SQL: Clarify approximate distinct count behavior. (#4000)

docs/content/querying/sql.md

@@ -104,10 +104,11 @@ You can access table and column metadata through JDBC using `connection.getMetaD
 
 The following SQL queries and features may be executed using approximate algorithms:
 
-- `COUNT(DISTINCT col)` and `APPROX_COUNT_DISTINCT(col)` aggregations use
+- `COUNT(DISTINCT col)` and `APPROX_COUNT_DISTINCT(col)` aggregations by default use
 [HyperLogLog](http://algo.inria.fr/flajolet/Publications/FlFuGaMe07.pdf), a fast approximate distinct counting
-algorithm. If you need exact distinct counts, set "useApproximateCountDistinct" to "false", either through query
-context or through broker configuration.
+algorithm. To disable this behavior for `COUNT(DISTINCT col)`, and use exact distinct counts, set
+"useApproximateCountDistinct" to "false", either through query context or through broker configuration.
+`APPROX_COUNT_DISTINCT(col)` is always approximate, regardless of this setting.
 - TopN-style queries with a single grouping column, like
 `SELECT col1, SUM(col2) FROM data_source GROUP BY col1 ORDER BY SUM(col2) DESC LIMIT 100`, by default will be executed
 as [TopN queries](topnquery.html), which use an approximate algorithm. To disable this behavior, and use exact

sql/src/test/java/io/druid/sql/calcite/CalciteQueryTest.java

@@ -1928,6 +1928,38 @@ public class CalciteQueryTest
     );
   }
 
+  @Test
+  public void testApproxCountDistinctWhenHllDisabled() throws Exception
+  {
+    // When HLL is disabled, APPROX_COUNT_DISTINCT is still approximate.
+
+    testQuery(
+        PLANNER_CONFIG_NO_HLL,
+        "SELECT APPROX_COUNT_DISTINCT(dim2) FROM druid.foo",
+        ImmutableList.<Query>of(
+            Druids.newTimeseriesQueryBuilder()
+                  .dataSource(CalciteTests.DATASOURCE1)
+                  .intervals(QSS(Filtration.eternity()))
+                  .granularity(Granularities.ALL)
+                  .aggregators(
+                      AGGS(
+                          new CardinalityAggregatorFactory(
+                              "a0",
+                              null,
+                              DIMS(new DefaultDimensionSpec("dim2", null)),
+                              false
+                          )
+                      )
+                  )
+                  .context(TIMESERIES_CONTEXT_DEFAULT)
+                  .build()
+        ),
+        ImmutableList.of(
+            new Object[]{3L}
+        )
+    );
+  }
+
   @Test
   public void testExactCountDistinctWithGroupingAndOtherAggregators() throws Exception
   {