MWL#68 Subquery optimization: Efficient NOT IN execution with NULLs

* Implemented a second partial matching strategy via table scan.
  This strategy is a fallback when there is no memory for rowid merging.

* Refactored the selection and creation of partial matching strategies,
  so that the choice of strategy is encapsulated in a separate method
  choose_partial_match_strategy().

* Refactored the representation of partial match strategies so that:
  - each strategy is represented by a polymorphic class, and
  - the base class for all partial match strategies contains common
    execution code.

* Added an estimate of the memory needed for the rowid merge strategy,
  and the system variable "rowid_merge_buff_size" to control the maximum
  memory to be used by the rowid merge algorithm.

* Added two optimizer_switch system variables to control the choice of
  partial match strategy:
  "partial_match_rowid_merge", "partial_match_table_scan".

* Fixed multiple problems with deallocation of resources by the partial
  match strategies.


sql/mysql_priv.h:
  * Added two optimizer_switch system variables to control the choice of
    partial match strategy:
    "partial_match_rowid_merge", "partial_match_table_scan".
sql/mysqld.cc:
  * Added two optimizer_switch system variables to control the choice of
    partial match strategy:
    "partial_match_rowid_merge", "partial_match_table_scan".
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
sql/set_var.cc:
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
sql/sql_class.h:
  * Added a system variable "rowid_merge_buff_size" to control the maximum
    memory to be used by the rowid merge algorithm.
support-files/build-tags:
  Newer versions of BZR require the recursive flag in order to list all files.

sql/item_subselect.cc

@@ -2910,13 +2910,7 @@ int subselect_uniquesubquery_engine::exec()
 
 
 /*
-  TIMOUR: this needs more thinking, as exec() is a wrong IMO because:
-  - we don't need empty_result_set, as it is == 1 <=> when
-    item->value == 0
-  - scan_table() returns >0 even when there was no actuall error,
-    but we only found EOF while scanning.
-  - scan_table should not check table->status, but it should check
-    HA_ERR_END_OF_FILE
+  TIMOUR: write comment
 */
 
 int subselect_uniquesubquery_engine::index_lookup()
@@ -2924,8 +2918,6 @@ int subselect_uniquesubquery_engine::index_lookup()
   DBUG_ENTER("subselect_uniquesubquery_engine::index_lookup");
   int error;
   TABLE *table= tab->table;
-  empty_result_set= TRUE;
-  table->status= 0;
  
   if (!table->file->inited)
     table->file->ha_index_init(tab->ref.key, 0);
@@ -2934,25 +2926,25 @@ int subselect_uniquesubquery_engine::index_lookup()
                                         make_prev_keypart_map(tab->
                                                               ref.key_parts),
                                         HA_READ_KEY_EXACT);
-
   DBUG_PRINT("info", ("lookup result: %i", error));
-  if (error &&
-      error != HA_ERR_KEY_NOT_FOUND && error != HA_ERR_END_OF_FILE)
-    error= report_error(table, error);
-  else
+
+  if (error && error != HA_ERR_KEY_NOT_FOUND && error != HA_ERR_END_OF_FILE)
   {
-    error= 0;
+    /*
+      TIMOUR: I don't understand at all when do we need to call report_error.
+      In most places where we access an index, we don't do this. Why here?
+    */
+    error= report_error(table, error);
+    DBUG_RETURN(error);
+  }
+
   table->null_row= 0;
-    if (!table->status && (!cond || cond->val_int()))
-    {
+  if (!error && (!cond || cond->val_int()))
     ((Item_in_subselect *) item)->value= 1;
-      empty_result_set= FALSE;
-    }
   else
     ((Item_in_subselect *) item)->value= 0;
-  }
 
-  DBUG_RETURN(error);
+  DBUG_RETURN(0);
 }
 
 
@@ -3415,19 +3407,24 @@ bool subselect_uniquesubquery_engine::no_tables()
   If max_keys > 1, then we need partial matching because there are
   more indexes than just the one we use during materialization to
   remove duplicates.
+
+  @note
+  TIMOUR: The schema-based analysis for partial matching can be done once for
+  prepared statement and remembered. It is done here to remove the need to
+  save/restore all related variables between each re-execution, thus making
+  the code simpler.
+
+  @retval PARTIAL_MATCH  if a partial match should be used
+  @retval COMPLETE_MATCH if a complete match (index lookup) should be used
 */
 
-void subselect_hash_sj_engine::set_strategy_using_schema()
+subselect_hash_sj_engine::exec_strategy
+subselect_hash_sj_engine::get_strategy_using_schema()
 {
   Item_in_subselect *item_in= (Item_in_subselect *) item;
 
-  DBUG_ENTER("subselect_hash_sj_engine::set_strategy_using_schema");
-
   if (item_in->is_top_level_item())
-  {
-    strategy= COMPLETE_MATCH;
-    DBUG_VOID_RETURN;
-  }
+    return COMPLETE_MATCH;
   else
   {
     List_iterator<Item> inner_col_it(*item_in->unit->get_unit_column_types());
@@ -3450,10 +3447,8 @@ void subselect_hash_sj_engine::set_strategy_using_schema()
 
   /* If no column contains NULLs use regular hash index lookups. */
   if (count_partial_match_columns)
-    strategy= PARTIAL_MATCH;
-  else
-    strategy= COMPLETE_MATCH;
-  DBUG_VOID_RETURN;
+    return PARTIAL_MATCH;
+  return COMPLETE_MATCH;
 }
 
 
@@ -3465,19 +3460,25 @@ void subselect_hash_sj_engine::set_strategy_using_schema()
   matching type of columns that cannot be NULL or that contain only NULLs.
   Based on this, the procedure determines the final execution strategy for
   the [NOT] IN predicate.
+
+  @retval PARTIAL_MATCH  if a partial match should be used
+  @retval COMPLETE_MATCH if a complete match (index lookup) should be used
 */
 
-void subselect_hash_sj_engine::set_strategy_using_data()
+subselect_hash_sj_engine::exec_strategy
+subselect_hash_sj_engine::get_strategy_using_data()
 {
   Item_in_subselect *item_in= (Item_in_subselect *) item;
   select_materialize_with_stats *result_sink=
     (select_materialize_with_stats *) result;
   Item *outer_col;
 
-  DBUG_ENTER("subselect_hash_sj_engine::set_strategy_using_data");
-
-  /* Call this procedure only if already selected partial matching. */
-  DBUG_ASSERT(strategy == PARTIAL_MATCH);
+  /*
+    If we already determined that a complete match is enough based on schema
+    information, nothing can be better.
+  */
+  if (strategy == COMPLETE_MATCH)
+    return COMPLETE_MATCH;
 
   for (uint i= 0; i < item_in->left_expr->cols(); i++)
   {
@@ -3501,9 +3502,117 @@ void subselect_hash_sj_engine::set_strategy_using_data()
 
   /* If no column contains NULLs use regular hash index lookups. */
   if (!count_partial_match_columns)
-    strategy= COMPLETE_MATCH;
+    return COMPLETE_MATCH;
+  return PARTIAL_MATCH;
+}
 
