95 lines
4.0 KiB
Markdown
95 lines
4.0 KiB
Markdown
---
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layout: default
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title: Limitations
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parent: SQL and PPL
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nav_order: 99
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---
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# Limitations
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The SQL plugin has the following limitations:
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## Aggregation over expression is not supported
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You can only apply aggregation to fields. Aggregations cannot accept an expression as a parameter. For example, `avg(log(age))` is not supported.
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## Subquery in the FROM clause
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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:
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```sql
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SELECT t.f, t.d
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FROM (
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SELECT FlightNum as f, DestCountry as d
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FROM opensearch_dashboards_sample_data_flights
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WHERE OriginCountry = 'US') t
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```
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But, if the outer query has `GROUP BY` or `ORDER BY`, then it's not supported.
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## JOIN does not support aggregations on the joined result
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The `join` query does not support aggregations on the joined result.
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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.
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## Pagination only supports basic queries
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The pagination query enables you to get back paginated responses.
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Currently, the pagination only supports basic queries. For example, the following query returns the data with cursor id.
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```json
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POST _plugins/_sql/
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{
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"fetch_size" : 5,
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"query" : "SELECT OriginCountry, DestCountry FROM opensearch_dashboards_sample_data_flights ORDER BY OriginCountry ASC"
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}
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```
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The response in JDBC format with cursor id.
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```json
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{
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"schema": [
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{
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"name": "OriginCountry",
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"type": "keyword"
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},
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{
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"name": "DestCountry",
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"type": "keyword"
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}
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],
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"cursor": "d:eyJhIjp7fSwicyI6IkRYRjFaWEo1UVc1a1JtVjBZMmdCQUFBQUFBQUFCSllXVTJKVU4yeExiWEJSUkhsNFVrdDVXVEZSYkVKSmR3PT0iLCJjIjpbeyJuYW1lIjoiT3JpZ2luQ291bnRyeSIsInR5cGUiOiJrZXl3b3JkIn0seyJuYW1lIjoiRGVzdENvdW50cnkiLCJ0eXBlIjoia2V5d29yZCJ9XSwiZiI6MSwiaSI6ImtpYmFuYV9zYW1wbGVfZGF0YV9mbGlnaHRzIiwibCI6MTMwNTh9",
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"total": 13059,
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"datarows": [[
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"AE",
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"CN"
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]],
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"size": 1,
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"status": 200
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}
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```
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The query with `aggregation` and `join` does not support pagination for now.
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## Query processing engines
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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.
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### V1 engine limitations
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* The select literal expression without `FROM` clause is not supported. For example, `SELECT 1` is not supported.
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* The `WHERE` clause does not support expressions. For example, `SELECT FlightNum FROM opensearch_dashboards_sample_data_flights where (AvgTicketPrice + 100) <= 1000` is not supported.
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* Most [relevancy search functions]({{site.url}}{{site.baseurl}}/search-plugins/sql/full-text/) are implemented in the `V2` engine only.
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Such queries are successfully executed by the `V2` engine unless they have `V1`-specific functions. You will likely never meet these limitations.
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### V2 engine limitations
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* The [cursor feature](#pagination-only-supports-basic-queries) is supported by the `V1` engine only.
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For support of `cursor`/`pagination` in the `V2` engine, track [GitHub issue #656](https://github.com/opensearch-project/sql/issues/656).
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* The `V2` engine does not track query execution time, so slow queries are not reported.
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* 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.
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* The `V2` engine does not support [`SCORE_QUERY`]({{site.url}}{{site.baseurl}}/search-plugins/sql/sql/functions#score-query) and [`WILDCARD_QUERY`]({{site.url}}{{site.baseurl}}/search-plugins/sql/sql/functions#wildcard-query) functions.
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