Backport of fix for Bug#52123

mysql-test/r/func_group.result

@@ -1739,6 +1739,16 @@ SELECT RELEASE_LOCK('aaaaaaaaaaaaaaaaa');
 #
 End of 5.1 tests
 #
+# Bug#52123 Assertion failed: aggregator == aggr->Aggrtype(),
+#           file .\item_sum.cc, line 587
+#
+CREATE TABLE t1(a int, KEY(a));
+INSERT INTO t1 VALUES (1), (2);
+SELECT 1 FROM t1 ORDER BY AVG(DISTINCT a);
+1
+1
+DROP TABLE t1;
+#
 # Bug#55648: Server crash on MIN/MAX on maximum time value
 #
 CREATE TABLE t1(c1 TIME NOT NULL);

mysql-test/t/func_group.test

@@ -1120,6 +1120,17 @@ SELECT RELEASE_LOCK('aaaaaaaaaaaaaaaaa');
 --echo #
 --echo End of 5.1 tests
 
+###
+--echo #
+--echo # Bug#52123 Assertion failed: aggregator == aggr->Aggrtype(),
+--echo #           file .\item_sum.cc, line 587
+--echo #
+
+CREATE TABLE t1(a int, KEY(a));
+INSERT INTO t1 VALUES (1), (2);
+SELECT 1 FROM t1 ORDER BY AVG(DISTINCT a);
+DROP TABLE t1;
+
 --echo #
 --echo # Bug#55648: Server crash on MIN/MAX on maximum time value
 --echo #

sql/item_sum.cc

@@ -556,24 +556,27 @@ Item *Item_sum::set_arg(uint i, THD *thd, Item *new_val)
 
 int Item_sum::set_aggregator(Aggregator::Aggregator_type aggregator)
 {
-  if (aggr)
-  {
   /*
     Dependent subselects may be executed multiple times, making
     set_aggregator to be called multiple times. The aggregator type
     will be the same, but it needs to be reset so that it is
     reevaluated with the new dependent data.
+    This function may also be called multiple times during query optimization.
+    In this case, the type may change, so we delete the old aggregator,
+    and create a new one.
   */
-    DBUG_ASSERT(aggregator == aggr->Aggrtype());
+  if (aggr && aggregator == aggr->Aggrtype())
+  {
     aggr->clear();
     return FALSE;
   }
+
+  delete aggr;
   switch (aggregator)
   {
   case Aggregator::DISTINCT_AGGREGATOR:
     aggr= new Aggregator_distinct(this);
     break;
-
   case Aggregator::SIMPLE_AGGREGATOR:
     aggr= new Aggregator_simple(this);
     break;

sql/item_sum.h

@@ -461,7 +461,6 @@ class Item_sum :public Item_result_field
   */
   virtual void no_rows_in_result()
   {
-    if (!aggr)
     set_aggregator(with_distinct ?
                    Aggregator::DISTINCT_AGGREGATOR :
                    Aggregator::SIMPLE_AGGREGATOR);
@@ -515,11 +514,10 @@ class Item_sum :public Item_result_field
     quick_group= with_distinct ? 0 : 1;
   }
 
-  /**
+  /*
     Set the type of aggregation : DISTINCT or not.
 
-    Called when the final determination is done about the aggregation
-    type and the object is about to be used.
+    May be called multiple times.
   */
 
   int set_aggregator(Aggregator::Aggregator_type aggregator);