Thanks for the answer ... the error described before is: "ORDER BY is only
supported when the partition key is restricted by an EQ or an IN."
But I don't see how I didn't respect the rule ...

The error message is indeed somewhat misleading and I've just committed a fix to return a better message. But at the end of the day, the limitation is that ORDER BY is just not supported with 2ndary indexes.
 
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Sylvain