mariadb/sql/opt_index_cond_pushdown.cc

#include "mysql_priv.h"
#include "sql_select.h"

/****************************************************************************
 * Index Condition Pushdown code starts
 ***************************************************************************/
/* 
  Check if given expression uses only table fields covered by the given index

  SYNOPSIS
    uses_index_fields_only()
      item           Expression to check
      tbl            The table having the index
      keyno          The index number
      other_tbls_ok  TRUE <=> Fields of other non-const tables are allowed

  DESCRIPTION
    Check if given expression only uses fields covered by index #keyno in the
    table tbl. The expression can use any fields in any other tables.
    
    The expression is guaranteed not to be AND or OR - those constructs are 
    handled outside of this function.

  RETURN
    TRUE   Yes
    FALSE  No
*/

bool uses_index_fields_only(Item *item, TABLE *tbl, uint keyno,
                            bool other_tbls_ok)
{
  if (item->const_item())
    return TRUE;

  /* 
    Don't push down the triggered conditions. Nested outer joins execution 
    code may need to evaluate a condition several times (both triggered and
    untriggered), and there is no way to put thi
    TODO: Consider cloning the triggered condition and using the copies for:
      1. push the first copy down, to have most restrictive index condition
         possible
      2. Put the second copy into tab->select_cond. 
  */
  if (item->type() == Item::FUNC_ITEM && 
      ((Item_func*)item)->functype() == Item_func::TRIG_COND_FUNC)
    return FALSE;

  if (!(item->used_tables() & tbl->map))
    return other_tbls_ok;

  Item::Type item_type= item->type();
  switch (item_type) {
  case Item::FUNC_ITEM:
    {
      /* This is a function, apply condition recursively to arguments */
      Item_func *item_func= (Item_func*)item;
      Item **child;
      Item **item_end= (item_func->arguments()) + item_func->argument_count();
      for (child= item_func->arguments(); child != item_end; child++)
      {
        if (!uses_index_fields_only(*child, tbl, keyno, other_tbls_ok))
          return FALSE;
      }
      return TRUE;
    }
  case Item::COND_ITEM:
    {
      /*
        This is a AND/OR condition. Regular AND/OR clauses are handled by
        make_cond_for_index() which will chop off the part that can be
        checked with index. This code is for handling non-top-level AND/ORs,
        e.g. func(x AND y).
      */
      List_iterator<Item> li(*((Item_cond*)item)->argument_list());
      Item *item;
      while ((item=li++))
      {
        if (!uses_index_fields_only(item, tbl, keyno, other_tbls_ok))
          return FALSE;
      }
      return TRUE;
    }
  case Item::FIELD_ITEM:
    {
      Item_field *item_field= (Item_field*)item;
      if (item_field->field->table != tbl) 
        return TRUE;
      /*
        The below is probably a repetition - the first part checks the
        other two, but let's play it safe:
      */
      return item_field->field->part_of_key.is_set(keyno) &&
             item_field->field->type() != MYSQL_TYPE_GEOMETRY &&
             item_field->field->type() != MYSQL_TYPE_BLOB;
    }
  case Item::REF_ITEM:
    return uses_index_fields_only(item->real_item(), tbl, keyno,
                                  other_tbls_ok);
  default:
    return FALSE; /* Play it safe, don't push unknown non-const items */
  }
}

#define ICP_COND_USES_INDEX_ONLY 10

/*
  Get a part of the condition that can be checked using only index fields

  SYNOPSIS
    make_cond_for_index()
      cond           The source condition
      table          The table that is partially available
      keyno          The index in the above table. Only fields covered by the index
                     are available
      other_tbls_ok  TRUE <=> Fields of other non-const tables are allowed

  DESCRIPTION
    Get a part of the condition that can be checked when for the given table 
    we have values only of fields covered by some index. The condition may
    refer to other tables, it is assumed that we have values of all of their 
    fields.

