MDEV-7904: ANALYZE FORMAT=JSON doesn't print r_rows for union output

Print r_rows. There is no table tracking for reading from tmp table, yet.

mysql-test/r/analyze_format_json.result

@@ -405,3 +405,61 @@ ANALYZE
   }
 }
 drop table t1,t2,t3,t4;
+#
+# MDEV-7904: ANALYZE FORMAT=JSON SELECT .. UNION SELECT doesn't print r_rows for union output
+#
+create table t0 (a int);
+INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (a int);
+INSERT INTO t1 select * from t0;
+analyze format=json (select * from t1 A where a<5) union (select * from t1 B where a in (2,3));
+ANALYZE
+{
+  "query_block": {
+    "union_result": {
+      "table_name": "<union1,2>",
+      "access_type": "ALL",
+      "r_loops": 1,
+      "r_rows": 5,
+      "query_specifications": [
+        {
+          "query_block": {
+            "select_id": 1,
+            "r_loops": 1,
+            "r_total_time_ms": "REPLACED",
+            "table": {
+              "table_name": "A",
+              "access_type": "ALL",
+              "r_loops": 1,
+              "rows": 10,
+              "r_rows": 10,
+              "r_total_time_ms": "REPLACED",
+              "filtered": 100,
+              "r_filtered": 50,
+              "attached_condition": "(A.a < 5)"
+            }
+          }
+        },
+        {
+          "query_block": {
+            "select_id": 2,
+            "r_loops": 1,
+            "r_total_time_ms": "REPLACED",
+            "table": {
+              "table_name": "B",
+              "access_type": "ALL",
+              "r_loops": 1,
+              "rows": 10,
+              "r_rows": 10,
+              "r_total_time_ms": "REPLACED",
+              "filtered": 100,
+              "r_filtered": 20,
+              "attached_condition": "(B.a in (2,3))"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+drop table t0, t1;

mysql-test/t/analyze_format_json.test

@@ -134,3 +134,19 @@ select * from t1, t2 where (t2.key1 between t1.lb1 and t1.rb1) and
                            (t2.key3=t1.c1  OR t2.key4=t1.c2);
 
 drop table t1,t2,t3,t4;
+
+--echo #
+--echo # MDEV-7904: ANALYZE FORMAT=JSON SELECT .. UNION SELECT doesn't print r_rows for union output
+--echo #
+create table t0 (a int);
+INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (a int);
+INSERT INTO t1 select * from t0;
+
+--replace_regex /"r_total_time_ms": [0-9]*[.]?[0-9]*/"r_total_time_ms": "REPLACED"/
+analyze format=json (select * from t1 A where a<5) union (select * from t1 B where a in (2,3));
+
+drop table t0, t1;
+
+

sql/sql_explain.cc

@@ -514,6 +514,23 @@ void Explain_union::print_explain_json(Explain_query *query,
   make_union_table_name(table_name_buffer);
   writer->add_member("table_name").add_str(table_name_buffer);
   writer->add_member("access_type").add_str("ALL"); // not very useful
+
+  /* r_loops (not present in tabular output) */
+  if (is_analyze)
+  {
+    writer->add_member("r_loops").add_ll(fake_select_lex_tracker.get_loops());
+  }
+
+  /* `r_rows` */
+  if (is_analyze)
+  {
+    writer->add_member("r_rows");
+    if (fake_select_lex_tracker.has_scans())
+      writer->add_double(fake_select_lex_tracker.get_avg_rows());
+    else
+      writer->add_null();
+  }
+
   writer->add_member("query_specifications").start_array();
 
   for (int i= 0; i < (int) union_members.elements(); i++)