-  DBUG_VOID_RETURN;
+
+void
+subselect_hash_sj_engine::choose_partial_match_strategy(
+  bool has_non_null_key, bool has_covering_null_row,
+  MY_BITMAP *partial_match_key_parts)
+{
+  size_t pm_buff_size;
+
+  DBUG_ASSERT(strategy == PARTIAL_MATCH);
+  /*
+    Choose according to global optimizer switch. If only one of the switches is
+    'ON', then the remaining strategy is the only possible one. The only cases
+    when this will be overriden is when the total size of all buffers for the
+    merge strategy is bigger than the 'rowid_merge_buff_size' system variable,
+    or if there isn't enough physical memory to allocate the buffers.
+  */
+  if (!optimizer_flag(thd, OPTIMIZER_SWITCH_PARTIAL_MATCH_ROWID_MERGE) &&
+       optimizer_flag(thd, OPTIMIZER_SWITCH_PARTIAL_MATCH_TABLE_SCAN))
+    strategy= PARTIAL_MATCH_SCAN;
+  else if
+     ( optimizer_flag(thd, OPTIMIZER_SWITCH_PARTIAL_MATCH_ROWID_MERGE) &&
+      !optimizer_flag(thd, OPTIMIZER_SWITCH_PARTIAL_MATCH_TABLE_SCAN))
+    strategy= PARTIAL_MATCH_MERGE;
+
+  /*
+    If both switches are ON, or both are OFF, we interpret that as "let the
+    optimizer decide". Perform a cost based choice between the two partial
+    matching strategies.
+  */
+  /*
+    TIMOUR: the above interpretation of the switch values could be changed to:
+    - if both are ON - let the optimizer decide,
+    - if both are OFF - do not use partial matching, therefore do not use
+      materialization in non-top-level predicates.
+    The problem with this is that we know for sure if we need partial matching
+    only after the subquery is materialized, and this is too late to revert to
+    the IN=>EXISTS strategy.
+  */
+  if (strategy == PARTIAL_MATCH)
+  {
+    /*
+      TIMOUR: Currently we use a super simplistic measure. This will be
+      addressed in a separate task.
+    */
+    if (tmp_table->file->stats.records < 100)
+      strategy= PARTIAL_MATCH_SCAN;
+    else
+      strategy= PARTIAL_MATCH_MERGE;
+  }
+
+  /* Check if there is enough memory for the rowid merge strategy. */
+  if (strategy == PARTIAL_MATCH_MERGE)
+  {
+    pm_buff_size= rowid_merge_buff_size(has_non_null_key,
+                                        has_covering_null_row,
+                                        partial_match_key_parts);
+    if (pm_buff_size > thd->variables.rowid_merge_buff_size)
+      strategy= PARTIAL_MATCH_SCAN;
+  }
+}
+
+
+/*
+  Compute the memory size of all buffers proportional to the number of rows
+  in tmp_table.
+
+  @details
+  If the result is bigger than thd->variables.rowid_merge_buff_size, partial
+  matching via merging is not applicable.
+*/
+
+size_t subselect_hash_sj_engine::rowid_merge_buff_size(
+  bool has_non_null_key, bool has_covering_null_row,
+  MY_BITMAP *partial_match_key_parts)
+{
+  size_t buff_size; /* Total size of all buffers used by partial matching. */
+  ha_rows row_count= tmp_table->file->stats.records;
+  uint rowid_length= tmp_table->file->ref_length;
+  select_materialize_with_stats *result_sink=
+    (select_materialize_with_stats *) result;
+
+  /* Size of the subselect_rowid_merge_engine::row_num_to_rowid buffer. */
+  buff_size= row_count * rowid_length * sizeof(uchar);
+
+  if (has_non_null_key)
+  {
+    /* Add the size of Ordered_key::key_buff of the only non-NULL key. */
+    buff_size+= row_count * sizeof(rownum_t);
+  }
+
+  if (!has_covering_null_row)
+  {
+    for (uint i= 0; i < partial_match_key_parts->n_bits; i++)
+    {
+      if (!bitmap_is_set(partial_match_key_parts, i) ||
+          result_sink->get_null_count_of_col(i) == row_count)
+        continue; /* In these cases we wouldn't construct Ordered keys. */
+
+      /* Add the size of Ordered_key::key_buff */
+      buff_size+= (row_count - result_sink->get_null_count_of_col(i)) *
+                         sizeof(rownum_t);
+      /* Add the size of Ordered_key::null_key */
+      buff_size+= bitmap_buffer_size(result_sink->get_max_null_of_col(i));
+    }
+  }
+
+  return buff_size;
 }
 
 
@@ -3561,7 +3670,6 @@ bool subselect_hash_sj_engine::init_permanent(List<Item> *tmp_columns)
                             thd->mem_root))
     DBUG_RETURN(TRUE);
 
-  set_strategy_using_schema();
   /*
     Create and initialize a select result interceptor that stores the
     result stream in a temporary table. The temporary table itself is
@@ -3623,7 +3731,9 @@ bool subselect_hash_sj_engine::init_permanent(List<Item> *tmp_columns)
               ((Item_in_subselect *) item)->left_expr->cols() ==
               tmp_table->key_info->key_parts);
 
-  if (make_semi_join_conds())
+  if (make_semi_join_conds() ||
+      /* A unique_engine is used both for complete and partial matching. */
+      !(lookup_engine= make_unique_engine()))
     DBUG_RETURN(TRUE);
 
   DBUG_RETURN(FALSE);
@@ -3691,7 +3801,7 @@ bool subselect_hash_sj_engine::make_semi_join_conds()
       DBUG_RETURN(TRUE);
     }
   }
-  if (semi_join_conds->fix_fields(thd, &semi_join_conds))
+  if (semi_join_conds->fix_fields(thd, (Item**)&semi_join_conds))
     DBUG_RETURN(TRUE);
 
   DBUG_RETURN(FALSE);
@@ -3791,7 +3901,7 @@ bool subselect_hash_sj_engine::init_runtime()
     clause of the query, and it is not 'fixed' during JOIN::prepare.
   */
   if (semi_join_conds && !semi_join_conds->fixed &&
-      semi_join_conds->fix_fields(thd, &semi_join_conds))
+      semi_join_conds->fix_fields(thd, (Item**)&semi_join_conds))
     return TRUE;
   /* Let our engine reuse this query plan for materialization. */
   materialize_join= materialize_engine->join;
@@ -3802,6 +3912,7 @@ bool subselect_hash_sj_engine::init_runtime()
 
 subselect_hash_sj_engine::~subselect_hash_sj_engine()
 {
+  delete lookup_engine;
   delete result;
   if (tmp_table)
     free_tmp_table(thd, tmp_table);
@@ -3817,9 +3928,30 @@ subselect_hash_sj_engine::~subselect_hash_sj_engine()
 
 void subselect_hash_sj_engine::cleanup()
 {
+  enum_engine_type lookup_engine_type= lookup_engine->engine_type();
   is_materialized= FALSE;
-  result->cleanup(); /* Resets the temp table as well. */
+  bitmap_clear_all(&non_null_key_parts);
+  bitmap_clear_all(&partial_match_key_parts);
+  count_partial_match_columns= 0;
+  count_null_only_columns= 0;
+  strategy= UNDEFINED;
   materialize_engine->cleanup();
+  if (lookup_engine_type == TABLE_SCAN_ENGINE ||
+      lookup_engine_type == ROWID_MERGE_ENGINE)
+  {
+    subselect_engine *inner_lookup_engine;
+    inner_lookup_engine=
+      ((subselect_partial_match_engine*) lookup_engine)->lookup_engine;
+    /*
+      Partial match engines are recreated for each PS execution inside
+      subselect_hash_sj_engine::exec().
+    */
+    delete lookup_engine;
+    lookup_engine= inner_lookup_engine;
+  }
+  DBUG_ASSERT(lookup_engine->engine_type() == UNIQUESUBQUERY_ENGINE);
+  lookup_engine->cleanup();
+  result->cleanup(); /* Resets the temp table as well. */
 }
 
