[SQL SERVER] [Performance] text data type conversion Effect varchar (max) of
User question, 2008 R2 (n) text transfer (n) the impact varchar (max) of?
From SQL2005, text, ntext and image data types will be varchar (max), nvarchar (max) and varbinary (max) data type substituted (below)
Most directly through the alter table ... alter column manner corresponding to the data type conversion,
But because the data had converted every 8kb restrictions,
So if you are converting varchar (8000) instead of varchar (max),
Then you need to pay attention to the content truncation problem (if the original text content length> 8000),
We recommend using a cast or convert to handle the conversion job to avoid truncation on conversion,
After the data type conversion is again obvious is the difference in performance,
After the case before a conversion all very pretty poor query performance,
Here I will simply simulate the original process, and made available to the users reference.
create table mytest
(
c1 text null
)
--datalength<=8000
insert into mytest
select '                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                '
Query c1 (text)
LOB read logic 5, a logic 1 is read.
C1 field data type is converted varchar (max)
LOB logical reads 4, a logic 1 is read.
Do not forget to perform the update
LOB logical reads 4, a logic 0 is read.
Why should I run update
Because the default data type is text content stored in the LOB structure, data tables only through the pointer (pointer) to a local LOB,
And varchar (max) The default data type is text content stored in the data table structure,
But the premise is that the content length must not exceed 8000, more than if it is stored in the LOB structure.
See above first query logic reads LOB 5 = (not equal to 0), indicating that the content is located in the LOB structure,
The second query logic reads LOB = 4 (content storage position to retain the original text), although significantly reduced but still not equal to 0,
But this time the content is stored in the LOB structure, you can imagine the amount if more than one query data,
The difference is the query performance extremely through update command, SQL Server will data length <= 8000 move to the contents of the data table structure,
The third query result you can see, the logic reads the LOB = 0 (the data content of all moved data table structure),
= 1 and the logic still read (logical reads reads small increases in accordance with the logic LOB).
reference
SQL Server 2005 has been replaced by the Database Engine features
Original: Large column [SQL SERVER] [Performance] text data type conversion impact varchar (max) of