Can't connect to a local postgres DB from SQL notebook

java.sql.SQLException: Cannot create PoolableConnectionFactory ([Amazon][JDBC](10100) Connection Refused: [Amazon][JDBC](11640) Required Connection Key(s): PWD, UID; [Amazon][JDBC](11480) Optional Connection Key(s): AccessKeyID, AuthMech, AutoCreate, BlockingRowsMode, ClusterID, DbGroups, DisableIsValidQuery, DriverLogLevel, EndpointUrl, FilterLevel, IAMDuration, Language, loginTimeout, OpenSourceSubProtocolOverride, plugin_name, profile, Region, SecretAccessKey, SessionToken, socketTimeout, ssl, sslcert, sslfactory, sslkey, sslpassword, sslrootcert, SSLTruststore , SSLTrustStorePath, tcpKeepAlive, TCPKeepAliveMinutes, unknownLength)

Reproduce:
1. Open a new SQL notebook
2. Add the following two lines:
%defaultDatasource jdbc:postgresql://localhost:5432/mydb
SELECT 1;