 
@@ -3838,6 +3970,7 @@ int subselect_hash_sj_engine::exec()
 {
   Item_in_subselect *item_in= (Item_in_subselect *) item;
   SELECT_LEX *save_select= thd->lex->current_select;
+  subselect_partial_match_engine *pm_engine= NULL;
   int res= 0;
 
   DBUG_ENTER("subselect_hash_sj_engine::exec");
@@ -3881,59 +4014,86 @@ int subselect_hash_sj_engine::exec()
     DBUG_RETURN(FALSE);
   }
 
-  if (strategy == PARTIAL_MATCH)
-    set_strategy_using_data();
-
-  /* A unique_engine is used both for complete and partial matching. */
-  if (!(lookup_engine= make_unique_engine()))
-  {
-    res= 1;
-    goto err;
-  }
-
+  /*
+    TIMOUR: The schema-based analysis for partial matching can be done once for
+    prepared statement and remembered. It is done here to remove the need to
+    save/restore all related variables between each re-execution, thus making
+    the code simpler.
+  */
+  strategy= get_strategy_using_schema();
+  /* This call may discover that we don't need partial matching at all. */
+  strategy= get_strategy_using_data();
   if (strategy == PARTIAL_MATCH)
   {
-    subselect_rowid_merge_engine *rowid_merge_engine;
-    uint count_pm_keys;
-    MY_BITMAP *nn_key_parts;
-    bool has_covering_null_row;
+    uint count_pm_keys; /* Total number of keys needed for partial matching. */
+    MY_BITMAP *nn_key_parts; /* The key parts of the only non-NULL index. */
+    uint covering_null_row_width;
     select_materialize_with_stats *result_sink=
       (select_materialize_with_stats *) result;
 
-    /* Total number of keys needed for partial matching. */
     nn_key_parts= (count_partial_match_columns < tmp_table->s->fields) ?
                   &non_null_key_parts : NULL;
 
-    has_covering_null_row= (result_sink->get_max_nulls_in_row() ==
+    if (result_sink->get_max_nulls_in_row() ==
         tmp_table->s->fields -
-                            (nn_key_parts ? bitmap_bits_set(nn_key_parts) : 0));
+        (nn_key_parts ? bitmap_bits_set(nn_key_parts) : 0))
+      covering_null_row_width= result_sink->get_max_nulls_in_row();
+    else
+      covering_null_row_width= 0;
 
-    if (has_covering_null_row)
+    if (covering_null_row_width)
       count_pm_keys= nn_key_parts ? 1 : 0;
     else
       count_pm_keys= count_partial_match_columns - count_null_only_columns +
         (nn_key_parts ? 1 : 0);
 
-    if (!(rowid_merge_engine=
+    choose_partial_match_strategy(test(nn_key_parts),
+                                  test(covering_null_row_width),
+                                  &partial_match_key_parts);
+    DBUG_ASSERT(strategy == PARTIAL_MATCH_MERGE ||
+                strategy == PARTIAL_MATCH_SCAN);
+    if (strategy == PARTIAL_MATCH_MERGE)
+    {
+      pm_engine=
         new subselect_rowid_merge_engine((subselect_uniquesubquery_engine*)
-                                           lookup_engine,
-                                           tmp_table,
+                                         lookup_engine, tmp_table,
                                          count_pm_keys,
-                                           has_covering_null_row,
-                                           item, result)) ||
-        rowid_merge_engine->init(nn_key_parts, &partial_match_key_parts))
+                                         covering_null_row_width,
+                                         item, result,
+                                         semi_join_conds->argument_list());
+      if (!pm_engine ||
+          ((subselect_rowid_merge_engine*) pm_engine)->
+            init(nn_key_parts, &partial_match_key_parts))
       {
+        /*
+          The call to init() would fail if there was not enough memory to allocate
+          all buffers for the rowid merge strategy. In this case revert to table
+          scanning which doesn't need any big buffers.
+        */
+        delete pm_engine;
+        pm_engine= NULL;
         strategy= PARTIAL_MATCH_SCAN;
-      delete rowid_merge_engine;
-      /* TIMOUR: setup execution structures for partial match via scanning. */
       }
-    else
+    }
+
+    if (strategy == PARTIAL_MATCH_SCAN)
+    {
+      if (!(pm_engine=
+            new subselect_table_scan_engine((subselect_uniquesubquery_engine*)
+                                            lookup_engine, tmp_table,
+                                            item, result,
+                                            semi_join_conds->argument_list(),
+                                            covering_null_row_width)))
       {
-      strategy= PARTIAL_MATCH_INDEX;
-      lookup_engine= rowid_merge_engine;
+        /* This is an irrecoverable error. */
+        res= 1;
+        goto err;
+      }
     }
   }
 
+  if (pm_engine)
+    lookup_engine= pm_engine;
   item_in->change_engine(lookup_engine);
 
 err:
@@ -4009,10 +4169,8 @@ Ordered_key::Ordered_key(uint keyid_arg, TABLE *tbl_arg, Item *search_key_arg,
 
 Ordered_key::~Ordered_key()
 {
-  /*
-    All data structures are allocated on thd->mem_root, thus we don't
-    free them here.
-  */
+  my_free((char*) key_buff, MYF(0));
+  bitmap_free(&null_key);
 }
 
 
@@ -4030,6 +4188,7 @@ void Ordered_key::cleanup()
   */
 }
 
+
 /*
   Initialize a multi-column index.
 */
@@ -4103,14 +4262,16 @@ bool Ordered_key::init(int col_idx)
 }
 
 
+/*
+  Allocate the buffers for both the row number, and the NULL-bitmap indexes.
+*/
+
 bool Ordered_key::alloc_keys_buffers()
 {
-  THD *thd= tbl->in_use;
-
   DBUG_ASSERT(key_buff_elements > 0);
 
-  if (!(key_buff= (rownum_t*) thd->alloc(key_buff_elements *
-                                         sizeof(rownum_t))))
+  if (!(key_buff= (rownum_t*) my_malloc(key_buff_elements * sizeof(rownum_t),
+                                        MYF(MY_WME))))
     return TRUE;
 
   /*
@@ -4118,10 +4279,8 @@ bool Ordered_key::alloc_keys_buffers()
     (max_null_row - min_null_row), and then use min_null_row as
     lookup offset.
   */
-  if (bitmap_init_memroot(&null_key,
-                          /* this is max array index, we need count, so +1. */
-                          max_null_row + 1,
-                          thd->mem_root))
+  /* Notice that max_null_row is max array index, we need count, so +1. */
+  if (bitmap_init(&null_key, NULL, max_null_row + 1, FALSE))
     return TRUE;
 
   cur_key_idx= HA_POS_ERROR;
@@ -4193,8 +4352,9 @@ void Ordered_key::sort_keys()
 
 
 /*
-  The probability that a certain row does not contain a NULL in some row in
-  a NULL-indexed column.
+  The fraction of rows that do not contain NULL in the columns indexed by
+  this key.
+
   @retval  1  if there are no NULLs
   @retval  0  if only NULLs
 */
@@ -4353,10 +4513,122 @@ void Ordered_key::print(String *str)
 }
 
