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:

  1. SELECT t.f, t.d
  2. FROM (
  3.     SELECT FlightNum as f, DestCountry as d
  4.     FROM opensearch_dashboards_sample_data_flights
  5.     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.

  1. POST _plugins/_sql/
  2. {
  3.   "fetch_size" : 5,
  4.   "query" : "SELECT OriginCountry, DestCountry FROM opensearch_dashboards_sample_data_flights ORDER BY OriginCountry ASC"
  5. }

The response in JDBC format with cursor id.

  1. {
  2.   "schema": [
  3.     {
  4.       "name": "OriginCountry",
  5.       "type": "keyword"
  6.     },
  7.     {
  8.       "name": "DestCountry",
  9.       "type": "keyword"
  10.     }
  11.   ],
  12.   "cursor": "d:eyJhIjp7fSwicyI6IkRYRjFaWEo1UVc1a1JtVjBZMmdCQUFBQUFBQUFCSllXVTJKVU4yeExiWEJSUkhsNFVrdDVXVEZSYkVKSmR3PT0iLCJjIjpbeyJuYW1lIjoiT3JpZ2luQ291bnRyeSIsInR5cGUiOiJrZXl3b3JkIn0seyJuYW1lIjoiRGVzdENvdW50cnkiLCJ0eXBlIjoia2V5d29yZCJ9XSwiZiI6MSwiaSI6ImtpYmFuYV9zYW1wbGVfZGF0YV9mbGlnaHRzIiwibCI6MTMwNTh9",
  13.   "total": 13059,
  14.   "datarows": [[
  15.     "AE",
  16.     "CN"
  17.   ]],
  18.   "size": 1,
  19.   "status": 200
  20. }

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.
  • json formatted output is supported in V1 engine only.
  • 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 complex queries. Accordingly, the explain output is no longer OpenSearch domain-specific language (DSL) but also includes query plan information from the V2 query engine.
  • The V2 query engine does not support aggregation queries such as histogram, date_histogram, percentiles, topHits, stats, extended_stats, terms, or range.
  • JOINs and sub-queries are not supported. To stay up to date on the development for JOINs and sub-queries, track GitHub issue #1441 and GitHub issue #892.
  • OpenSearch does not natively support the array data type but does allow multi-value fields implicitly. The SQL/PPL plugin adheres strictly to the data type semantics defined in index mappings. When parsing OpenSearch responses, it expects data to match the declared type and does not interpret all data in an array. If the plugins.query.field_type_tolerance setting is enabled, the SQL/PPL plugin handles array datasets by returning scalar data types, allowing basic queries (for example, SELECT * FROM tbl WHERE condition). However, using multi-value fields in expressions or functions will result in exceptions. If this setting is disabled or not set, only the first element of an array is returned, preserving the default behavior.
  • PartiQL syntax for nested queries is not supported.