MDEV-6689: valgrind errors in view.test in 10.1

SHOW COLUMNS and SHOW KEYS commands fill IS_table_read_plan
in a special way - they don't set or use lookup_field_vals
member.

Added a "trivial_show_command" flag that signals that
lookup_field_vals has no valid data, made EXPLAIN code honor it.

sql/sql_select.cc

@@ -23490,16 +23490,19 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
       const char *tmp_buff;
       int f_idx;
       StringBuffer<64> key_name_buf;
-      if (is_table_read_plan->has_db_lookup_value())
+      if (is_table_read_plan->trivial_show_command ||
+          is_table_read_plan->has_db_lookup_value())
       {
         /* The "key" has the name of the column referring to the database */
         f_idx= table_list->schema_table->idx_field1;
         tmp_buff= table_list->schema_table->fields_info[f_idx].field_name;
         key_name_buf.append(tmp_buff, strlen(tmp_buff), cs);
       }          
-      if (is_table_read_plan->has_table_lookup_value())
+      if (is_table_read_plan->trivial_show_command ||
+          is_table_read_plan->has_table_lookup_value())
       {
-        if (is_table_read_plan->has_db_lookup_value())
+        if (is_table_read_plan->trivial_show_command ||
+            is_table_read_plan->has_db_lookup_value())
           key_name_buf.append(',');
 
         f_idx= table_list->schema_table->idx_field2;
@@ -23630,8 +23633,9 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
       else
         eta->push_extra(ET_OPEN_FULL_TABLE);
       /* psergey-note: the following has a bug.*/
-      if (table_list->is_table_read_plan->has_db_lookup_value() &&
-          table_list->is_table_read_plan->has_table_lookup_value())
+      if (table_list->is_table_read_plan->trivial_show_command ||
+          (table_list->is_table_read_plan->has_db_lookup_value() &&
+           table_list->is_table_read_plan->has_table_lookup_value()))
         eta->push_extra(ET_SCANNED_0_DATABASES);
       else if (table_list->is_table_read_plan->has_db_lookup_value() ||
                table_list->is_table_read_plan->has_table_lookup_value())

sql/sql_show.cc

@@ -8035,6 +8035,7 @@ static bool optimize_for_get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond
   if (lsel && lsel->table_list.first)
   {
     /* These do not need to have a query plan */
+    plan->trivial_show_command= true;
     goto end;
   }
 

sql/sql_show.h

@@ -192,9 +192,17 @@ typedef struct st_lookup_field_values
 class IS_table_read_plan : public Sql_alloc
 {
 public:
-  IS_table_read_plan() : no_rows(false) {}
+  IS_table_read_plan() : no_rows(false), trivial_show_command(FALSE) {}
 
   bool no_rows;
+  /*
+    For EXPLAIN only: For SHOW KEYS and SHOW COLUMNS, we know which
+    db_name.table_name will be read, however for some reason we don't
+    set the fields in this->lookup_field_vals.
+    In order to not have JOIN::save_explain_data() walking over uninitialized
+    data, we set trivial_show_command=true.
+  */
+  bool trivial_show_command;
 
   LOOKUP_FIELD_VALUES lookup_field_vals;
   Item *partial_cond;