    Example:
      make_cond_for_index(
         "cond(t1.field) AND cond(t2.key1) AND cond(t2.non_key) AND cond(t2.key2)",
          t2, keyno(t2.key1)) 
      will return
        "cond(t1.field) AND cond(t2.key2)"

  RETURN
    Index condition, or NULL if no condition could be inferred.
*/

Item *make_cond_for_index(Item *cond, TABLE *table, uint keyno,
                          bool other_tbls_ok)
{
  if (!cond)
    return NULL;
  if (cond->type() == Item::COND_ITEM)
  {
    uint n_marked= 0;
    if (((Item_cond*) cond)->functype() == Item_func::COND_AND_FUNC)
    {
      table_map used_tables= 0;
      Item_cond_and *new_cond=new Item_cond_and;
      if (!new_cond)
	return (COND*) 0;
      List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
      Item *item;
      while ((item=li++))
      {
	Item *fix= make_cond_for_index(item, table, keyno, other_tbls_ok);
	if (fix)
        {
	  new_cond->argument_list()->push_back(fix);
          used_tables|= fix->used_tables();
        }
        if (test(item->marker == ICP_COND_USES_INDEX_ONLY))
        {
          n_marked++;
          item->marker= 0;
        } 
      }
      if (n_marked ==((Item_cond*)cond)->argument_list()->elements)
        cond->marker= ICP_COND_USES_INDEX_ONLY;
      switch (new_cond->argument_list()->elements) {
      case 0:
	return (COND*) 0;
      case 1:
        new_cond->used_tables_cache= used_tables;
	return new_cond->argument_list()->head();
      default:
	new_cond->quick_fix_field();
        new_cond->used_tables_cache= used_tables;
	return new_cond;
      }
    }
    else /* It's OR */
    {
      Item_cond_or *new_cond=new Item_cond_or;
      if (!new_cond)
	return (COND*) 0;
      List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
      Item *item;
      while ((item=li++))
      {
	Item *fix= make_cond_for_index(item, table, keyno, other_tbls_ok);
	if (!fix)
	  return (COND*) 0;
	new_cond->argument_list()->push_back(fix);
        if (test(item->marker == ICP_COND_USES_INDEX_ONLY))
        {
          n_marked++;
          item->marker= 0;
        } 
      }
      if (n_marked ==((Item_cond*)cond)->argument_list()->elements)
        cond->marker= ICP_COND_USES_INDEX_ONLY;
      new_cond->quick_fix_field();
      new_cond->used_tables_cache= ((Item_cond_or*) cond)->used_tables_cache;
      new_cond->top_level_item();
      return new_cond;
    }
  }

  if (!uses_index_fields_only(cond, table, keyno, other_tbls_ok))
    return (COND*) 0;
  cond->marker= ICP_COND_USES_INDEX_ONLY;
  return cond;
}


Item *make_cond_remainder(Item *cond, bool exclude_index)
{
  if (exclude_index && cond->marker == ICP_COND_USES_INDEX_ONLY)
    return 0; /* Already checked */

  if (cond->type() == Item::COND_ITEM)
  {
    table_map tbl_map= 0;
    if (((Item_cond*) cond)->functype() == Item_func::COND_AND_FUNC)
    {
      /* Create new top level AND item */
      Item_cond_and *new_cond=new Item_cond_and;
      if (!new_cond)
	return (COND*) 0;
      List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
      Item *item;
      while ((item=li++))
      {
	Item *fix= make_cond_remainder(item, exclude_index);
	if (fix)
        {
	  new_cond->argument_list()->push_back(fix);
          tbl_map |= fix->used_tables();
        }
      }
      switch (new_cond->argument_list()->elements) {
      case 0:
	return (COND*) 0;
      case 1:
	return new_cond->argument_list()->head();
      default:
	new_cond->quick_fix_field();
        ((Item_cond*)new_cond)->used_tables_cache= tbl_map;
	return new_cond;
      }
    }
    else /* It's OR */
    {
      Item_cond_or *new_cond=new Item_cond_or;
      if (!new_cond)
	return (COND*) 0;
      List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
      Item *item;
      while ((item=li++))
      {
	Item *fix= make_cond_remainder(item, FALSE);
	if (!fix)
	  return (COND*) 0;
	new_cond->argument_list()->push_back(fix);
        tbl_map |= fix->used_tables();
      }
      new_cond->quick_fix_field();
      ((Item_cond*)new_cond)->used_tables_cache= tbl_map;
      new_cond->top_level_item();
      return new_cond;
    }
  }
  return cond;
}


