MDEV-5106: Server crashes in Explain_union::print_explain on ER_TOO_BIG_SELECT

- Don't save UNION's EXPLAIN data if optimization failed with 
  an error. We could end up saving incomplete plan, which will 
  cause a crash when we attempt to print it.

mysql-test/r/explain_slowquerylog.result

@@ -26,3 +26,16 @@ SELECT 1;
 EXPLAIN PARTITIONS SELECT 1 ;
 id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+#
+# MDEV-5106: Server crashes in Explain_union::print_explain on ER_TOO_BIG_SELECT with explain in slow log
+#
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES
+(1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+SET max_join_size = 10;
+( SELECT ta.* FROM t1 ta, t1 tb ) UNION ( SELECT * FROM t1 );
+ERROR 42000: The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay
+SELECT 'Server still alive?' as 'Yes';
+Yes
+Server still alive?
+DROP TABLE t1;

mysql-test/t/explain_non_select.test

@@ -169,4 +169,3 @@ explain insert into t2 values
   (11, 1+(select max(a+1) from t1));
 
 drop table t1,t2;
-

mysql-test/t/explain_slowquerylog.test

@@ -35,3 +35,15 @@ SELECT 1;
 EXPLAIN PARTITIONS SELECT 1 ;
 
 
+--echo #
+--echo # MDEV-5106: Server crashes in Explain_union::print_explain on ER_TOO_BIG_SELECT with explain in slow log
+--echo #
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES
+(1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+SET max_join_size = 10;
+--error ER_TOO_BIG_SELECT
+( SELECT ta.* FROM t1 ta, t1 tb ) UNION ( SELECT * FROM t1 );
+SELECT 'Server still alive?' as 'Yes';
+
+DROP TABLE t1;

sql/sql_explain.cc

@@ -157,6 +157,10 @@ bool print_explain_query(LEX *lex, THD *thd, String *str)
 }
 
 
+/* 
+  Return tabular EXPLAIN output as a text string
+*/
+
 bool Explain_query::print_explain_str(THD *thd, String *out_str)
 {
   List<Item> fields;

sql/sql_union.cc

@@ -630,7 +630,7 @@ bool st_select_lex_unit::exec()
   
   saved_error= optimize();
 
-  if (!was_executed && thd->lex->explain)
+  if (!saved_error && !was_executed && thd->lex->explain)
     save_union_explain(thd->lex->explain);
 
   if (uncacheable || !item || !item->assigned() || describe)