Large OBject (LOB) data types are datatypes that can store large data. These datatypes can hold values larger than 8K row size.Prior to SQL Server 2005, we have only TEXT, NTEXT and IMAGE to hold large data. With SQL 2005, Microsoft has introduced additional three datatypes to hold large values like VARCHAR(MAX),NVARCHAR(MAX) and VARBINARY(MAX).

How to identify LOB columns in your database?

The below query is useful to identify LOB columns. Please note that the query contains a filter not to fetch CDC schema objects. Feel free to add more filters as required.

SELECT T.TABLE_CATALOG DATABASE_NAME,T.TABLE_SCHEMA AS SCHEMA_NAME,
    T.TABLE_NAME, 
    C.COLUMN_NAME,C.DATA_TYPE DATA_TYPE,C.CHARACTER_MAXIMUM_LENGTH MAXIMUM_LENGTH,C.COLLATION_NAME
FROM INFORMATION_SCHEMA.COLUMNS C
    INNER JOIN INFORMATION_SCHEMA.TABLES T
        ON C.TABLE_SCHEMA = T.TABLE_SCHEMA
        AND C.TABLE_NAME = T.TABLE_NAME
WHERE T.TABLE_TYPE = 'BASE TABLE' 
AND ((C.DATA_TYPE IN ('VARCHAR', 'NVARCHAR') AND C.CHARACTER_MAXIMUM_LENGTH = -1)
OR DATA_TYPE IN ('TEXT', 'NTEXT', 'IMAGE', 'VARBINARY', 'XML', 'FILESTREAM'))
AND T.TABLE_SCHEMA NOT IN('CDC') -- EXCEPTION LIST
ORDER BY T.TABLE_SCHEMA, T.TABLE_NAME

How do we identify the length or size of LOB datatype in SQL Server

At times, you may need to understand the length or size of your LOB columns. One example is to set a right value for “max text repl size (B)” to avoid certain issues like this. Usually, we use a function LEN to get the length of the data, however, LEN is not compatible certain datatypes like TEXT,NTEXT and IMAGE.

To resolve, SQL Server has another function – DATALENGTH(). The function returns the number of length or size of the data in bytes. The below query will help to identify the length or size of LOB datatypes like TEXT, NTEXT, IMAGE etc.

--Table creation script
Create Table tablename (columnname image)

--Sample data insert (The data has been trimmed for readability)
Insert into tablename values('0xFAEFCD8FFE000104A46494600010100000100010000FFDB00840009060')

SELECT  DATALENGTH(columnname) as inBytes,
        DATALENGTH(columnname) / 1024.0 as inKb,      
        DATALENGTH(columnname) / 1024.0 / 1024.0 as inMb FROM   tablename

DROP Table tablename

Thats it with this, hoping to see you with another blog post soon!