This functionality is in beta and is subject to change. The design and code is less mature than official GA features and is being provided as-is with no warranties. Beta features are not subject to the support SLA of official GA features.
Elasticsearch has a special type of relationship fields called
nested fields. In Elasticsearch SQL they can be used by referencing their inner
sub-fields. Even though
SYS COLUMNS in non-driver mode (in the CLI and in REST calls) and
DESCRIBE TABLE will still display
them as having the type
NESTED, they cannot be used in a query. One can only reference its sub-fields in the form:
SELECT dep.dep_name.keyword FROM test_emp GROUP BY languages;
Elasticsearch SQL doesn’t support multi-nested documents, so a query cannot reference more than one nested field in an index. This applies to multi-level nested fields, but also multiple nested fields defined on the same level. For example, for this index:
column | type | mapping ----------------------+---------------+------------- nested_A |STRUCT |NESTED nested_A.nested_X |STRUCT |NESTED nested_A.nested_X.text|VARCHAR |KEYWORD nested_A.text |VARCHAR |KEYWORD nested_B |STRUCT |NESTED nested_B.text |VARCHAR |KEYWORD
nested_B cannot be used at the same time, nor
For such situations, Elasticsearch SQL will display an error message.
When SELECTing a nested field, pagination will not work as expected, Elasticsearch SQL will return at least the page size records. This is because of the way nested queries work in Elasticsearch: the root nested field will be returned and it’s matching inner nested fields as well, pagination taking place on the root nested document and not on its inner hits.
keyword fields in Elasticsearch can be normalized by defining a
normalizer. Such fields are not supported in Elasticsearch SQL.
Array fields are not supported due to the "invisible" way in which Elasticsearch handles an array of values: the mapping doesn’t indicate whether a field is an array (has multiple values) or not, so without reading all the data, Elasticsearch SQL cannot know whether a field is a single or multi value.
When doing aggregations (
GROUP BY) Elasticsearch SQL relies on Elasticsearch’s
composite aggregation for its support for paginating results.
But this type of aggregation does come with a limitation: sorting can only be applied on the key used for the aggregation’s buckets. This
means that queries like
SELECT * FROM test GROUP BY age ORDER BY COUNT(*) are not possible.
Using sub-selects (
SELECT X FROM (SELECT Y)) is supported to a small degree: any sub-select that can be "flattened" into a single
SELECT is possible with Elasticsearch SQL. For example:
SELECT * FROM (SELECT first_name, last_name FROM emp WHERE last_name NOT LIKE '%a%') WHERE first_name LIKE 'A%' ORDER BY 1; first_name | last_name ---------------+--------------- Alejandro |McAlpine Anneke |Preusig Anoosh |Peyn Arumugam |Ossenbruggen
The query above is possible because it is equivalent with:
SELECT first_name, last_name FROM emp WHERE last_name NOT LIKE '%a%' AND first_name LIKE 'A%' ORDER BY 1;
But, if the sub-select would include a
GROUP BY or
HAVING or the enclosing
SELECT would be more complex than
FROM (SELECT ...) WHERE [simple_condition], this is currently un-supported.