# Limitations and considerations

The following limitations apply when using Google Cloud BigQuery as a target in a Replicate task:

* The following DDLs are not supported:

  * Drop column
  * Rename column
  * Change column data type

* Capturing changes from source tables without a Primary Key or Unique Index is not supported. If you need to capture changes from such tables, you can add a Primary Key [using a transformation](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/tasks_useTransTable.htm). Additionally, Primary Key or Unique Index columns cannot contain NULLs. If you are aware that such columns will be populated with NULL values, [define a transformation](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/tasks_useTransTable.htm) to change the NULLs to non-nullable values.

* The [Transactional apply Change Processing Mode](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/tasks_applychangtunestab.htm) is not supported.

* The [Allow unlimited LOB size](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/target_metadata.htm) LOB handling option is not supported.

* With the following task settings configuration, the replication task will perform a DELETE operation followed by an INSERT instead of an UPDATE:

  * [Change Processing Mode](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/tasks_applychangtunestab.htm) is set to Batch optimized apply.

  * The Apply changes using SQL MERGE option is not selected.

  * [UPDATEs error handling for Apply Conflicts](https://help.qlik.com/en-US/replicate/November2024/Content/Global_Common/Content/SharedEMReplicate/Customize%20Tasks/tasks_applyErrorStab.htm) is set to: No record found for applying an UPDATE: INSERT the missing target record.

  As Google Cloud BigQuery does not support rollback, in the event that the replication task is unable to insert the updated row, the data will be deleted from the target.