 
+subselect_partial_match_engine::subselect_partial_match_engine(
+  subselect_uniquesubquery_engine *engine_arg,
+  TABLE *tmp_table_arg, Item_subselect *item_arg,
+  select_result_interceptor *result_arg,
+  List<Item> *equi_join_conds_arg,
+  uint covering_null_row_width_arg)
+  :subselect_engine(item_arg, result_arg),
+   tmp_table(tmp_table_arg), lookup_engine(engine_arg),
+   equi_join_conds(equi_join_conds_arg),
+   covering_null_row_width(covering_null_row_width_arg)
+{}
+
+
+int subselect_partial_match_engine::exec()
+{
+  Item_in_subselect *item_in= (Item_in_subselect *) item;
+  int res;
+
+  /* Try to find a matching row by index lookup. */
+  res= lookup_engine->copy_ref_key_simple();
+  if (res == -1)
+  {
+    /* The result is FALSE based on the outer reference. */
+    item_in->value= 0;
+    item_in->null_value= 0;
+    return 0;
+  }
+  else if (res == 0)
+  {
+    /* Search for a complete match. */
+    if ((res= lookup_engine->index_lookup()))
+    {
+      /* An error occured during lookup(). */
+      item_in->value= 0;
+      item_in->null_value= 0;
+      return res;
+    }
+    else if (item_in->value)
+    {
+      /*
+        A complete match was found, the result of IN is TRUE.
+        Notice: (this->item == lookup_engine->item)
+      */
+      return 0;
+    }
+  }
+
+  if (covering_null_row_width == tmp_table->s->fields)
+  {
+    /*
+      If there is a NULL-only row that coveres all columns the result of IN
+      is UNKNOWN. 
+    */
+    item_in->value= 0;
+    /*
+      TIMOUR: which one is the right way to propagate an UNKNOWN result?
+      Should we also set empty_result_set= FALSE; ???
+    */
+    //item_in->was_null= 1;
+    item_in->null_value= 1;
+    return 0;
+  }
+
+  /*
+    There is no complete match. Look for a partial match (UNKNOWN result), or
+    no match (FALSE).
+  */
+  if (tmp_table->file->inited)
+    tmp_table->file->ha_index_end();
+
+  if (partial_match())
+  {
+    /* The result of IN is UNKNOWN. */
+    item_in->value= 0;
+    /*
+      TIMOUR: which one is the right way to propagate an UNKNOWN result?
+      Should we also set empty_result_set= FALSE; ???
+    */
+    //item_in->was_null= 1;
+    item_in->null_value= 1;
+  }
+  else
+  {
+    /* The result of IN is FALSE. */
+    item_in->value= 0;
+    /*
+      TIMOUR: which one is the right way to propagate an UNKNOWN result?
+      Should we also set empty_result_set= FALSE; ???
+    */
+    //item_in->was_null= 0;
+    item_in->null_value= 0;
+  }
+
+  return 0;
+}
+
+
+void subselect_partial_match_engine::print(String *str,
+                                           enum_query_type query_type)
+{
+  /*
+    Should never be called as the actual engine cannot be known at query
+    optimization time.
+  */
+  DBUG_ASSERT(FALSE);
+}
+
+
 /*
   @param non_null_key_parts  
   @param partial_match_key_parts  A union of all single-column NULL key parts.
   @param count_partial_match_columns Number of NULL keyparts (set bits above).
+
+  @retval FALSE  the engine was initialized successfully
+  @retval TRUE   there was some (memory allocation) error during initialization,
+                 such errors should be interpreted as revert to other strategy
 */
 
 bool
@@ -4379,14 +4651,17 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
     return FALSE;
   }
 
-  DBUG_ASSERT(!has_covering_null_row || (has_covering_null_row &&
+  DBUG_ASSERT(!covering_null_row_width || (covering_null_row_width &&
                                            keys_count == 1 &&
                                            non_null_key_parts));
-
+  /*
+    Allocate buffers to hold the merged keys and the mapping between rowids and
+    row numbers.
+  */
   if (!(merge_keys= (Ordered_key**) thd->alloc(keys_count *
                                                sizeof(Ordered_key*))) ||
-      !(row_num_to_rowid= (uchar*) thd->alloc(row_count * rowid_length *
-                                              sizeof(uchar))))
+      !(row_num_to_rowid= (uchar*) my_malloc(row_count * rowid_length *
+                                             sizeof(uchar), MYF(MY_WME))))
     return TRUE;
 
   /* Create the only non-NULL key if there is any. */
@@ -4395,10 +4670,7 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
     non_null_key= new Ordered_key(cur_keyid, tmp_table, item_in->left_expr,
                                   0, 0, 0, row_num_to_rowid);
     if (non_null_key->init(non_null_key_parts))
-    {
-      // TIMOUR: revert to partial matching via scanning
       return TRUE;
-    }
     merge_keys[cur_keyid]= non_null_key;
     merge_keys[cur_keyid]->first();
     ++cur_keyid;
@@ -4406,9 +4678,10 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
 
   /*
     If there is a covering NULL row, the only key that is needed is the
-    only non-NULL key that is already created above.
+    only non-NULL key that is already created above. We create keys on
+    NULL-able columns only if there is no covering NULL row.
   */
-  if (!has_covering_null_row)
+  if (!covering_null_row_width)
   {
     if (bitmap_init_memroot(&matching_keys, keys_count, thd->mem_root) ||
         bitmap_init_memroot(&matching_outer_cols, keys_count, thd->mem_root) ||
@@ -4436,10 +4709,7 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
                                      result_sink->get_max_null_of_col(i),
                                      row_num_to_rowid);
         if (merge_keys[cur_keyid]->init(i))
-        {
-          // TIMOUR: revert to partial matching via scanning
           return TRUE;
-        }
         merge_keys[cur_keyid]->first();
       }
       ++cur_keyid;
@@ -4510,10 +4780,7 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
 
   if (init_queue(&pq, keys_count, 0, FALSE,
                  subselect_rowid_merge_engine::cmp_keys_by_cur_rownum, NULL))
-  {
-    // TIMOUR: revert to partial matching via scanning
     return TRUE;
-  }
 
   return FALSE;
 }
@@ -4521,26 +4788,21 @@ subselect_rowid_merge_engine::init(MY_BITMAP *non_null_key_parts,
 
 subselect_rowid_merge_engine::~subselect_rowid_merge_engine()
 {
+  /* None of the resources below is allocated if there are no ordered keys. */
+  if (keys_count)
+  {
+    my_free((char*) row_num_to_rowid, MYF(0));
+    for (uint i= 0; i < keys_count; i++)
+      delete merge_keys[i];
     delete_queue(&pq);
-}
-
-
-void subselect_rowid_merge_engine::cleanup()
-{
-  lookup_engine->cleanup();
-  /* Tell handler we don't need the index anymore */
-  if (tmp_table->file->inited)
+    if (tmp_table->file->inited == handler::RND)
       tmp_table->file->ha_rnd_end();
-  queue_remove_all(&pq);
+  }
 }
 
 
-void subselect_rowid_merge_engine::print(String *str, enum_query_type query_type)
+void subselect_rowid_merge_engine::cleanup()
 {
-  str->append(STRING_WITH_LEN("<rowid_merge>("));
-  for (uint i= 0; i < keys_count; i++)
-    merge_keys[i]->print(str);
-  str->append(')');
 }
 
 
@@ -4627,20 +4889,31 @@ bool subselect_rowid_merge_engine::partial_match()
   Ordered_key *cur_key;
   rownum_t cur_row_num;
   uint count_nulls_in_search_key= 0;
+  bool res= FALSE;
 
   /* If there is a non-NULL key, it must be the first key in the keys array. */
   DBUG_ASSERT(!non_null_key || (non_null_key && merge_keys[0] == non_null_key));
+
+  /* All data accesses during execution are via handler::ha_rnd_pos() */
+  tmp_table->file->ha_rnd_init(0);
+
   /* Check if there is a match for the columns of the only non-NULL key. */
   if (non_null_key && !non_null_key->lookup())
-    return FALSE;
+  {
+    res= FALSE;
+    goto end;
+  }
 
