Fixed bug lp:809245

In addition to the bug fix explained below, the patch performs
few renames, and adds some comments to avoid similar problems.

Analysis:
The failed assert was due to a bug in MWL#68, where it was
incorrectly assumed that the size of the bitmap
subselect_rowid_merge_engine::null_only_columns should be
the same as the size of the array of Ordered_keys.

The bitmap null_only_columns contains bits to mark columns
that contain only NULLs. Therefore the indexes of the bits
to be set in null_only_columns are different from the indexes
of the Ordered_keys. If there is a NULL-only column that appears
in a table after the last partial match column with Ordered_key,
this NULL-only column would require setting a bit with index
bigger than the size of the bitmap null_only_columns.

Accessing such a bit caused the failed assert.

Solution:
Upon analysis, it turns out that null_only_columns is not needed
at all, because we are looking for partial matches, and having
such columns guarantees that there is a partial match for any
corresponding outer value.

Therefore the patch removes
  subselect_rowid_merge_engine::null_only_columns.

1 files changed, 5 insertions, 10 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 26a45a14dbe..42be1635b6f 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -1192,11 +1192,6 @@ protected:
   */
   MY_BITMAP matching_outer_cols;
   /*
-    Columns that consist of only NULLs. Such columns match any value.
-    Computed once per query execution.
-  */
-  MY_BITMAP null_only_columns;
-  /*
     Indexes of row numbers, sorted by <column_value, row_number>. If an
     index may contain NULLs, the NULLs are stored efficiently in a bitmap.
 
@@ -1205,13 +1200,13 @@ protected:
     non-NULL columns, it is contained in keys[0].
   */
   Ordered_key **merge_keys;
-  /* The number of elements in keys. */
-  uint keys_count;
+  /* The number of elements in merge_keys. */
+  uint merge_keys_count;
   /*
     An index on all non-NULL columns of 'tmp_table'. The index has the
     logical form: <[v_i1 | ... | v_ik], rownum>. It allows to find the row
     number where the columns c_i1,...,c1_k contain the values v_i1,...,v_ik.
-    If such an index exists, it is always the first element of 'keys'.
+    If such an index exists, it is always the first element of 'merge_keys'.
   */
   Ordered_key *non_null_key;
   /*
@@ -1236,7 +1231,7 @@ protected:
 public:
   subselect_rowid_merge_engine(THD *thd_arg,
                                subselect_uniquesubquery_engine *engine_arg,
-                               TABLE *tmp_table_arg, uint keys_count_arg,
+                               TABLE *tmp_table_arg, uint merge_keys_count_arg,
                                uint covering_null_row_width_arg,
                                Item_subselect *item_arg,
                                select_result_interceptor *result_arg,
@@ -1244,7 +1239,7 @@ public:
     :subselect_partial_match_engine(thd_arg, engine_arg, tmp_table_arg,
                                     item_arg, result_arg, equi_join_conds_arg,
                                     covering_null_row_width_arg),
-    keys_count(keys_count_arg), non_null_key(NULL)
+    merge_keys_count(merge_keys_count_arg), non_null_key(NULL)
   {}
   ~subselect_rowid_merge_engine();
   bool init(MY_BITMAP *non_null_key_parts, MY_BITMAP *partial_match_key_parts);