Original idea from Zardosht Kasheff to add HA_CLUSTERED_INDEX

- Added a lot of code comments
- Updated get_best_ror_intersec() to prefer index scan on not clustered keys before clustered keys.
- Use HA_CLUSTERED_INDEX to define if one should use HA_MRR_INDEX_ONLY
- For test of using index or filesort to resolve ORDER BY, use HA_CLUSTERED_INDEX flag instead of primary_key_is_clustered()
- Use HA_TABLE_SCAN_ON_INDEX instead of primary_key_is_clustered() to decide if ALTER TABLE ... ORDER BY will have any effect.

sql/ha_partition.h:
  Added comment with warning for code unsafe to use with multiple storage engines at the same time
sql/handler.h:
  Added HA_CLUSTERED_INDEX.
  Documented primary_key_is_clustered()
sql/opt_range.cc:
  Added code comments
  Updated get_best_ror_intersec() to ignore clustered keys.
  Optimized away cpk_scan_used and one instance of current_thd (Simpler code)
  Use HA_CLUSTERED_INDEX to define if one should use HA_MRR_INDEX_ONLY
sql/sql_select.cc:
  Changed comment to #ifdef
  For test of using index or filesort to resolve ORDER BY, use HA_CLUSTERED_INDEX flag instead of primary_key_is_clustered()
  (Change is smaller than what it looks beause of indentation change)
sql/sql_table.cc:
  Use HA_TABLE_SCAN_ON_INDEX instead of primary_key_is_clustered() to decide if ALTER TABLE ... ORDER BY will have any effect.
storage/innobase/handler/ha_innodb.h:
  Added support for HA_CLUSTERED_INDEX
storage/innodb_plugin/handler/ha_innodb.cc:
  Added support for HA_CLUSTERED_INDEX
storage/xtradb/handler/ha_innodb.cc:
  Added support for HA_CLUSTERED_INDEX

1 files changed, 5 insertions, 2 deletions

diff --git a/storage/innodb_plugin/handler/ha_innodb.cc b/storage/innodb_plugin/handler/ha_innodb.cc
index 07805f88c8f..41d9b93b3fa 100644
--- a/storage/innodb_plugin/handler/ha_innodb.cc
+++ b/storage/innodb_plugin/handler/ha_innodb.cc
@@ -2995,12 +2995,15 @@ UNIV_INTERN
 ulong
 ha_innobase::index_flags(
 /*=====================*/
-	uint,
+	uint index,
 	uint,
 	bool)
 const
 {
-	return(HA_READ_NEXT | HA_READ_PREV | HA_READ_ORDER
+       ulong extra_flag= 0;
+       if (table && index == table->s->primary_key)
+             extra_flag= HA_CLUSTERED_INDEX;
+	return(HA_READ_NEXT | HA_READ_PREV | HA_READ_ORDER | extra_flag
 	       | HA_READ_RANGE | HA_KEYREAD_ONLY);
 }