MDEV-23767: IN-to-subquery conversion is not visible in optimizer trace

Add the printout

mysql-test/main/opt_trace.result

@@ -8684,5 +8684,103 @@ JSON_DETAILED(JSON_EXTRACT(trace, '$**.mark_join_nest_as_const'))
     }
 ]
 drop table t0, t1, t2, t3;
+#
+# MDEV-23767: IN-to-subquery conversion is not visible in optimizer trace
+#
+create table t0 (a int);
+INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+set @tmp=@@in_predicate_conversion_threshold;
+set in_predicate_conversion_threshold=3;
+explain select * from t0 where a in (1,2,3,4,5,6);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+1	PRIMARY	<derived3>	ref	key0	key0	4	test.t0.a	2	FirstMatch(t0)
+3	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+[
+    
+    {
+        "item": "t0.a in (1,2,3,4,5,6)",
+        "conversion": 
+        [
+            
+            {
+                "join_preparation": 
+                {
+                    "select_id": 2,
+                    "steps": 
+                    [
+                        
+                        {
+                            "derived": 
+                            {
+                                "table": "tvc_0",
+                                "select_id": 3,
+                                "algorithm": "materialized"
+                            }
+                        },
+                        
+                        {
+                            "transformation": 
+                            {
+                                "select_id": 2,
+                                "from": "IN (SELECT)",
+                                "to": "materialization",
+                                "sjm_scan_allowed": true,
+                                "possible": true
+                            }
+                        },
+                        
+                        {
+                            "transformation": 
+                            {
+                                "select_id": 2,
+                                "from": "IN (SELECT)",
+                                "to": "semijoin",
+                                "chosen": true
+                            }
+                        },
+                        
+                        {
+                            "expanded_query": "/* select#2 */ select tvc_0._col_1 from (values (1),(2),(3),(4),(5),(6)) tvc_0"
+                        }
+                    ]
+                }
+            }
+        ]
+    }
+]
+explain select * from t0 where a in (1,2,3,4,5,a+1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+[
+    
+    {
+        "item": "t0.a in (1,2,3,4,5,t0.a + 1)",
+        "done": false,
+        "reason": "non-constant element in the IN-list"
+    }
+]
+explain select * from t0 where a in ('1','2','3','4','5','6');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+[
+    
+    {
+        "item": "t0.a in ('1','2','3','4','5','6')",
+        "done": false,
+        "reason": "type mismatch"
+    }
+]
+set in_predicate_conversion_threshold=@tmp;
+drop table t0;
 # End of 10.5 tests
 set optimizer_trace='enabled=off';

mysql-test/main/opt_trace.test

@@ -654,5 +654,31 @@ from information_schema.optimizer_trace;
 
 drop table t0, t1, t2, t3;
 
+--echo #
+--echo # MDEV-23767: IN-to-subquery conversion is not visible in optimizer trace
+--echo #
+create table t0 (a int);
+INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+set @tmp=@@in_predicate_conversion_threshold;
+set in_predicate_conversion_threshold=3;
+
+explain select * from t0 where a in (1,2,3,4,5,6);
+
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+
+explain select * from t0 where a in (1,2,3,4,5,a+1);
+
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+
+explain select * from t0 where a in ('1','2','3','4','5','6');
+select json_detailed(json_extract(trace, '$**.in_to_subquery_conversion'))
+from information_schema.optimizer_trace;
+
+set in_predicate_conversion_threshold=@tmp;
+drop table t0;
+
 --echo # End of 10.5 tests
 set optimizer_trace='enabled=off';

sql/sql_tvc.cc

@@ -22,6 +22,7 @@
 #include "sql_explain.h"
 #include "sql_parse.h"
 #include "sql_cte.h"
+#include "my_json_writer.h"
 
 
 /**
@@ -903,6 +904,10 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
   if (!transform_into_subq)
     return this;
   
+  Json_writer_object trace_wrapper(thd);
+  Json_writer_object trace_conv(thd, "in_to_subquery_conversion");
+  trace_conv.add("item", this);
+
   transform_into_subq= false;
 
   List<List_item> values;
@@ -922,13 +927,29 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
   uint32 length= max_length_of_left_expr();
   if (!length  || length > tmp_table_max_key_length() ||
       args[0]->cols() > tmp_table_max_key_parts())
+  {
+    trace_conv.add("done", false);
+    trace_conv.add("reason", "key is too long");
     return this;
-  
+  }
+
   for (uint i=1; i < arg_count; i++)
   {
-    if (!args[i]->const_item() || cmp_row_types(args[0], args[i]))
+    if (!args[i]->const_item())
+    {
+      trace_conv.add("done", false);
+      trace_conv.add("reason", "non-constant element in the IN-list");
       return this;
+    }
+    
+    if (cmp_row_types(args[0], args[i]))
+    {
+      trace_conv.add("done", false);
+      trace_conv.add("reason", "type mismatch");
+      return this;
+    }
   }
+  Json_writer_array trace_nested_obj(thd, "conversion");
 
   Query_arena backup;
   Query_arena *arena= thd->activate_stmt_arena_if_needed(&backup);
@@ -1020,6 +1041,7 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
     goto err;
 
   parent_select->curr_tvc_name++;
+
   return sq;
 
 err: