MWL#182: Explain running statements: address review feedback

- Add a testcase showing that queries specified in a charset that's different
  from the charset used for warnings, are converted.

mysql-test/r/show_explain.result

@@ -1045,5 +1045,29 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using filesort
 DROP TABLE t1,t2;
+#
+# Check if queries in non-default charsets work.
+#
+set names cp1251;
+select charset('<27><>');
+charset('<27><>')
+cp1251
+select hex('<27><>');
+hex('<27><>')
+E3FB
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+select * from t0 where length('<27><>') = a;
+set names utf8;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+Warnings:
+Note	1003	select * from t0 where length('гы') = a
+set names default;
+a
+2
+set debug_dbug='';
+set names default;
 # End
 drop table t0;

mysql-test/t/show_explain.test

@@ -1077,5 +1077,33 @@ WHERE ( 8, 89 ) IN ( SELECT b, SUM( DISTINCT b ) FROM t2 GROUP BY b );
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # Check if queries in non-default charsets work.
+--echo #
+
+set names cp1251;
+# The below are two Russian letters with codes E3FB in cp1251 encoding.
+select charset('<27><>');
+select hex('<27><>');
+
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+
+send
+select * from t0 where length('<27><>') = a;
+
+connection default;
+set names utf8;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+set names default;
+
+connection con1;
+# The constant should be two letters, the last looking like 'bl'
+reap;
+
+set debug_dbug='';
+set names default;
+
 --echo # End
 drop table t0;