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**Description**

Update source_code field of dag_code table to MEDIUMTEXT
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**Use case / motivation**

Lot of dags exceed the limit of 65K characters limit giving error `"Data too long for column 'source_code' at row 1"` when enabling webserver to fetch dag_code from db. 
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**Related Issues**

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airflow/migrations/versions/4addfa1236f1_add_fractional_seconds_to_mysql_tables.py
op.alter_column(table_name='dag_pickle', column_name='created_dttm', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='dag_run', column_name='execution_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='dag_run', column_name='start_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='dag_run', column_name='end_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='import_error', column_name='timestamp', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='job', column_name='start_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='job', column_name='end_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='job', column_name='latest_heartbeat', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='log', column_name='dttm', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='log', column_name='execution_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(
            table_name='sla_miss', column_name='execution_date', type_=mysql.DATETIME(fsp=6), nullable=False
        )
        op.alter_column(table_name='sla_miss', column_name='timestamp', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='task_fail', column_name='execution_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='task_fail', column_name='start_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='task_fail', column_name='end_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(
            table_name='task_instance',
            column_name='execution_date',
            type_=mysql.DATETIME(fsp=6),
            nullable=False,
        )
        op.alter_column(table_name='task_instance', column_name='start_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='task_instance', column_name='end_date', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='task_instance', column_name='queued_dttm', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='xcom', column_name='timestamp', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='xcom', column_name='execution_date', type_=mysql.DATETIME(fsp=6))
def downgrade():  
    conn = op.get_bind()