Fixed the bug mdev12992.

When the SELECT query from a trigger that used a subquery in its SELECT list was prepared the counter select_n_having_items was incremented in the constructor Item::Item(THD *thd). As a result each invocation of the trigger required more and more memory for the ref_pointer_array for this SELECT. Made sure that the counter st_select_lex::select_n_having_items would be incremented only at the first execution of such trigger.

Author: igorbabaev

mysql-test/r/trigger.result

@@ -2368,3 +2368,14 @@ tr1	1	2016-01-01 10:10:10.33
 tr2	2	2016-01-01 10:10:10.99
 drop table t1;
 set time_zone= @@global.time_zone;
+#
+# MDEV-12992: Increasing memory consumption
+with each invocation of trigger
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (b INT);
+CREATE TRIGGER tr 
+AFTER UPDATE ON t1 FOR EACH ROW SELECT (SELECT b FROM t2) INTO @x;
+# Running 20000 queries
+DROP TABLE t1,t2;

mysql-test/t/trigger.test

@@ -2676,3 +2676,27 @@ select trigger_name, action_order, created from information_schema.triggers
  where event_object_table = 't1' and trigger_schema='test';
 drop table t1;
 set time_zone= @@global.time_zone;
+
+--echo #
+--echo # MDEV-12992: Increasing memory consumption
+--echo with each invocation of trigger
+--echo #
+
+--let $n= 20000
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (b INT);
+CREATE TRIGGER tr 
+ AFTER UPDATE ON t1 FOR EACH ROW SELECT (SELECT b FROM t2) INTO @x;
+ 
+--disable_query_log
+--echo # Running $n queries
+while ($n)
+{
+ UPDATE t1 SET a = 2;
+ --dec $n
+}
+--enable_query_log
+ 
+DROP TABLE t1,t2;

sql/item.cc

@@ -490,7 +490,8 @@ Item::Item(THD *thd):
  command => we should check thd->lex->current_select on zero (thd->lex
  can be uninitialised)
  */
- if (thd->lex->current_select)
+ if (thd->lex->current_select &&
+ thd->stmt_arena->is_stmt_prepare_or_first_sp_execute())
  {
  enum_parsing_place place= 
  thd->lex->current_select->parsing_place;