We started development on a project with PostgreSQL as our database. However midway we had to switch to SQL Server for a variety of reasons.

Here are some of the issues we noticed while migrating to SQL Server.

Unique constraint on a column which has multiple NULL values

As per the ANSI SQL standard unique constraint should allow multiple NULL values.

PostgreSQL documentation on unique constraint states following.

In general, a unique constraint is violated when there is more than one row in
the table where the values of all of the columns included in the constraint are
equal.

However, two null values are not considered equal in this comparison.

That means even in the presence of a unique constraint it is possible to store
duplicate rows that contain a null value in at least one of the constrained
columns.

This behavior conforms to the SQL standard, but we have heard that
other SQL databases might not follow this rule. So be careful when developing
applications that are intended to be portable.

In SQL Server a unique constraint does not allow multiple NULL values.

Devise by default adds unique index on reset_password_token column.

add_index :users, :reset_password_token, :unique => true

Devise is doing the right thing by enforcing a unique index on reset_password_token so that when a user clicks on a link to reset password the application would know who the user is.

However here is the problem. If we add a new user then by default the value of reset_password_token is NULL. If we add another user then we have two records with NULL value in reset_password_token. This works in PostgreSQL.

But SQL Server would not allow to have two records with NULL in reset_password_token column.

So how do we solve this problem.

Partial index to rescue. It is also known as Filtered index. Both PostgreSQL and SQL server support it. Rails also supports partial index by allowing us to pass where option as shown below.

add_index :users, :reset_password_token,
                  unique: true,
                  where: 'reset_password_token IS NOT NULL'

Please visit this issue if you want to see detailed discussion on this topic.

This behavior of SQL Server comes in play in various forms. Let’s say that we are adding api_auth_token to an existing users table.

Typically a migration for that might look like as shown below.

add_column :users, :api_auth_token, :string,
                                    :null => true,
                                    :unique => true

In this case we have plenty of records in the users table so the above migration will fail in PostgreSQL. We will have to resort to usage of partial index to fix this issue.

Adding not null constraint on a column with an index

In PostgreSQL following case will work just fine.

add_column :users, :email, :string, :unique => true
change_column :users, :email, :null => false

Above migration will fail with SQL Server.

In SQL Server a “not null constraint” cannot be added on a column which has a index on it. We need to first remove the unique index, then add the “not null” constraint and then add the unique index back.

The other solution is to add not NULL constraint first in the migration and then add any index.

Serialize array into a string column

ActiveRecord supports Array Datatype for PostgreSQL. We were using this feature to store a list of IDs.

After switching to SQL server we converted the column into string type and serialized the array.

serialize :user_ids, Array