EXPLAIN PLAN statements return a `RESOURCES` column that describes the resource being queried as well as a `PLAN` column that contains a JSON array of native queries that Druid will run.
For example, consider the following query:
```sql
EXPLAIN PLAN FOR
SELECT
channel,
COUNT(*)
FROM wikipedia
WHERE channel IN (SELECT page FROM wikipedia GROUP BY page ORDER BY COUNT(*) DESC LIMIT 10)
- [Timeseries](timeseriesquery.md) is used for queries that GROUP BY `FLOOR(__time TO unit)` or `TIME_FLOOR(__time, period)`, have no other grouping expressions, no HAVING clause, no nesting, and either no ORDER BY, or an
-`FLOOR(__time TO DAY) = TIMESTAMP '2000-01-01 00:00:00'` (specific day)
Refer to the [Interpreting EXPLAIN PLAN output](#interpreting-explain-plan-output) section for details on confirming
that time filters are being translated as you expect.
## Joins
SQL join operators are translated to native join datasources as follows:
1. Joins that the native layer can handle directly are translated literally, to a [join datasource](datasource.md#join)
whose `left`, `right`, and `condition` are faithful translations of the original SQL. This includes any SQL join where
the right-hand side is a lookup or subquery, and where the condition is an equality where one side is an expression based
on the left-hand table, the other side is a simple column reference to the right-hand table, and both sides of the
equality are the same data type.
2. If a join cannot be handled directly by a native [join datasource](datasource.md#join) as written, Druid SQL
will insert subqueries to make it runnable. For example, `foo INNER JOIN bar ON foo.abc = LOWER(bar.def)` cannot be
directly translated, because there is an expression on the right-hand side instead of a simple column access. A subquery
will be inserted that effectively transforms this clause to
`foo INNER JOIN (SELECT LOWER(def) AS def FROM bar) t ON foo.abc = t.def`.
3. Druid SQL does not currently reorder joins to optimize queries.
Refer to the [Interpreting EXPLAIN PLAN output](#interpreting-explain-plan-output) section for details on confirming
that joins are being translated as you expect.
Refer to the [Query execution](query-execution.md#join) page for information about how joins are executed.
## Subqueries
Subqueries in SQL are generally translated to native query datasources. Refer to the
[Query execution](query-execution.md#query) page for information about how subqueries are executed.
> Note: Subqueries in the WHERE clause, like `WHERE col1 IN (SELECT foo FROM ...)` are translated to inner joins.
## Approximations
Druid SQL will use approximate algorithms in some situations:
- The `COUNT(DISTINCT col)` aggregation functions by default uses a variant of
[HyperLogLog](http://algo.inria.fr/flajolet/Publications/FlFuGaMe07.pdf), a fast approximate distinct counting
algorithm. Druid SQL will switch to exact distinct counts if you set "useApproximateCountDistinct" to "false", either
through query context or through Broker configuration.
- GROUP BY queries over a single column with ORDER BY and LIMIT may be executed using the TopN engine, which uses an
approximate algorithm. Druid SQL will switch to an exact grouping algorithm if you set "useApproximateTopN" to "false",
either through query context or through Broker configuration.
- Aggregation functions that are labeled as using sketches or approximations, such as APPROX_COUNT_DISTINCT, are always
approximate, regardless of configuration.
**A known issue with approximate functions based on data sketches**
The `APPROX_QUANTILE_DS` and `DS_QUANTILES_SKETCH` functions can fail with an `IllegalStateException` if one of the sketches for
the query hits `maxStreamLength`: the maximum number of items to store in each sketch.
See [GitHub issue 11544](https://github.com/apache/druid/issues/11544) for more details.
To workaround the issue, increase value of the maximum string length with the `approxQuantileDsMaxStreamLength` parameter
in the query context. Since it is set to 1,000,000,000 by default, you don't need to override it in most cases.
See [accuracy information](https://datasketches.apache.org/docs/Quantiles/OrigQuantilesSketch) in the DataSketches documentation for how many bytes are required per stream length.
This query context parameter is a temporary solution to avoid the known issue. It may be removed in a future release after the bug is fixed.
