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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
#
#
#
"""Add fractional seconds to mysql tables
Revision ID: 4addfa1236f1
Revises: f2ca10b85618
Create Date: 2016-09-11 13:39:18.592072
"""
from alembic import op
from sqlalchemy.dialects import mysql
revision = '4addfa1236f1'
down_revision = 'f2ca10b85618'
branch_labels = None
depends_on = None
def upgrade():  
    conn = op.get_bind()
    if conn.dialect.name == "mysql":
        op.alter_column(table_name='dag', column_name='last_scheduler_run', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='dag', column_name='last_pickled', type_=mysql.DATETIME(fsp=6))
        op.alter_column(table_name='dag', column_name='last_expired', type_=mysql.DATETIME(fsp=6))