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Altering column from varchar(2048) to varchar(max) performance


Altering column from varchar(2048) to varchar(max) performance

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Mike Byrd
Mike Byrd
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If I run following TSQL (original column defined as VARCHAR(2048)

ALTER TABLE dbo.SystemLog
ALTER COLUMN LogData VARCHAR(MAX);

It takes ~75 minutes to complete, but if I run (original column defined as VARCHAR(2048)

ALTER TABLE dbo.SystemLog
ALTER COLUMN LogData VARCHAR(8000);

It only takes 2 seconds to complete (refreshed copy of original table).

Can anybody tell me why the conversion from VARCHAR(2048) to VARCHAR(MAX) takes so long. It was my understanding that data within a varchar(max) column was retained in the table data if it fit within the page, but if not, then moved elsewhere like the deprecated TEXT field was. Obviously something is going on.

Any ideas please? Thanks in advance.

Mike Byrd
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altering to varchar(8000) is only a catalog operation (and maybe an index op if the column is indexed) because it stays the same datatype, only the var length changes.
Changing to datatype varchar(max) is actually a change to an other datatype ! (max characteristics)

Johan


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Jeff Moden
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ALZDBA (7/29/2009)
altering to varchar(8000) is only a catalog operation (and maybe an index op if the column is indexed) because it stays the same datatype, only the var length changes.
Changing to datatype varchar(max) is actually a change to an other datatype ! (max characteristics)


This also drives the data to an "out of row" condition. It has to find a reference and then go outside the row and page to find the data. All of that takes extra time.

--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.
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