The Serialize option only applies to the Stream operator. Use the Serialize option when correct sequencing of transactions is required. For example, when a job contains a transaction that inserts a row to open a new account, and another transaction updates the balance for the account, then the sequencing of the transactions is critical.
Using the Serialize option in APPLY statements, the Stream operator ensures that operations for a given row occur in the order they are received from the input stream.
To use this option, associate a sequencing key (usually the primary index) with the target table. Each input data row is hashed based on the key to determine the session assigned to process each input row. This allows all rows with the same key to be processed in sequence by the same session, which is especially important if rows are distributed among many sessions.
When using the Serialize option, only one instance of the Stream operator is allowed. Specifying more than one instance causes the Stream operator to terminate with an error.