I would do a select statement like this:
curl -X POST "localhost:9200/_sql?format=txt&pretty" -H 'Content-Type: application/json' -d'
"query": "SELECT * FROM greylog_10"
But that gives me an error that indicates that this works entirely different. According to the linked documentation i would expect a list of results and not a write attempt to the table/index/frankly i don't know where it tried to write.
According to the linked article the index name should be usable like a table name but that does not seem to be the case. What am i doing wrong?