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**Description**

Update source_code field of dag_code table to MEDIUMTEXT
<!-- A short description of your feature -->

**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/d2ae31099d61_increase_text_size_for_mysql.py
#
#
#
"""Increase text size for MySQL (not relevant for other DBs' text types)
Revision ID: d2ae31099d61
Revises: 947454bf1dff
Create Date: 2017-08-18 17:07:16.686130
"""
from alembic import op
from sqlalchemy.dialects import mysql
revision = 'd2ae31099d61'
down_revision = '947454bf1dff'
branch_labels = None
depends_on = None
def upgrade():  
    conn = op.get_bind()
    if conn.dialect.name == "mysql":
        op.alter_column(table_name='variable', column_name='val', type_=mysql.MEDIUMTEXT)
def downgrade():  
    conn = op.get_bind()
    if conn.dialect.name == "mysql":
        op.alter_column(table_name='variable', column_name='val', type_=mysql.TEXT)