Frozen Indices

Elasticsearch frozen indices are a useful and powerful tool for hot/warm architecture introduced in Elasticsearch 6.6, essentially by trading speed for memory. Elasticsearch SQL supports frozen indices and similar to Elasticsearch, due to their performance characteristics, allows searches on them only when explicitly told so by user - in other words, by default, frozen indices are not included in searches.

One can toggle the use of frozen indices through:

dedicated configuration parameter
Set to true properties index_include_frozen in the SQL REST API or index.include.frozen in the drivers to include frozen indices.
dedicated keyword
Explicitly perform the inclusion through the dedicated FROZEN keyword in the FROM clause or INCLUDE FROZEN in the SHOW commands:
SHOW TABLES INCLUDE FROZEN;

     name      |     type      |     kind
---------------+---------------+---------------
archive        |BASE TABLE     |FROZEN INDEX
emp            |BASE TABLE     |INDEX
employees      |VIEW           |ALIAS
library        |BASE TABLE     |INDEX
SELECT * FROM FROZEN archive LIMIT 1;

     author      |        name        |  page_count   |    release_date
-----------------+--------------------+---------------+--------------------
James S.A. Corey |Leviathan Wakes     |561            |2011-06-02T00:00:00Z

Unless enabled, frozen indices are completely ignored; it is as if they do not exist and as such, queries ran against them are likely to fail.