I don't believe stored procedures are supported on any open source database platforms, unfortunately. See the quote from the following: Stored Procedures
A stored procedure is a subroutine available to applications that access a relational database system. When you connect to a SAP Sybase ASE, Microsoft SQL Server, or Teradata database with Tableau, you can use a stored procedure to define the connection.
SQL Server has a free edition, SQL Server Express, but there a lots of limitations to it. That being the case, you may need to consider a different option. Could you schedule a stored procedure to build some tables which can then be consumed in Tableau?