Warning! The maximum key length is 900 bytes. The index has maximum length of 1000 bytes

Question

For indexing columns that have Large sizes ,sqlserver indexes only
columns that have size up to 900 bytes.

To solve this problem

Firstly : I added a column hashCol to generate hashcode of Type SHA1 for MyCol

alter table myTable
    add 
    hashCol AS HASHBYTES('SHA1', CONVERT(VARCHAR(90), MyCol))

Secondly : I added a unique constrain for hashCol to uniquely Identify MyCol

 ALTER  TABLE  myTable
     ADD CONSTRAINT hashCol_Unique UNIQUE (hashCol)

By this way I overcame the problem of Indexing columns that have large size

references

Generate Unique hash for a field in SQL Server in-sql-server