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Additional documentation for query caching (#10503) 

* Add documentation for when caching is unsupported

* Minor changes

* Minor doc fix

* Review comments

* Add more details

* Fix spelling check

* Fix doc for union query

* Trailing dot
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Abhishek Agarwal 2020-10-21 02:19:13 +05:30 committed by GitHub
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14
docs/querying/caching.md
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@ -82,3 +82,17 @@ Note that the task executor processes only support caches that keep their data l
This restriction exists because the cache stores results at the level of intermediate partial segments generated by the
ingestion tasks. These intermediate partial segments will not necessarily be identical across task replicas, so
remote cache types such as `memcached` will be ignored by task executor processes.
## Unsupported queries
Query caching is not available for following:
- Queries, that involve a `union` datasource, do not support result-level caching. Refer to the
[related issue](https://github.com/apache/druid/issues/8713) for details. Please note that not all union SQL queries are executed using a union datasource. You can use the `explain` operation to see how the union query in sql will be executed.
- Queries, that involve an `Inline` datasource or a `Lookup` datasource, do not support any caching.
- Queries, with a sub-query in them, do not support any caching though the output of sub-queries itself may be cached.
Refer to the [Query execution](query-execution.md#query) page for more details on how sub-queries are executed.
- Join queries do not support any caching on the broker [More details](https://github.com/apache/druid/issues/10444).
- GroupBy v2 queries do not support any caching on broker [More details](https://github.com/apache/druid/issues/3820).
- Data Source Metadata queries are not cached anywhere.
- Queries, that have `bySegment` set in the query context, are not cached on the broker. They are currently cached on
historical but this behavior will potentially be removed in the future.

12
docs/querying/datasource.md
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@ -113,6 +113,16 @@ use table datasources.
### `union`
<!--DOCUSAURUS_CODE_TABS-->
<!--SQL-->
```sql
SELECT col1, COUNT(*)
FROM (
SELECT col1, col2, col3 FROM tbl1
UNION ALL
SELECT col1, col2, col3 FROM tbl2
)
GROUP BY col1
```
<!--Native-->
```json
{
@ -134,8 +144,6 @@ another will be treated as if they contained all null values in the tables where
The list of "dataSources" must be nonempty. If you want to query an empty dataset, use an [`inline` datasource](#inline)
instead.
Union datasources are not available in Druid SQL.
Refer to the [Query execution](query-execution.md#union) page for more details on how queries are executed when you
use union datasources.