ALTER TABLE fails while converting INT64 column type to STRING

Question

I have table homes_passed which has a column street_number which is not nullable and is of type INT64. I wanted to change the column to STRING using alter table command, but for some reason I am unable to achieve it.

Is there any rules in changing the types of a column? I couldn't locate the rules in the document. Any pointer would be helpful.

Please note that, the table is empty and the column is not a part of any index.

Here is the Operation Status I receive.

 {  
       "name":"projects/*****************/instances/my-instance/databases/my-database/operations/_auto_15*************",
       "metadata":{  
          "@type":"type.googleapis.com/google.spanner.admin.database.v1.UpdateDatabaseDdlMetadata",
          "database":"projects/*****************/instances/my-instance/databases/my-database",
          "statements":[  
             "ALTER TABLE homes_passed ALTER COLUMN street_number STRING(130) NOT NULL"
          ]
       },
       "done":true,
       "error":{  
          "code":3,
          "message":"Cannot change type of column homes_passed.street_number from INT64 to STRING."
       }
    }

Answer 1

This modification is not allowed in a Google Cloud Spanner database. Only the following alterations are allowed: https://cloud.google.com/spanner/docs/schema-updates#supported_schema_updates