   /*
     If there is a NULL (sub)row that covers all NULL-able columns,
     then there is a guranteed partial match, and we don't need to search
     for the matching row.
    */
-  if (has_covering_null_row)
-    return TRUE;
+  if (covering_null_row_width)
+  {
+    res= TRUE;
+    goto end;
+  }
 
   if (non_null_key)
     queue_insert(&pq, (uchar *) non_null_key);
@@ -4667,14 +4940,20 @@ bool subselect_rowid_merge_engine::partial_match()
   if (count_nulls_in_search_key ==
       ((Item_in_subselect *) item)->left_expr->cols() -
       (non_null_key ? non_null_key->get_column_count() : 0))
-    return TRUE;
+  {
+    res= TRUE;
+    goto end;
+  }
 
   /*
     If there is no NULL (sub)row that covers all NULL columns, and there is no
     single match for any of the NULL columns, the result is FALSE.
   */
   if (pq.elements - test(non_null_key) == 0)
-    return FALSE;
+  {
+    res= FALSE;
+    goto end;
+  }
 
   DBUG_ASSERT(pq.elements);
 
@@ -4692,10 +4971,8 @@ bool subselect_rowid_merge_engine::partial_match()
       Check the only matching row of the only key min_key for NULL matches
       in the other columns.
     */
-    if (test_null_row(min_row_num))
-      return TRUE;
-    else
-      return FALSE;
+    res= test_null_row(min_row_num);
+    goto end;
   }
 
   while (TRUE)
@@ -4710,7 +4987,10 @@ bool subselect_rowid_merge_engine::partial_match()
       /* Follows from the correct use of priority queue. */
       DBUG_ASSERT(cur_row_num > min_row_num);
       if (test_null_row(min_row_num))
-        return TRUE;
+      {
+        res= TRUE;
+        goto end;
+      }
       else
       {
         min_key= cur_key;
@@ -4727,99 +5007,112 @@ bool subselect_rowid_merge_engine::partial_match()
     if (pq.elements == 0)
     {
       /* Check the last row of the last column in PQ for NULL matches. */
-      if (test_null_row(min_row_num))
-        return TRUE;
-      else
-        return FALSE;
+      res= test_null_row(min_row_num);
+      goto end;
     }
   }
 
-  /* We should never get here. */
+  /* We should never get here - all branches must be handled explicitly above. */
   DBUG_ASSERT(FALSE);
-  return FALSE;
+
+end:
+  tmp_table->file->ha_rnd_end();
+  return res;
 }
 
 
-int subselect_rowid_merge_engine::exec()
-{
-  Item_in_subselect *item_in= (Item_in_subselect *) item;
-  int res;
+subselect_table_scan_engine::subselect_table_scan_engine(
+  subselect_uniquesubquery_engine *engine_arg,
+  TABLE *tmp_table_arg,
+  Item_subselect *item_arg,
+  select_result_interceptor *result_arg,
+  List<Item> *equi_join_conds_arg,
+  uint covering_null_row_width_arg)
+  :subselect_partial_match_engine(engine_arg, tmp_table_arg, item_arg,
+                                  result_arg, equi_join_conds_arg,
+                                  covering_null_row_width_arg)
+{}
 
-  /* Try to find a matching row by index lookup. */
-  res= lookup_engine->copy_ref_key_simple();
-  if (res == -1)
-  {
-    /* The result is FALSE based on the outer reference. */
-    item_in->value= 0;
-    item_in->null_value= 0;
-    return 0;
-  }
-  else if (res == 0)
+
+/*
+  TIMOUR:
+  This method is based on subselect_uniquesubquery_engine::scan_table().
+  Consider refactoring somehow, 80% of the code is the same.
+
+  for each row_i in tmp_table
   {
-    if ((res= lookup_engine->index_lookup()))
+    count_matches= 0;
+    for each row element row_i[j]
     {
-      /* An error occured during lookup(). */
-      item_in->value= 0;
-      item_in->null_value= 0;
-      return res;
+      if (outer_ref[j] is NULL || row_i[j] is NULL || outer_ref[j] == row_i[j])
+        ++count_matches;
     }
-    else if (item_in->value)
-    {
+    if (count_matches == outer_ref.elements)
+      return TRUE
+  }
+  return FALSE
+*/
+
+bool subselect_table_scan_engine::partial_match()
+{
+  List_iterator_fast<Item> equality_it(*equi_join_conds);
+  Item *cur_eq;
+  uint count_matches;
+  int error;
+  bool res;
+
+  tmp_table->file->ha_rnd_init(1);
+  tmp_table->file->extra_opt(HA_EXTRA_CACHE,
+                             current_thd->variables.read_buff_size);
   /*
-        A complete match was found, the result of IN is TRUE.
-        Notice: (this->item == lookup_engine->item)
+  TIMOUR:
+  scan_table() also calls "table->null_row= 0;", why, do we need it?
   */
-      return 0;
+  for (;;)
+  {
+    error= tmp_table->file->ha_rnd_next(tmp_table->record[0]);
+    if (error) {
+      if (error == HA_ERR_RECORD_DELETED)
+      {
+        error= 0;
+        continue;
       }
+      if (error == HA_ERR_END_OF_FILE)
+      {
+        error= 0;
+        break;
       }
-
-  if (has_covering_null_row && !keys_count)
+      else
       {
-    /*
-      If there is a NULL-only row that coveres all columns the result of IN
-      is UNKNOWN. 
-    */
-    item_in->value= 0;
-    /*
-      TIMOUR: which one is the right way to propagate an UNKNOWN result?
-      Should we also set empty_result_set= FALSE; ???
-    */
-    //item_in->was_null= 1;
-    item_in->null_value= 1;
-    return 0;
+        error= report_error(tmp_table, error);
+        break;
+      }
     }
 
-  /* All data accesses during execution are via handler::ha_rnd_pos() */
-  if (tmp_table->file->inited)
-    tmp_table->file->ha_index_end();
-  tmp_table->file->ha_rnd_init(0);
-  /*
-    There is no complete match. Look for a partial match (UNKNOWN result), or
-    no match (FALSE).
-  */
-  if (partial_match())
+    equality_it.rewind();
+    count_matches= 0;
+    while ((cur_eq= equality_it++))
     {
-    /* The result of IN is UNKNOWN. */
-    item_in->value= 0;
-    /*
-      TIMOUR: which one is the right way to propagate an UNKNOWN result?
-      Should we also set empty_result_set= FALSE; ???
-    */
-    //item_in->was_null= 1;
-    item_in->null_value= 1;
+      DBUG_ASSERT(cur_eq->type() == Item::FUNC_ITEM &&
+                  ((Item_func*)cur_eq)->functype() == Item_func::EQ_FUNC);
+      if (!cur_eq->val_int() && !cur_eq->null_value)
+        break;
+      ++count_matches;
     }
-  else
+    if (count_matches == tmp_table->s->fields)
     {
-    /* The result of IN is FALSE. */
-    item_in->value= 0;
-    /*
-      TIMOUR: which one is the right way to propagate an UNKNOWN result?
-      Should we also set empty_result_set= FALSE; ???
-    */
-    //item_in->was_null= 0;
-    item_in->null_value= 0;
+      res= TRUE; /* Found a matching row. */
+      goto end;
+    }
   }
+
+  res= FALSE;
+end:
   tmp_table->file->ha_rnd_end();
+  return res;
+}
 
