CountDistinctRewriteProc (Hive 3.1.1 API)

java.lang.Object
- org.apache.hadoop.hive.ql.optimizer.Transform
- - org.apache.hadoop.hive.ql.optimizer.CountDistinctRewriteProc

```
public class CountDistinctRewriteProc
extends Transform
```
Queries of form : select max(c), count(distinct c) from T; generates a plan of form TS->mGBy->RS->rGBy->FS This plan suffers from a problem that vertex containing rGBy->FS necessarily need to have 1 task. This limitation results in slow execution because that task gets all the data. This optimization if successful will rewrite above plan to mGby1-rs1-mGby2-mGby3-rs2-rGby1 This introduces extra vertex of mGby2-mGby3-rs2. Note this vertex can have multiple tasks and since we are doing aggregation, output of this must necessarily be smaller than its input, which results in much less data going in to original rGby->FS vertex, which continues to have single task. Also note on calcite tree we have HiveExpandDistinctAggregatesRule rule which does similar plan transformation but has different conditions which needs to be satisfied. Additionally, we don't do any costing here but this is possibly that this transformation may slow down query a bit since if data is small enough to fit in a single task of last reducer, injecting additional vertex in pipeline may make query slower. If this happens, users can use the configuration hive.optimize.countdistinct to turn it off.

Nested Classes
Modifier and Type	Class and Description
`class`	`CountDistinctRewriteProc.CountDistinctProcessor` CountDistinctProcessor.

Constructors
Constructor and Description
`CountDistinctRewriteProc()`

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`ParseContext`	`transform(ParseContext pctx)` All transformation steps implement this interface.

Class CountDistinctRewriteProc