Limitations

The SQL plugin has the following limitations:

Aggregation over expression is not supported

You can only apply aggregation to fields. Aggregations cannot accept an expression as a parameter. For example, avg(log(age)) is not supported.

Subquery in the FROM clause

Subquery in the FROM clause in this format: SELECT outer FROM (SELECT inner) is supported only when the query is merged into one query. For example, the following query is supported:

SELECT t.f, t.d
FROM (
    SELECT FlightNum as f, DestCountry as d
    FROM opensearch_dashboards_sample_data_flights
    WHERE OriginCountry = 'US') t

But, if the outer query has GROUP BY or ORDER BY, then it's not supported.

JOIN does not support aggregations on the joined result

The join query does not support aggregations on the joined result. For example, e.g. SELECT depo.name, avg(empo.age) FROM empo JOIN depo WHERE empo.id == depo.id GROUP BY depo.name is not supported.

Pagination only supports basic queries

The pagination query enables you to get back paginated responses.

Currently, the pagination only supports basic queries. For example, the following query returns the data with cursor id.

POST _plugins/_sql/
{
  "fetch_size" : 5,
  "query" : "SELECT OriginCountry, DestCountry FROM opensearch_dashboards_sample_data_flights ORDER BY OriginCountry ASC"
}

The response in JDBC format with cursor id.

{
  "schema": [
    {
      "name": "OriginCountry",
      "type": "keyword"
    },
    {
      "name": "DestCountry",
      "type": "keyword"
    }
  ],
  "cursor": "d:eyJhIjp7fSwicyI6IkRYRjFaWEo1UVc1a1JtVjBZMmdCQUFBQUFBQUFCSllXVTJKVU4yeExiWEJSUkhsNFVrdDVXVEZSYkVKSmR3PT0iLCJjIjpbeyJuYW1lIjoiT3JpZ2luQ291bnRyeSIsInR5cGUiOiJrZXl3b3JkIn0seyJuYW1lIjoiRGVzdENvdW50cnkiLCJ0eXBlIjoia2V5d29yZCJ9XSwiZiI6MSwiaSI6ImtpYmFuYV9zYW1wbGVfZGF0YV9mbGlnaHRzIiwibCI6MTMwNTh9",
  "total": 13059,
  "datarows": [[
    "AE",
    "CN"
  ]],
  "size": 1,
  "status": 200
}

The query with aggregation and join does not support pagination for now.

Query processing engines

The SQL plugin has two query processing engines, V1 and V2. Most of the features are supported by both engines, but only the new engine is actively being developed. A query that is first executed on the V2 engine falls back to the V1 engine in case of failure. If a query is supported in V2 but not included in V1, the query will fail with an error response.

V1 engine limitations

The select literal expression without FROM clause is not supported. For example, SELECT 1 is not supported.
The WHERE clause does not support expressions. For example, SELECT FlightNum FROM opensearch_dashboards_sample_data_flights where (AvgTicketPrice + 100) <= 1000 is not supported.
Most relevancy search functions are implemented in the V2 engine only.

Such queries are successfully executed by the V2 engine unless they have V1-specific functions. You will likely never meet these limitations.

V2 engine limitations

The cursor feature is supported by the V1 engine only. For support of cursor/pagination in the V2 engine, track GitHub issue #656.
The V2 engine does not track query execution time, so slow queries are not reported.
The V2 query engine not only runs queries in the OpenSearch engine but also supports post-processing for complicated queries. Accordingly, the explain output is no longer pure OpenSearch domain-specific language (DSL) but also includes query plan information from the V2 query engine.
The V2 engine does not support SCORE_QUERY and WILDCARD_QUERY functions.

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