-  return 0;
+
+void subselect_table_scan_engine::cleanup()
+{
 }

sql/item_subselect.h

@@ -436,7 +436,7 @@ public:
   friend class Item_in_optimizer;
   friend class subselect_indexsubquery_engine;
   friend class subselect_hash_sj_engine;
-  friend class subselect_rowid_merge_engine;
+  friend class subselect_partial_match_engine;
 };
 
 
@@ -472,7 +472,7 @@ public:
   enum enum_engine_type {ABSTRACT_ENGINE, SINGLE_SELECT_ENGINE,
                          UNION_ENGINE, UNIQUESUBQUERY_ENGINE,
                          INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE,
-                         ROR_INTERSECT_ENGINE};
+                         ROWID_MERGE_ENGINE, TABLE_SCAN_ENGINE};
 
   subselect_engine(Item_subselect *si, select_result_interceptor *res)
     :thd(0)
@@ -716,6 +716,109 @@ inline bool Item_subselect::is_uncacheable() const
 }
 
 
+/**
+  Compute an IN predicate via a hash semi-join. This class is responsible for
+  the materialization of the subquery, and the selection of the correct and
+  optimal execution method (e.g. direct index lookup, or partial matching) for
+  the IN predicate.
+*/
+
+class subselect_hash_sj_engine : public subselect_engine
+{
+protected:
+  /* The table into which the subquery is materialized. */
+  TABLE *tmp_table;
+  /* TRUE if the subquery was materialized into a temp table. */
+  bool is_materialized;
+  /*
+    The old engine already chosen at parse time and stored in permanent memory.
+    Through this member we can re-create and re-prepare materialize_join for
+    each execution of a prepared statement. We also reuse the functionality
+    of subselect_single_select_engine::[prepare | cols].
+  */
+  subselect_single_select_engine *materialize_engine;
+  /* The engine used to compute the IN predicate. */
+  subselect_engine *lookup_engine;
+  /*
+    QEP to execute the subquery and materialize its result into a
+    temporary table. Created during the first call to exec().
+  */
+  JOIN *materialize_join;
+
+  /* Keyparts of the only non-NULL composite index in a rowid merge. */
+  MY_BITMAP non_null_key_parts;
+  /* Keyparts of the single column indexes with NULL, one keypart per index. */
+  MY_BITMAP partial_match_key_parts;
+  uint count_partial_match_columns;
+  uint count_null_only_columns;
+  /*
+    A conjunction of all the equality condtions between all pairs of expressions
+    that are arguments of an IN predicate. We need these to post-filter some
+    IN results because index lookups sometimes match values that are actually
+    not equal to the search key in SQL terms.
+ */
+  Item_cond_and *semi_join_conds;
+  /* Possible execution strategies that can be used to compute hash semi-join.*/
+  enum exec_strategy {
+    UNDEFINED,
+    COMPLETE_MATCH, /* Use regular index lookups. */
+    PARTIAL_MATCH,  /* Use some partial matching strategy. */
+    PARTIAL_MATCH_MERGE, /* Use partial matching through index merging. */
+    PARTIAL_MATCH_SCAN,  /* Use partial matching through table scan. */
+    IMPOSSIBLE      /* Subquery materialization is not applicable. */
+  };
+  /* The chosen execution strategy. Computed after materialization. */
+  exec_strategy strategy;
+protected:
+  exec_strategy get_strategy_using_schema();
+  exec_strategy get_strategy_using_data();
+  size_t rowid_merge_buff_size(bool has_non_null_key,
+                               bool has_covering_null_row,
+                               MY_BITMAP *partial_match_key_parts);
+  void choose_partial_match_strategy(bool has_non_null_key,
+                                     bool has_covering_null_row,
+                                     MY_BITMAP *partial_match_key_parts);
+  bool make_semi_join_conds();
+  subselect_uniquesubquery_engine* make_unique_engine();
+
+public:
+  subselect_hash_sj_engine(THD *thd, Item_subselect *in_predicate,
+                           subselect_single_select_engine *old_engine)
+    :subselect_engine(in_predicate, NULL), tmp_table(NULL),
+    is_materialized(FALSE), materialize_engine(old_engine), lookup_engine(NULL),
+    materialize_join(NULL), count_partial_match_columns(0),
+    count_null_only_columns(0), semi_join_conds(NULL), strategy(UNDEFINED)
+  {
+    set_thd(thd);
+  }
+  ~subselect_hash_sj_engine();
+
+  bool init_permanent(List<Item> *tmp_columns);
+  bool init_runtime();
+  void cleanup();
+  int prepare() { return 0; } /* Override virtual function in base class. */
+  int exec();
+  virtual void print(String *str, enum_query_type query_type);
+  uint cols()
+  {
+    return materialize_engine->cols();
+  }
+  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT; }
+  table_map upper_select_const_tables() { return 0; }
+  bool no_rows() { return !tmp_table->file->stats.records; }
+  virtual enum_engine_type engine_type() { return HASH_SJ_ENGINE; }
+  /*
+    TODO: factor out all these methods in a base subselect_index_engine class
+    because all of them have dummy implementations and should never be called.
+  */
+  void fix_length_and_dec(Item_cache** row);//=>base class
+  void exclude(); //=>base class
+  //=>base class
+  bool change_result(Item_subselect *si, select_result_interceptor *result);
+  bool no_tables();//=>base class
+};
+
+
 /*
   Distinguish the type od (0-based) row numbers from the type of the index into
   an array of row numbers.
@@ -745,7 +848,7 @@ typedef ha_rows rownum_t;
     PS (re)execution, however most of the comprising objects can be reused.
 */
 
-class Ordered_key
+class Ordered_key : public Sql_alloc
 {
 protected:
   /*
@@ -761,6 +864,8 @@ protected:
   uint key_column_count;
   /*
     An expression, or sequence of expressions that forms the search key.
+    The search key is a sequence when it is Item_row. Each element of the
+    sequence is accessible via Item::element_index(int i).
   */
   Item *search_key;
 
@@ -808,8 +913,6 @@ protected:
   int cmp_key_with_search_key(rownum_t row_num);
 
 public:
-  static void *operator new(size_t size) throw ()
-  { return sql_alloc(size); }
   Ordered_key(uint keyid_arg, TABLE *tbl_arg,
               Item *search_key_arg, ha_rows null_count_arg,
               ha_rows min_null_row_arg, ha_rows max_null_row_arg,
@@ -828,6 +931,10 @@ public:
     DBUG_ASSERT(i < key_column_count);
     return key_columns[i]->field->field_index;
   }
+  /*
+    Get the search key element that corresponds to the i-th key part of this
+    index.
+  */
   Item *get_search_key(uint i)
   {
     return search_key->element_index(key_columns[i]->field->field_index);
@@ -899,7 +1006,7 @@ public:
 };
 
