Method to update LOB data with minimal log

Use the .WRITE (expression, @Offset,@Length) clause to perform a partial or full update of varchar(max)nvarchar(max), and varbinary(max) data types.

For example, a partial update of a varchar(max) column might delete or modify only the first 200 characters of the column, whereas a full update would delete or modify all the data in the column. .WRITE updates that insert or append new data are minimally logged if the database recovery model is set to bulk-logged or simple.

UPDATE <table_or_view_name>
SET column_name.WRITE (expression, @offset, @length)
FROM <table_source>
WHERE <search_condition>

expression is the value that is copied to column_nameexpression must evaluate to or be able to be implicitly cast to the column_name type. If expression is set to NULL, @Length is ignored, and the value in column_nameis truncated at the specified @Offset.

@Offset is the starting point in the value of column_name at which expression is written. @Offset is a zero-based ordinal position, is bigint, and cannot be a negative number. If @Offset is NULL, the update operation appends expression at the end of the existing column_name value and @Length is ignored. If @Offset is greater than the length of the column_name value, the Database Engine returns an error. If @Offset plus @Length exceeds the end of the underlying value in the column, the deletion occurs up to the last character of the value. If @Offset plus LEN(expression) is greater than the underlying declared size, an error is raised.

@Length is the length of the section in the column, starting from @Offset, that is replaced by expression@Length is bigint and cannot be a negative number. If @Length is NULL, the update operation removes all data from @Offset to the end of the column_name value.

The regular update statement results in overwriting the entire string using full logging and it’s very inefficient when dealing with large value updates.

Note: The update using WRITE method will fail if the value is null.

More information about performance in my tip on MSSQLTips

 

 

How update works in transactional replication?

In a post about how update works I showed what happened when run an update with the same values. SQL Server is smart enough to see that and not changing anything and register minimum log.

Another day I saw a comment to avoid updating records when none of the values are changing.

Avoid updating records when none of the values are changing. This still does a write and, if the table is in replication or change tracking, still causes the row to be propagated out to other servers. If you are updating a potentially large number of records, make sure to only update the ones where the new value doesn’t equal the old value.

Let’s see how the update behavior when update 10 millions rows but without any change and see if will be any row propagation to another server.

objectexplorer

My Lab contains two servers and a demo database replicated from SQL01 to SQL02.

It’s configured a transaction replication to send the articles and I’m using a table created with the code:

CREATE TABLE tblMillionsRows (
    id BIGINT NOT NULL IDENTITY PRIMARY KEY,
    largeColumn NVARCHAR(MAX) DEFAULT REPLICATE('TESTE',100),
    smallColumn NVARCHAR(150) DEFAULT 'DEMO',
    dateColumn DATETIME
        DEFAULT GETDATE()
);
GO

Inserting rows:

INSERT INTO dbo.tblMillionsRows (
    largeColumn,
    smallColumn,
    dateColumn
)
DEFAULT VALUES
GO 2000000

It will take a while to insert all rows. The next step is add this table in the replication.

DECLARE @publication    AS sysname;
DECLARE @table AS sysname;
DECLARE @filterclause AS nvarchar(500);
DECLARE @filtername AS nvarchar(386);
DECLARE @schemaowner AS sysname;
SET @publication = N'SQL01_demo_tb01'; 
SET @table = N'tblMillionsRows';
SET @schemaowner = N'dbo';

EXEC sp_addarticle 
	@publication = @publication, 
	@article = @table, 
	@source_object = @table,
	@source_owner = @schemaowner, 
	@schema_option = 0x80030F3,
	@vertical_partition = N'true', 
	@type = N'logbased',
	@filter_clause = @filterclause;

EXEC sp_articlecolumn 
	@publication = @publication, 
	@article = @table;

EXEC sp_startpublication_snapshot 
	@publication = 'SQL01_demo_tb01', 
	@publisher =  'SQL01'   

With all set, let’s do some tests. First updating the heap table and seeing if something is replicated.

UPDATE tblMillionsRows SET smallColumn = N'DEMO' WHERE id < 10000
GO

After the update, I ran the script below to see the transactions and commands available to replicate and nothing changed.

SELECT * FROM distribution.dbo.MSrepl_Commands 
SELECT * FROM distribution.dbo.MSrepl_Transactions 
EXEC sp_browsereplcmds 

But, when I change one row running the update below I could see a new command to replicate in the another server.

UPDATE tblMillionsRows SET smallColumn = N'DEMO1' WHERE id < 10000
GO

replicationcommands

In conclusion, SQL Server won’t replicate updates that don’t change the value. You can see more about updates in my post.

Be careful when doing updates, because if the table is replicated and you change millions rows the transaction replication will create one command for each updated row.