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Diff between select single and select upto 1 rows.

Hello aLL,

PL tell what is technical diff between select single and select upto 1 rows and how it is affecting the performance.

Rushikesh

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    author's profile photo Former Member
    Former Member
    Posted on Dec 03, 2007 at 08:53 AM

    SELECT SINGLE returns the first matching row for the given condition and it may not be unique, if there are more matching rows for the given condition.

    SELECT ... UP TO 1 ROWS retrieves all the matching records and applies aggregation and ordering and returns the first record.

    Inorder to check for the existence of a record then it is better to use SELECT SINGLE than using SELECT ... UP TO 1 ROWS since it uses low memory and has better performance.

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  • Posted on Dec 03, 2007 at 08:31 AM

    Hi, it has been discussed before .

    Statement select single will always bypass the table buffer, and only retrieve single record at any select clause .

    Statement select up to 1 record will have a buffer lookup, and it would retrieve all data suitable to select-clause before selecting the top most record to fetch back to application server.

    Both case has different runtime benefit depend on your query case .

    Rgds,

    TS WINEDYA

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  • Posted on Dec 03, 2007 at 09:56 AM

    > Statement select single will always bypass the table buffer, and only retrieve

    > single record at any select clause .

    > Statement select up to 1 record will have a buffer lookup, and

    This is incorrect the opposite is the case, single record buffers can only be accessed if the keyword SINGLE is used!

    There is no relevant performance difference, use SAINGLE if you specify the full primary key, use up to 1 row if the key is more general but you need only 1 row.

    Overall, it is not a performance issue.

    Never use aggregates with up to 1 row as aggregates have higher priority than the up to 1 rows.

    Siegfried

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  • author's profile photo Former Member
    Former Member
    Posted on Dec 03, 2007 at 10:00 AM

    Hi

    Knowing when to use SELECT SINGLE or SELECT ... UP TO 1 ROWS

    A lot of people use the SELECT SINGLE statement to check for the existence of a value in a database. Other people prefer to use the 'UP TO 1 ROWS' variant of the SELECT statement.

    So what's the difference between using 'SELECT SINGLE' statement as against a 'SELECT .... UP TO 1 ROWS' statement ?

    If you're considering the statements

    SELECT SINGLE field INTO w_field FROM table.

    and

    SELECT field INTO w_field FROM table UP TO 1 ROWS. ENDSELECT.

    then looking at the result, not much apart from the extra ENDSELECT statement. Look at the run time and memory usage and they may be worlds apart.

    Why is this ?? The answer is simple.

    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.

    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.

    Get the difference ??

    If not, here is a good example, credit for this example goes to Richard Harper, a friend of mine on sapfans.com :

    Create a Ztable called ZDifference with 2 fields in it, MANDT of type MANDT and POSNR of type POSNR. Make sure both of these are keys. Also create a table maintenance dialog for it (SE11->Utilities->Table Maintenance Generator). Fill the table with ten rows 000001-000010.

    Then run the program shown below:

    Code:

  • Program: Z_Difference

  • Purpose: A program that demonstrates the difference

  • between SELECT SINGLE and SELECT UP TO n ROWS.

  • This program requires the data table Z_DIFFERENCE

  • to have been created according to the structure

  • outlined in the text above and populated with

  • at least 10 records.

  • Creation Date: 21/04/2004

  • Requested By:

  • Reference Doc:

  • Author: R Harper

  • Modification History:

  • Date Reason Transport Who

    Report Z_Difference

    Message-id 38

    Line-Size 80

    Line-Count 0

    No Standard Page Heading.

    Start-Of-Selection.

    Data: w_Single type Posnr,

    t_Rows type standard table of Posnr

    initial size 0

    with header line.

    Select single Posnr

    from zDifference

    into w_Single.

    Select Posnr

    into table t_Rows

    from zDifference

    up to 1 rows

    order by Posnr descending.

    Write :/ 'Select single:', w_Single.

    Skip 1.

    Write :/ 'Up to 1 rows :'.

    Loop at t_Rows.

    Write t_Rows.

    EndLoop.

    You should see the output:

    Select single: 000001

    Up to 1 rows : 000010

    The first 'SELECT' statement selected the first record in the database according to any selection criterion in the 'WHERE' clause. This is what a 'SELECT SINGLE' does. The second 'SELECT' has asked the database to reverse the order of the records before returning the first row of the result.

    In order to be able to do this the database has read the entire table, sort it and then return the first record. If there was no ORDER BY clause then the results would have been identical (ie both '000001') but the second select if given a big enough table to look at would be far slower.

    Note that this causes a problem in the Extended Program Check if the full key is not specified in a 'SELECT SINGLE'. Replacing the 'SELECT SINGLE' by an "UP TO 1 ROWS" will give the same exact results without any warning but the program will run slower and consume more memory. This is a good example of a warning that we should ignore... considering you are sure of what you are doing !!

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  • author's profile photo Former Member
    Former Member
    Posted on Dec 03, 2007 at 10:00 AM

    hi,

    Let me give you example

    Table: AAA

    No NAME

    1 AAA

    2 AAA

    3 BBB

    4 AAA

    SELECT SINGLE * "Where name = 'AAA '.

    Immediately it will search when serch condition is true it will display the data and stops further searching.

    So, O/p = 1 , AAA

    upto 1 row. "Where name = 'AAA '.

    It will fetch all the that are matched with name 'AAA'.

    And then display the first row.

    (Something like this

    1 AAA

    2 AAA

    4 AAA)

    So, performance wise when more number of records exists with same conditions it is better to use select single *

    But we use this queries depending on the requirment.

    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.

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  • Posted on Dec 03, 2007 at 03:24 PM
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