I'm having troubles to find the correct query to get all documents
where a specific field is not set at all.
My test index has 3 text fields: field1, field2, field3. Some
documents, have all fields filled, some only one or two of the fields.
What would be the query to get all documents with an empty field2?
Let's say time goes by, the index grew, and I like to add another
field "field4". Do I have to reindex all documents in the index again
for such a schema change?