MDEV-31721: Cursor protocol increases the counter of "Empty_queries" for select

Problem:
  Empty queries are incremented if no rows are sent to the client in the
  EXECUTE phase of select query. With cursor protocol, rows are not sent
  during EXECUTE phase; they are sent later in FETCH phase. Hence,
  queries executed with cursor protocol are always falsely treated as
  empty in EXECUTE phase.

Fix:
  For cursor protocol, empty queries are now counted during the FETCH
  phase. This ensures counter correctly reflects whether any rows were
  actually sent to the client.

Tests included in `mysql-test/main/show.test`.

mysql-test/main/show.result

@@ -65,3 +65,37 @@ drop table t1;
 #
 # End of 10.3 tests
 #
+#
+# MDEV-31721: Cursor protocol increases the counter of  "Empty_queries" for select
+#
+FLUSH STATUS;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+SELECT COUNT(*) FROM t1;
+COUNT(*)
+1
+SELECT * FROM t1;
+a
+1
+SELECT * FROM t1 LIMIT 0;
+a
+SHOW STATUS LIKE "Empty_queries";
+Variable_name	Value
+Empty_queries	1
+DROP TABLE t1;
+#------------------------------
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (2);
+SELECT * FROM t1 UNION SELECT * FROM t2;
+a
+1
+2
+SELECT * FROM t1 UNION SELECT * FROM t2 LIMIT 0;
+a
+SHOW STATUS LIKE "Empty_queries";
+Variable_name	Value
+Empty_queries	2
+DROP TABLE t1, t2;
+# End of 10.11 tests

mysql-test/main/show.test

@@ -56,3 +56,40 @@ drop table t1;
 --echo #
 --echo # End of 10.3 tests
 --echo #
+
+--echo #
+--echo # MDEV-31721: Cursor protocol increases the counter of  "Empty_queries" for select
+--echo #
+
+FLUSH STATUS;
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+
+--disable_ps2_protocol
+SELECT COUNT(*) FROM t1;
+SELECT * FROM t1;
+SELECT * FROM t1 LIMIT 0;
+--enable_ps2_protocol
+
+SHOW STATUS LIKE "Empty_queries";
+
+DROP TABLE t1;
+
+--echo #------------------------------
+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (2);
+
+--disable_ps2_protocol
+SELECT * FROM t1 UNION SELECT * FROM t2;
+SELECT * FROM t1 UNION SELECT * FROM t2 LIMIT 0;
+--enable_ps2_protocol
+
+SHOW STATUS LIKE "Empty_queries";
+
+DROP TABLE t1, t2;
+
+--echo # End of 10.11 tests

sql/sql_class.cc

@@ -65,6 +65,7 @@
 #include "lock.h"
 #include "wsrep_mysqld.h"
 #include "sql_connect.h"
+#include "sql_cursor.h"                         //Select_materialize
 #ifdef WITH_WSREP
 #include "wsrep_thd.h"
 #include "wsrep_trans_observer.h"
@@ -8737,3 +8738,9 @@ void Charset_loader_server::raise_not_applicable_error(const char *cs,
 {
   my_error(ER_COLLATION_CHARSET_MISMATCH, MYF(0), cl, cs);
 }
+
+
+bool THD::is_cursor_execution() const
+{
+  return dynamic_cast<Select_materialize*>(this->lex->result);
+}

sql/sql_class.h

@@ -5850,6 +5850,17 @@ public:
       return false;
     return !is_set_timestamp_forbidden(this);
   }
+
+  /**
+    @brief
+    Return true if current statement uses cursor protocol for execution.
+
+    @details
+    Cursor protocol execution is determined by checking if lex->result is a
+    Select_materialize object, which is exclusively used by the server for
+    cursor result set materialization.
+  */
+  bool is_cursor_execution() const;
 };
 
 

sql/sql_cursor.cc

@@ -24,72 +24,6 @@
 #include "probes_mysql.h"
 #include "sql_parse.h"                        // mysql_execute_command
 
