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Indexing Computed Columns

  • November 8, 2018 at 10:42 pm

    #416476

    Comments posted to this topic are about the item Indexing Computed Columns

  • November 8, 2018 at 10:53 pm

    #2012929

    Really interesting question, thanks Steve

    forgot that  the results of this datetime function is non-deterministic....

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  • November 9, 2018 at 1:54 am

    #2012933

    Stewart "Arturius" Campbell - Thursday, November 8, 2018 10:53 PM

    Really interesting question, thanks Steve

    forgot that  the results of this datetime function is non-deterministic....

    DATEPART not always is nondeterministic.
    e.g. datepart(day,'20180131') is deterministic because it returns always 31
    e.g. datepart(dw,'20180131') is NONdeterministic, because it depends from localization.

  • November 10, 2018 at 8:00 am

    #2013066

    I believe you meant "not" rather than "now".

    However, your first example is also non-deterministic.  Here's the proof.

    Create table dbo.SalesOrderHeader (RN INT IDENTITY(1,1),SomeDate DATETIME);
    GO
    ALTER TABLE dbo.SalesOrderHeader ADD WhichDay AS datepart(day,'20180131') PERSISTED;

    Msg 4936, Level 16, State 1, Line 1
    Computed column 'WhichDay' in table 'SalesOrderHeader' cannot be persisted because the column is non-deterministic.

    --Jeff Moden

    RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
    First step towards the paradigm shift of writing Set Based code:
    ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

    Change is inevitable... Change for the better is not.

    Helpful Links:
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  • November 14, 2018 at 5:57 am

    #2013420

    nice question steve, it had me stumped

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  • November 14, 2018 at 6:43 am

    #2013442

    Jeff Moden - Saturday, November 10, 2018 8:00 AM

    I believe you meant "not" rather than "now".

    However, your first example is also non-deterministic.  Here's the proof.

    Create table dbo.SalesOrderHeader (RN INT IDENTITY(1,1),SomeDate DATETIME);
    GO
    ALTER TABLE dbo.SalesOrderHeader ADD WhichDay AS datepart(day,'20180131') PERSISTED;

    Msg 4936, Level 16, State 1, Line 1
    Computed column 'WhichDay' in table 'SalesOrderHeader' cannot be persisted because the column is non-deterministic.

    Here an example where it is deterministic:
    DROP table dbo.SalesOrderHeader
    Create table dbo.SalesOrderHeader (RN INT IDENTITY(1,1),SomeDate DATETIME NOT NULL);
    ALTER TABLE dbo.SalesOrderHeader ADD WhichDay AS datepart(d,SomeDate) PERSISTED;
    create index idx_SalesOrderHeader ON SalesOrderHeader(WhichDay)
    Commands completed successfully.

    The difference is the second input parameter:
    with string is non-deterministic
    with a valid  datetime is deterministic

  • November 15, 2018 at 4:53 am

    #2013622

    Carlo Romagnano - Wednesday, November 14, 2018 6:43 AM

    Jeff Moden - Saturday, November 10, 2018 8:00 AM

    I believe you meant "not" rather than "now".

    However, your first example is also non-deterministic.  Here's the proof.

    Create table dbo.SalesOrderHeader (RN INT IDENTITY(1,1),SomeDate DATETIME);
    GO
    ALTER TABLE dbo.SalesOrderHeader ADD WhichDay AS datepart(day,'20180131') PERSISTED;

    Msg 4936, Level 16, State 1, Line 1
    Computed column 'WhichDay' in table 'SalesOrderHeader' cannot be persisted because the column is non-deterministic.

    Here an example where it is deterministic:
    DROP table dbo.SalesOrderHeader
    Create table dbo.SalesOrderHeader (RN INT IDENTITY(1,1),SomeDate DATETIME NOT NULL);
    ALTER TABLE dbo.SalesOrderHeader ADD WhichDay AS datepart(d,SomeDate) PERSISTED;
    create index idx_SalesOrderHeader ON SalesOrderHeader(WhichDay)
    Commands completed successfully.

    The difference is the second input parameter:
    with string is non-deterministic
    with a valid  datetime is deterministic

    Wow, that just gave me a headache.LOL But hey I forgot the deterministic rule. OLD AGE is A bummer.
    Thanks for the schooling!
    Paul

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