/*
  Try to extract and push the index condition

  SYNOPSIS
    push_index_cond()
      tab            A join tab that has tab->table->file and its condition
                     in tab->select_cond
      keyno          Index for which extract and push the condition

  DESCRIPTION
    Try to extract and push the index condition down to table handler
*/

void push_index_cond(JOIN_TAB *tab, uint keyno)
{
  DBUG_ENTER("push_index_cond");
  Item *idx_cond;
  bool do_index_cond_pushdown=
    ((tab->table->file->index_flags(keyno, 0, 1) &
      HA_DO_INDEX_COND_PUSHDOWN) &&
     optimizer_flag(tab->join->thd, OPTIMIZER_SWITCH_INDEX_COND_PUSHDOWN));

  /*
    Do not try index condition pushdown on indexes which have partially-covered
    columns. Unpacking from a column prefix into index tuple is not a supported 
    operation in some engines, see e.g. MySQL BUG#42991.
    TODO: a better solution would be not to consider partially-covered columns
    as parts of the index and still produce/check index condition for
    fully-covered index columns.
  */
  KEY *key_info= tab->table->key_info + keyno;
  for (uint kp= 0; kp < key_info->key_parts; kp++)
  {
    if ((key_info->key_part[kp].key_part_flag & HA_PART_KEY_SEG))
    {
      do_index_cond_pushdown= FALSE;
      break;
    }
  }

  if (do_index_cond_pushdown)
  {
    DBUG_EXECUTE("where",
                 print_where(tab->select_cond, "full cond", QT_ORDINARY););

    idx_cond= make_cond_for_index(tab->select_cond, tab->table, keyno,
                                  tab->icp_other_tables_ok);

    DBUG_EXECUTE("where",
                 print_where(idx_cond, "idx cond", QT_ORDINARY););

    if (idx_cond)
    {
      Item *idx_remainder_cond= 0;
      tab->pre_idx_push_select_cond= tab->select->cond;
      /*
        For BKA cache we store condition to special BKA cache field
        because evaluation of the condition requires additional operations
        before the evaluation. This condition is used in 
        JOIN_CACHE_BKA[_UNIQUE]::skip_index_tuple() functions.
      */
      if (tab->use_join_cache &&
          /*
            if cache is used then the value is TRUE only 
            for BKA[_UNIQUE] cache (see check_join_cache_usage func).
          */
          tab->icp_other_tables_ok &&
          (idx_cond->used_tables() &
           ~(tab->table->map | tab->join->const_table_map)))
        tab->cache_idx_cond= idx_cond;
      else
        idx_remainder_cond= tab->table->file->idx_cond_push(keyno, idx_cond);

      /*
        Disable eq_ref's "lookup cache" if we've pushed down an index
        condition. 
        TODO: This check happens to work on current ICP implementations, but
        there may exist a compliant implementation that will not work 
        correctly with it. Sort this out when we stabilize the condition
        pushdown APIs.
      */
      if (idx_remainder_cond != idx_cond)
        tab->ref.disable_cache= TRUE;

      Item *row_cond= tab->idx_cond_fact_out ? 
                        make_cond_remainder(tab->select_cond, TRUE) :
	                tab->pre_idx_push_select_cond;

      DBUG_EXECUTE("where",
                   print_where(row_cond, "remainder cond", QT_ORDINARY););
      
      if (row_cond)
      {
        if (!idx_remainder_cond)
          tab->select_cond= row_cond;
        else
        {
          COND *new_cond= new Item_cond_and(row_cond, idx_remainder_cond);
          tab->select_cond= new_cond;
	  tab->select_cond->quick_fix_field();
          ((Item_cond_and*)tab->select_cond)->used_tables_cache= 
            row_cond->used_tables() | idx_remainder_cond->used_tables();
        }
      }
      else
        tab->select_cond= idx_remainder_cond;
      if (tab->select)
      {
        DBUG_EXECUTE("where",
                     print_where(tab->select->cond,
                                 "select_cond",
                                 QT_ORDINARY););

        tab->select->cond= tab->select_cond;
        tab->select->pre_idx_push_select_cond= tab->pre_idx_push_select_cond;
      }
    }
  }
  DBUG_VOID_RETURN;
}