single versus select up to n rows ... different!

arguments nonsense!

database optimizer .... not involved in the whole stuff.

Re: Database optimiser

Former Member — Mon, 21 Dec 2009 11:58:07 GMT

They will soon be gone. We are just investigating for a while still to see that we catch all the "guests" in the "ponits gamer pool". It looks like a "nest" of them...

Yes, basic questions with irrelvant answers are often a symptom of a bias, as are not recognizing correct answers. If they knew their stuff then they would not need to cheat!

Cheers,

Julius