planner: do not eliminate group_concat in aggregate elimination (#9967)

cmd/explaintest/r/explain.result

@@ -22,3 +22,18 @@ c1 timestamp YES NULL
 desc t id;
 Field Type Null Key Default Extra
 id int(11) YES NULL 
+drop table if exists t;
+create table t(id int primary key, a int, b int);
+explain select group_concat(a) from t group by id;
+id count task operator info
+StreamAgg_8 8000.00 root group by:col_1, funcs:group_concat(col_0, ",")
+└─Projection_18 10000.00 root cast(test.t.a), test.t.id
+ └─TableReader_15 10000.00 root data:TableScan_14
+ └─TableScan_14 10000.00 cop table:t, range:[-inf,+inf], keep order:true, stats:pseudo
+explain select group_concat(a, b) from t group by id;
+id count task operator info
+StreamAgg_8 8000.00 root group by:col_2, funcs:group_concat(col_0, col_1, ",")
+└─Projection_18 10000.00 root cast(test.t.a), cast(test.t.b), test.t.id
+ └─TableReader_15 10000.00 root data:TableScan_14
+ └─TableScan_14 10000.00 cop table:t, range:[-inf,+inf], keep order:true, stats:pseudo
+drop table t;

cmd/explaintest/t/explain.test

@@ -6,3 +6,9 @@ describe t;
 desc t;
 desc t c1;
 desc t id;
+
+drop table if exists t;
+create table t(id int primary key, a int, b int);
+explain select group_concat(a) from t group by id;
+explain select group_concat(a, b) from t group by id;
+drop table t;

executor/aggregate_test.go

@@ -582,6 +582,7 @@ func (s *testSuite) TestAggEliminator(c *C) {
  tk.MustQuery("select min(b) from t").Check(testkit.Rows("-2"))
  tk.MustQuery("select max(b*b) from t").Check(testkit.Rows("4"))
  tk.MustQuery("select min(b*b) from t").Check(testkit.Rows("1"))
+ tk.MustQuery("select group_concat(b, b) from t group by a").Check(testkit.Rows("-1-1", "-2-2", "11", "<nil>"))
 }
 
 func (s *testSuite) TestMaxMinFloatScalaFunc(c *C) {

planner/core/rule_aggregation_push_down.go

@@ -391,6 +391,20 @@ func (a *aggregationOptimizer) aggPushDown(p LogicalPlan) LogicalPlan {
 // For count(expr), sum(expr), avg(expr), count(distinct expr, [expr...]) we may need to rewrite the expr. Details are shown below.
 // If we can eliminate agg successful, we return a projection. Else we return a nil pointer.
 func (a *aggregationOptimizer) tryToEliminateAggregation(agg *LogicalAggregation) *LogicalProjection {
+ for _, af := range agg.AggFuncs {
+ // TODO(issue #9968): Actually, we can rewrite GROUP_CONCAT when all the
+ // arguments it accepts are promised to be NOT-NULL.
+ // When it accepts only 1 argument, we can extract this argument into a
+ // projection.
+ // When it accepts multiple arguments, we can wrap the arguments with a
+ // function CONCAT_WS and extract this function into a projection.
+ // BUT, GROUP_CONCAT should truncate the final result according to the
+ // system variable `group_concat_max_len`. To ensure the correctness of
+ // the result, we close the elimination of GROUP_CONCAT here.
+ if af.Name == ast.AggFuncGroupConcat {
+ return nil
+ }
+ }
  schemaByGroupby := expression.NewSchema(agg.groupByCols...)
  coveredByUniqueKey := false
  for _, key := range agg.children[0].Schema().Keys {