MDEV-37195:

For all degenerate select queries having sub-queries in them,
the field rows_examined in the slow query log is always being set to 0.

The problem is that, although sub-queries increment the rows_examined field
of the thd object correctly, the degenerate outer select query is resetting
the rows_examined to zero after it has finished execution, by
invoking thd->set_examined_row_count(0).

The solution is to remove the thd->set_examined_row_count(0) in the
degenerate select queries.

Author: bsrikanth-mariadb

mysql-test/main/log_state.result

@@ -392,6 +392,57 @@ disconnect con2;
 connection default;
 DROP TABLE t1;
 TRUNCATE TABLE mysql.slow_log;
+#
+# MDEV-37195 Rows_examined is always 0 in the slow query log 
+# for queries with a subquery and degenerate select
+#
+SET GLOBAL log_output = "TABLE";
+SET GLOBAL slow_query_log = ON;
+SET GLOBAL long_query_time = 0.0;
+TRUNCATE TABLE mysql.slow_log;
+CREATE TABLE t1 (id INT);
+INSERT INTO t1(id) SELECT seq FROM seq_1_to_10;
+connect  con2,localhost,root,,;
+SELECT 100 in (SELECT id FROM t1) AS res;
+res
+0
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+rows_examined	sql_text
+10	SELECT 100 in (SELECT id FROM t1) AS res
+TRUNCATE TABLE mysql.slow_log;
+SELECT 100 in (
+SELECT id FROM t1 
+UNION
+SELECT id FROM t1
+) AS res;
+res
+0
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+rows_examined	sql_text
+20	SELECT 100 in (
+SELECT id FROM t1 
+UNION
+SELECT id FROM t1
+) AS res
+TRUNCATE TABLE mysql.slow_log;
+SELECT 100 in (
+SELECT id FROM t1 
+UNION ALL
+SELECT id FROM t1
+) AS res;
+res
+0
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+rows_examined	sql_text
+20	SELECT 100 in (
+SELECT id FROM t1 
+UNION ALL
+SELECT id FROM t1
+) AS res
+disconnect con2;
+connection default;
+DROP TABLE t1;
+TRUNCATE TABLE mysql.slow_log;
 End of 10.11 tests
 SET GLOBAL long_query_time = @save_long_query_time;
 SET GLOBAL log_output = @old_log_output;

mysql-test/main/log_state.test

@@ -441,6 +441,57 @@ DROP TABLE t1;
 
 TRUNCATE TABLE mysql.slow_log;
 
+###########################################################################
+
+--echo #
+--echo # MDEV-37195 Rows_examined is always 0 in the slow query log 
+--echo # for queries with a subquery and degenerate select
+--echo #
+
+SET GLOBAL log_output = "TABLE";
+SET GLOBAL slow_query_log = ON;
+SET GLOBAL long_query_time = 0.0;
+
+# clear slow_log of any residual slow queries
+TRUNCATE TABLE mysql.slow_log;
+
+CREATE TABLE t1 (id INT);
+ 
+INSERT INTO t1(id) SELECT seq FROM seq_1_to_10;
+
+connect (con2,localhost,root,,);
+--disable_ps_protocol
+
+SELECT 100 in (SELECT id FROM t1) AS res;
+
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+
+TRUNCATE TABLE mysql.slow_log;
+
+SELECT 100 in (
+    SELECT id FROM t1 
+    UNION
+    SELECT id FROM t1
+) AS res;
+
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+
+TRUNCATE TABLE mysql.slow_log;
+
+SELECT 100 in (
+    SELECT id FROM t1 
+    UNION ALL
+    SELECT id FROM t1
+) AS res;
+
+SELECT rows_examined,sql_text FROM mysql.slow_log WHERE sql_text LIKE '%SELECT 100%';
+
+disconnect con2;
+connection default;
+DROP TABLE t1;
+
+TRUNCATE TABLE mysql.slow_log;
+
 --echo End of 10.11 tests
 ###########################################################################
 

sql/sql_select.cc

@@ -4865,7 +4865,6 @@ void JOIN::exec_inner()
     }
     /* Single select (without union) always returns 0 or 1 row */
     thd->limit_found_rows= send_records;
-    thd->set_examined_row_count(0);
     DBUG_VOID_RETURN;
   }