Airflow heartbeat FATAL: sorry, too many clients already - sqlalchemy

Question

Has anyone encountered "sorry, too many clients already" error on heartbeat phase while running a subdag?
[2017-11-22 08:21:23,747] {jobs.py:2136} ERROR - Exception while
trying to heartbeat! Sleeping for 5.0s Traceback (most recent call
last): File
"/usr/local/lib/python2.7/site-packages/airflow/jobs.py", line 2131,
in _execute
self.heartbeat() File "/usr/local/lib/python2.7/site-packages/airflow/jobs.py", line 180, in
heartbeat
job = session.query(BaseJob).filter(BaseJob.id == self.id).first() File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py",
line 2755, in first
ret = list(self[0:1]) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line
2547, in getitem
return list(res) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line
2855, in iter
return self._execute_and_instances(context) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line
2876, in _execute_and_instances
close_with_result=True) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line
2885, in _get_bind_args
**kw File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line
2867, in _connection_from_session
conn = self.session.connection(**kw) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py",
line 998, in connection
execution_options=execution_options) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py",
line 1003, in _connection_for_bind
engine, execution_options) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py",
line 403, in _connection_for_bind
conn = bind.contextual_connect() File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py",
line 2112, in contextual_connect
self._wrap_pool_connect(self.pool.connect, None), File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py",
line 2151, in _wrap_pool_connect
e, dialect, self) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py",
line 1465, in _handle_dbapi_exception_noconnection
exc_info File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py",
line 203, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb, cause=cause) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py",
line 2147, in _wrap_pool_connect
return fn() File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 387,
in connect
return _ConnectionFairy._checkout(self) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 766,
in _checkout
fairy = _ConnectionRecord.checkout(pool) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 516,
in checkout
rec = pool._do_get() File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line
1229, in _do_get
return self._create_connection() File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 333,
in _create_connection
return _ConnectionRecord(self) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 461,
in init
self.connect(first_connect_check=True) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 651,
in __connect
connection = pool._invoke_creator(self) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/strategies.py",
line 105, in connect
return dialect.connect(*cargs, **cparams) File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py",
line 393, in connect
return self.dbapi.connect(*cargs, **cparams) File "/usr/local/lib/python2.7/site-packages/psycopg2/__init.py", line
130, in connect
conn = _connect(dsn, connection_factory=connection_factory, **kwasync) OperationalError: (psycopg2.OperationalError) FATAL: sorry, too many clients already
from airflow.cfg:
on airflow.cfg:
sql_alchemy_pool_size = 100
sql_alchemy_pool_recycle = 3600
Is there a known solution to this problem?
I'm using airflow 1.8.2 with LocalExecutor.