-/****************************************************************************
-  Declarations.
-****************************************************************************/
-
-/**
-  Materialized_cursor -- an insensitive materialized server-side
-  cursor. The result set of this cursor is saved in a temporary
-  table at open. The cursor itself is simply an interface for the
-  handler of the temporary table.
-*/
-
-class Materialized_cursor: public Server_side_cursor
-{
-  MEM_ROOT main_mem_root;
-  /* A fake unit to supply to select_send when fetching */
-  SELECT_LEX_UNIT fake_unit;
-  TABLE *table;
-  List<Item> item_list;
-  ulong fetch_limit;
-  ulong fetch_count;
-  bool is_rnd_inited;
-public:
-  Materialized_cursor(select_result *result, TABLE *table);
-
-  int send_result_set_metadata(THD *thd, List<Item> &send_result_set_metadata);
-  bool is_open() const override { return table != 0; }
-  int open(JOIN *join __attribute__((unused))) override;
-  void fetch(ulong num_rows) override;
-  void close() override;
-  bool export_structure(THD *thd, Row_definition_list *defs) override
-  {
-    return table->export_structure(thd, defs);
-  }
-  ~Materialized_cursor() override;
-
-  void on_table_fill_finished();
-};
-
-
-/**
-  Select_materialize -- a mediator between a cursor query and the
-  protocol. In case we were not able to open a non-materialzed
-  cursor, it creates an internal temporary HEAP table, and insert
-  all rows into it. When the table reaches max_heap_table_size,
-  it's converted to a MyISAM table. Later this table is used to
-  create a Materialized_cursor.
-*/
-
-class Select_materialize: public select_unit
-{
-  select_result *result; /**< the result object of the caller (PS or SP) */
-public:
-  Materialized_cursor *materialized_cursor;
-  Select_materialize(THD *thd_arg, select_result *result_arg):
-    select_unit(thd_arg), result(result_arg), materialized_cursor(0) {}
-  bool send_result_set_metadata(List<Item> &list, uint flags) override;
-  bool send_eof() override { return false; }
-  bool view_structure_only() const override
-  {
-    return result->view_structure_only();
-  }
-};
-
-
-/**************************************************************************/
-
 /**
   Attempt to open a materialized cursor.
 

sql/sql_cursor.h

@@ -68,6 +68,66 @@ public:
 };
 
 
+/**
+  Materialized_cursor -- an insensitive materialized server-side
+  cursor. The result set of this cursor is saved in a temporary
+  table at open. The cursor itself is simply an interface for the
+  handler of the temporary table.
+*/
+
+class Materialized_cursor: public Server_side_cursor
+{
+  MEM_ROOT main_mem_root;
+  /* A fake unit to supply to select_send when fetching */
+  SELECT_LEX_UNIT fake_unit;
+  TABLE *table;
+  List<Item> item_list;
+  ulong fetch_limit;
+  ulong fetch_count;
+  bool is_rnd_inited;
+public:
+  Materialized_cursor(select_result *result, TABLE *table);
+
+  int send_result_set_metadata(THD *thd, List<Item> &send_result_set_metadata);
+  bool is_open() const override { return table != 0; }
+  int open(JOIN *join __attribute__((unused))) override;
+  void fetch(ulong num_rows) override;
+  void close() override;
+  bool export_structure(THD *thd, Row_definition_list *defs) override
+  {
+    return table->export_structure(thd, defs);
+  }
+  ~Materialized_cursor() override;
+
+  void on_table_fill_finished();
+};
+
+
+/**
+  Select_materialize -- a mediator between a cursor query and the
+  protocol. In case we were not able to open a non-materialzed
+  cursor, it creates an internal temporary HEAP table, and insert
+  all rows into it. When the table reaches max_heap_table_size,
+  it's converted to a MyISAM table. Later this table is used to
+  create a Materialized_cursor.
+*/
+
+class Select_materialize: public select_unit
+{
+  select_result *result; /**< the result object of the caller (PS or SP) */
+public:
+  Materialized_cursor *materialized_cursor;
+  Select_materialize(THD *thd_arg, select_result *result_arg):
+    select_unit(thd_arg), result(result_arg), materialized_cursor(0) {}
+  bool send_result_set_metadata(List<Item> &list, uint flags) override;
+  bool send_eof() override { return false; }
+  bool view_structure_only() const override
+  {
+    return result->view_structure_only();
+  }
+};
+
+
 int mysql_open_cursor(THD *thd, select_result *result,
                       Server_side_cursor **res);
 

sql/sql_parse.cc

@@ -56,6 +56,7 @@
 #include "sql_test.h"         // mysql_print_status
 #include "sql_select.h"       // handle_select, mysql_select,
                               // mysql_explain_union
+#include "sql_cursor.h"       // Select_materialzie
 #include "sql_load.h"         // mysql_load
 #include "sql_servers.h"      // create_servers, alter_servers,
                               // drop_servers, servers_reload
@@ -6443,7 +6444,7 @@ static bool execute_sqlcom_select(THD *thd, TABLE_LIST *all_tables)
     }
   }
   /* Count number of empty select queries */
-  if (!thd->get_sent_row_count() && !res)
+  if (!thd->is_cursor_execution() && !thd->get_sent_row_count() && !res)
     status_var_increment(thd->status_var.empty_queries);
   else
     status_var_add(thd->status_var.rows_sent, thd->get_sent_row_count());

sql/sql_prepare.cc

@@ -3767,6 +3767,9 @@ void mysqld_stmt_fetch(THD *thd, char *packet, uint packet_length)
 
   cursor->fetch(num_rows);
 
+  if (!thd->get_sent_row_count())
+    status_var_increment(thd->status_var.empty_queries);
+
   if (!cursor->is_open())
   {
     stmt->close_cursor();