Derived tables and union can now create distinct keys

The idea is that instead of marking all select_lex's with DISTINCT, we
only mark those that really need distinct result.

Benefits of this change:
- Temporary tables used with derived tables, UNION, IN are now smaller
  as duplicates are removed already on the insert phase.
- The optimizer can now produce better plans with EQ_REF. This can be
  seen from the tests where several queries does not anymore materialize
  derived tables twice.
- Queries affected by 'in_predicate_conversion_threshold' where large IN
  lists are converted to sub query produces better plans.

Other things:
- Removed on duplicate call to sel->init_select() in
  LEX::add_primary_to_query_expression_body()
- I moved the testing of
  tab->table->pos_in_table_list->is_materialized_derived()
  in join_read_const_table() to the caller as it caused problems for
  derived tables that could be proven to be const tables.
  This also is likely to fix some bugs as if join_read_const_table()
  was aborted, the table was left marked as JT_CONST, which cannot
  be good.  I added an ASSERT there for now that can be removed when
  the code has been properly tested.

1 files changed, 7 insertions, 5 deletions

diff --git a/sql/sql_derived.cc b/sql/sql_derived.cc
index 6f0857239dd..1fa5179875c 100644
--- a/sql/sql_derived.cc
+++ b/sql/sql_derived.cc
@@ -870,18 +870,20 @@ bool mysql_derived_prepare(THD *thd, LEX *lex, TABLE_LIST *derived)
     goto exit;
 
   /*
-    Temp table is created so that it hounours if UNION without ALL is to be 
+    Temp table is created so that it honors if UNION without ALL is to be
     processed
 
-    As 'distinct' parameter we always pass FALSE (0), because underlying
-    query will control distinct condition by itself. Correct test of
-    distinct underlying query will be is_unit_op &&
+    As 'distinct' parameter we pass unit->distinct, which tells us if
+    the values should be uniq.
+    Note that the underlying query will also control distinct condition.
+    Correct test of distinct underlying query will be is_unit_op &&
     !unit->union_distinct->next_select() (i.e. it is union and last distinct
     SELECT is last SELECT of UNION).
   */
   thd->create_tmp_table_for_derived= TRUE;
   if (!(derived->table) &&
-      derived->derived_result->create_result_table(thd, &unit->types, FALSE,
+      derived->derived_result->create_result_table(thd, &unit->types,
+                                                   unit->distinct,
                                                    (first_select->options |
                                                    thd->variables.option_bits |
                                                    TMP_TABLE_ALL_COLUMNS),