Simplified code in generate_derived_keys() and when using pos_in_tables

Added comments that not used keys of derivied tables will be deleted.
Added some comments about checking if pos_in_table_list is 0.

Other things:
- Added a marker (DBTYPE_IN_PREDICATE) in TABLE_LIST->derived_type
  to indicate that the table was generated from IN (list). This is
  useful for debugging and can later be used by explain if needed.
- Removed a not needed test of table->pos_in_table_list as it should
  always be valid at this point in time.

1 files changed, 8 insertions, 0 deletions

diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index a716cae6955..05d0f601434 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -8899,6 +8899,10 @@ bool optimize_schema_tables_reads(JOIN *join)
        tab; 
        tab= next_linear_tab(join, tab, WITH_BUSH_ROOTS))
   {
+    /*
+      The following is true for the temporary table that will hold the
+      final result.
+    */
     if (!tab->table || !tab->table->pos_in_table_list)
       continue;
 
@@ -8970,6 +8974,10 @@ bool get_schema_tables_result(JOIN *join,
        tab; 
        tab= next_linear_tab(join, tab, WITH_BUSH_ROOTS))
   {
+    /*
+      The following is true for the temporary table that will hold the
+      final result.
+    */
     if (!tab->table || !tab->table->pos_in_table_list)
       break;