 
-class subselect_rowid_merge_engine: public subselect_engine
+class subselect_partial_match_engine : public subselect_engine
 {
 protected:
   /* The temporary table that contains a materialized subquery. */
@@ -910,6 +1017,51 @@ protected:
     FALSE and UNKNOWN.
   */
   subselect_uniquesubquery_engine *lookup_engine;
+  /* A list of equalities between each pair of IN operands. */
+  List<Item> *equi_join_conds;
+  /*
+    If there is a row, such that all its NULL-able components are NULL, this
+    member is set to the number of covered columns. If there is no covering
+    row, then this is 0.
+  */
+  uint covering_null_row_width;
+protected:
+  virtual bool partial_match()= 0;
+public:
+  subselect_partial_match_engine(subselect_uniquesubquery_engine *engine_arg,
+                                 TABLE *tmp_table_arg, Item_subselect *item_arg,
+                                 select_result_interceptor *result_arg,
+                                 List<Item> *equi_join_conds_arg,
+                                 uint covering_null_row_width_arg);
+  int prepare() { return 0; }
+  int exec();
+  void fix_length_and_dec(Item_cache**) {}
+  uint cols() { /* TODO: what is the correct value? */ return 1; }
+  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT; }
+  void exclude() {}
+  table_map upper_select_const_tables() { return 0; }
+  bool change_result(Item_subselect*, select_result_interceptor*)
+  { DBUG_ASSERT(FALSE); return false; }
+  bool no_tables() { return false; }
+  bool no_rows()
+  {
+    /*
+      TODO: It is completely unclear what is the semantics of this
+      method. The current result is computed so that the call to no_rows()
+      from Item_in_optimizer::val_int() sets Item_in_optimizer::null_value
+      correctly.
+    */
+    return !(((Item_in_subselect *) item)->null_value);
+  }
+  void print(String*, enum_query_type);
+
+  friend void subselect_hash_sj_engine::cleanup();
+};
+
+
+class subselect_rowid_merge_engine: public subselect_partial_match_engine
+{
+protected:
   /*
     Mapping from row numbers to row ids. The rowids are stored sequentially
     in the array - rowid[i] is located in row_num_to_rowid + i * rowid_length.
@@ -953,8 +1105,6 @@ protected:
     This queue is used by the partial match algorithm in method exec().
   */
   QUEUE pq;
-  /* True if there is a NULL (sub)row that covers all NULLable columns. */
-  bool has_covering_null_row;
 protected:
   /*
     Comparison function to compare keys in order of decreasing bitmap
@@ -972,143 +1122,34 @@ protected:
 public:
   subselect_rowid_merge_engine(subselect_uniquesubquery_engine *engine_arg,
                                TABLE *tmp_table_arg, uint keys_count_arg,
-                               uint has_covering_null_row_arg,
+                               uint covering_null_row_width_arg,
                                Item_subselect *item_arg,
-                               select_result_interceptor *result_arg)
-    :subselect_engine(item_arg, result_arg),
-    tmp_table(tmp_table_arg), lookup_engine(engine_arg),
-    keys_count(keys_count_arg), non_null_key(NULL),
-    has_covering_null_row(has_covering_null_row_arg)
+                               select_result_interceptor *result_arg,
+                               List<Item> *equi_join_conds_arg)
+    :subselect_partial_match_engine(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)
   {
     thd= lookup_engine->get_thd();
   }
   ~subselect_rowid_merge_engine();
   bool init(MY_BITMAP *non_null_key_parts, MY_BITMAP *partial_match_key_parts);
   void cleanup();
-  int prepare() { return 0; }
-  void fix_length_and_dec(Item_cache**) {}
-  int exec();
-  uint cols() { /* TODO: what is the correct value? */ return 1; }
-  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT; }
-  void exclude() {}
-  table_map upper_select_const_tables() { return 0; }
-  void print(String*, enum_query_type);
-  bool change_result(Item_subselect*, select_result_interceptor*)
-  { DBUG_ASSERT(FALSE); return false; }
-  bool no_tables() { return false; }
-  bool no_rows()
-  {
-    /*
-      TODO: It is completely unclear what is the semantics of this
-      method. The current result is computed so that the call to no_rows()
-      from Item_in_optimizer::val_int() sets Item_in_optimizer::null_value
-      correctly.
-    */
-    return !(((Item_in_subselect *) item)->null_value);
-  }
+  virtual enum_engine_type engine_type() { return ROWID_MERGE_ENGINE; }
 };
 
 
-/**
-  Compute an IN predicate via a hash semi-join. This class is responsible for
-  the materialization of the subquery, and the selection of the correct and
-  optimal execution method (e.g. direct index lookup, or partial matching) for
-  the IN predicate.
-*/
-
-class subselect_hash_sj_engine : public subselect_engine
+class subselect_table_scan_engine: public subselect_partial_match_engine
 {
 protected:
-  /* The table into which the subquery is materialized. */
-  TABLE *tmp_table;
-  /* TRUE if the subquery was materialized into a temp table. */
-  bool is_materialized;
-  /*
-    The old engine already chosen at parse time and stored in permanent memory.
-    Through this member we can re-create and re-prepare materialize_join for
-    each execution of a prepared statement. We also reuse the functionality
-    of subselect_single_select_engine::[prepare | cols].
-  */
-  subselect_single_select_engine *materialize_engine;
-  /* The engine used to compute the IN predicate. */
-  subselect_engine *lookup_engine;
-  /*
-    QEP to execute the subquery and materialize its result into a
-    temporary table. Created during the first call to exec().
-  */
-  JOIN *materialize_join;
-  /*
-    TRUE if the subquery result has an all-NULL column, which means that
-    there at best can be a partial match for any IN execution.
-  */
-  bool inner_partial_match;
-  /*
-    TRUE if the materialized subquery contains a whole row only of NULLs.
-  */
-  bool has_null_row;
-
-  /* Keyparts of the only non-NULL composite index in a rowid merge. */
-  MY_BITMAP non_null_key_parts;
-  /* Keyparts of the single column indexes with NULL, one keypart per index. */
-  MY_BITMAP partial_match_key_parts;
-  uint count_partial_match_columns;
-  uint count_null_only_columns;
-  /*
-    A conjunction of all the equality condtions between all pairs of expressions
-    that are arguments of an IN predicate. We need these to post-filter some
-    IN results because index lookups sometimes match values that are actually
-    not equal to the search key in SQL terms.
- */
-  Item *semi_join_conds;
-  /* Possible execution strategies that can be used to compute hash semi-join.*/
-  enum exec_strategy {
-    COMPLETE_MATCH, /* Use regular index lookups. */
-    PARTIAL_MATCH,  /* Use some partial matching strategy. */
-    PARTIAL_MATCH_INDEX, /* Use partial matching through index merging. */
-    PARTIAL_MATCH_SCAN,  /* Use partial matching through table scan. */
-    IMPOSSIBLE      /* Subquery materialization is not applicable. */
-  };
-  /* The chosen execution strategy. Computed after materialization. */
-  exec_strategy strategy;
-protected:
-  void set_strategy_using_schema();
-  void set_strategy_using_data();
-  bool make_semi_join_conds();
-  subselect_uniquesubquery_engine* make_unique_engine();
-
+  bool partial_match();
 public:
-  subselect_hash_sj_engine(THD *thd, Item_subselect *in_predicate,
-                           subselect_single_select_engine *old_engine)
-    :subselect_engine(in_predicate, NULL), tmp_table(NULL),
-    is_materialized(FALSE), materialize_engine(old_engine), lookup_engine(NULL),
-    materialize_join(NULL), count_partial_match_columns(0),
-    count_null_only_columns(0), semi_join_conds(NULL)
-  {
-    set_thd(thd);
-  }
-  ~subselect_hash_sj_engine();
-
-  bool init_permanent(List<Item> *tmp_columns);
-  bool init_runtime();
+  subselect_table_scan_engine(subselect_uniquesubquery_engine *engine_arg,
+                              TABLE *tmp_table_arg, Item_subselect *item_arg,
+                              select_result_interceptor *result_arg,
+                              List<Item> *equi_join_conds_arg,
+                              uint covering_null_row_width_arg);
   void cleanup();
-  int prepare() { return 0; } /* Override virtual function in base class. */
-  int exec();
-  virtual void print (String *str, enum_query_type query_type);
-  uint cols()
-  {
-    return materialize_engine->cols();
-  }
-  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT; }
-  table_map upper_select_const_tables() { return 0; }
-  bool no_rows() { return !tmp_table->file->stats.records; }
-  virtual enum_engine_type engine_type() { return HASH_SJ_ENGINE; }
-  /*
-    TODO: factor out all these methods in a base subselect_index_engine class
-    because all of them have dummy implementations and should never be called.
-  */
-  void fix_length_and_dec(Item_cache** row);//=>base class
-  void exclude(); //=>base class
-  //=>base class
-  bool change_result(Item_subselect *si, select_result_interceptor *result);
-  bool no_tables();//=>base class
+  virtual enum_engine_type engine_type() { return TABLE_SCAN_ENGINE; }
 };

sql/mysql_priv.h

@@ -552,12 +552,14 @@ protected:
 #define OPTIMIZER_SWITCH_LOOSE_SCAN 64
 #define OPTIMIZER_SWITCH_MATERIALIZATION 128
 #define OPTIMIZER_SWITCH_SEMIJOIN 256
+#define OPTIMIZER_SWITCH_PARTIAL_MATCH_ROWID_MERGE 512
+#define OPTIMIZER_SWITCH_PARTIAL_MATCH_TABLE_SCAN 1024
 
