How to do join in Opensearch 2.3 with SQL

Question

This is driving me crazy. I am using Opensearch 2.3 (on a AWS managed domain). I have 2 indices which looks something like this:

orders
.userId

sessions
.userId

And I want to do the following SQL query:

SELECT COUNT(*) FROM orders JOIN sessions ON orders.userId = sessions.userId 

but that gives me

Index type [orders AS orders_0] does not exist

And I read somewhere that I need to alias index if doing joins, so I try this:

SELECT COUNT(*) FROM orders o JOIN sessions s ON o.userId = s.userId 

but that gives me

Index type [orders AS o] does not exist

I don't know what else to try.

(Running SELECT * FROM orders gives rows and the same goes for SELECT * FROM sessions. So the indices do exists.)