 #ifdef DBUG_OFF
-#  define OPTIMIZER_SWITCH_LAST 512
+#  define OPTIMIZER_SWITCH_LAST 2048
 #else
-#  define OPTIMIZER_SWITCH_TABLE_ELIMINATION 512
-#  define OPTIMIZER_SWITCH_LAST 1024
+#  define OPTIMIZER_SWITCH_TABLE_ELIMINATION 2048
+#  define OPTIMIZER_SWITCH_LAST 4096
 #endif
 
 #ifdef DBUG_OFF 
@@ -570,7 +572,9 @@ protected:
                                     OPTIMIZER_SWITCH_FIRSTMATCH | \
                                     OPTIMIZER_SWITCH_LOOSE_SCAN | \
                                     OPTIMIZER_SWITCH_MATERIALIZATION | \
-                                    OPTIMIZER_SWITCH_SEMIJOIN)
+                                    OPTIMIZER_SWITCH_SEMIJOIN | \
+                                    OPTIMIZER_SWITCH_PARTIAL_MATCH_ROWID_MERGE|\
+                                    OPTIMIZER_SWITCH_PARTIAL_MATCH_TABLE_SCAN)
 #else
 #  define OPTIMIZER_SWITCH_DEFAULT (OPTIMIZER_SWITCH_INDEX_MERGE | \
                                     OPTIMIZER_SWITCH_INDEX_MERGE_UNION | \
@@ -581,7 +585,9 @@ protected:
                                     OPTIMIZER_SWITCH_FIRSTMATCH | \
                                     OPTIMIZER_SWITCH_LOOSE_SCAN | \
                                     OPTIMIZER_SWITCH_MATERIALIZATION | \
-                                    OPTIMIZER_SWITCH_SEMIJOIN)
+                                    OPTIMIZER_SWITCH_SEMIJOIN | \
+                                    OPTIMIZER_SWITCH_PARTIAL_MATCH_ROWID_MERGE|\
+                                    OPTIMIZER_SWITCH_PARTIAL_MATCH_TABLE_SCAN)
 #endif
 
 /*

sql/mysqld.cc

@@ -302,6 +302,8 @@ static const char *optimizer_switch_names[]=
   "index_merge_intersection",
   "index_condition_pushdown",
   "firstmatch","loosescan","materialization", "semijoin",
+  "partial_match_rowid_merge",
+  "partial_match_table_scan",
 #ifndef DBUG_OFF
   "table_elimination",
 #endif
@@ -320,6 +322,8 @@ static const unsigned int optimizer_switch_names_len[]=
   sizeof("loosescan") - 1,
   sizeof("materialization") - 1,
   sizeof("semijoin") - 1,
+  sizeof("partial_match_rowid_merge") - 1,
+  sizeof("partial_match_table_scan") - 1,
 #ifndef DBUG_OFF
   sizeof("table_elimination") - 1,
 #endif
@@ -5794,7 +5798,8 @@ enum options_mysqld
   OPT_RECORD_RND_BUFFER, OPT_DIV_PRECINCREMENT, OPT_RELAY_LOG_SPACE_LIMIT,
   OPT_RELAY_LOG_PURGE,
   OPT_SLAVE_NET_TIMEOUT, OPT_SLAVE_COMPRESSED_PROTOCOL, OPT_SLOW_LAUNCH_TIME,
-  OPT_SLAVE_TRANS_RETRIES, OPT_READONLY, OPT_DEBUGGING, OPT_DEBUG_FLUSH,
+  OPT_SLAVE_TRANS_RETRIES, OPT_READONLY, OPT_ROWID_MERGE_BUFF_SIZE,
+  OPT_DEBUGGING, OPT_DEBUG_FLUSH,
   OPT_SORT_BUFFER, OPT_TABLE_OPEN_CACHE, OPT_TABLE_DEF_CACHE,
   OPT_THREAD_CONCURRENCY, OPT_THREAD_CACHE_SIZE,
   OPT_TMP_TABLE_SIZE, OPT_THREAD_STACK,
@@ -7130,6 +7135,11 @@ The minimum value for this variable is 4096.",
    (uchar**) &max_system_variables.range_alloc_block_size, 0, GET_ULONG,
    REQUIRED_ARG, RANGE_ALLOC_BLOCK_SIZE, RANGE_ALLOC_BLOCK_SIZE,
    (longlong) ULONG_MAX, 0, 1024, 0},
+  {"rowid_merge_buff_size", OPT_ROWID_MERGE_BUFF_SIZE,
+   "The size of the buffers used [NOT] IN evaluation via partial matching.",
+   (uchar**) &global_system_variables.rowid_merge_buff_size,
+   (uchar**) &max_system_variables.rowid_merge_buff_size, 0, GET_ULONG,
+   REQUIRED_ARG, 8*1024*1024L, 0, MAX_MEM_TABLE_SIZE/2, 0, 1, 0},
   {"read_buffer_size", OPT_RECORD_BUFFER,
    "Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.",
    (uchar**) &global_system_variables.read_buff_size,

sql/set_var.cc

@@ -540,6 +540,9 @@ static sys_var_long_ptr	sys_query_cache_size(&vars, "query_cache_size",
 
 static sys_var_thd_ulong	sys_range_alloc_block_size(&vars, "range_alloc_block_size",
 						   &SV::range_alloc_block_size);
+static sys_var_thd_ulong	sys_rowid_merge_buff_size(&vars, "rowid_merge_buff_size",
+					   &SV::rowid_merge_buff_size);
+
 static sys_var_thd_ulong	sys_query_alloc_block_size(&vars, "query_alloc_block_size",
 						   &SV::query_alloc_block_size,
 						   0, fix_thd_mem_root);

sql/sql_class.h

@@ -343,6 +343,8 @@ struct system_variables
   ulong mrr_buff_size;
   ulong div_precincrement;
   ulong sortbuff_size;
+  /* Total size of all buffers used by the subselect_rowid_merge_engine. */
+  ulong rowid_merge_buff_size;
   ulong thread_handling;
   ulong tx_isolation;
   ulong completion_type;

support-files/build-tags

@@ -4,7 +4,7 @@ rm -f TAGS
 filter='\.cc$\|\.c$\|\.h$\|\.yy$'
 
 list="find . -type f"
-bzr root >/dev/null 2>/dev/null && list="bzr ls --from-root --kind=file --versioned"
+bzr root >/dev/null 2>/dev/null && list="bzr ls --from-root -R --kind=file --versioned"
 
 $list |grep $filter |while read f; 
 do