Function table_after_join_selectivity

sql/sql_select.cc:11267–11525

*/

static
double table_after_join_selectivity(JOIN *join, uint idx, JOIN_TAB *s,
                                    table_map rem_tables,
                                    double *new_records_out)
{
  uint16 ref_keyuse_steps_buf[MAX_REF_PARTS];
  uint   ref_keyuse_size= MAX_REF_PARTS;
  uint16 *ref_keyuse_steps= ref_keyuse_steps_buf;
  Field *field;
  TABLE *table= s->table;
  MY_BITMAP *read_set= table->read_set;
  POSITION *pos= &join->positions[idx];
  double sel, records_out= pos->records_out;
  uint keyparts= 0;
  uint found_part_ref_or_null= 0;

  if (pos->key != 0)
  {
    sel= table->cond_selectivity;
    /* 
      A ref access or hash join is used for this table. ref access is created
      from

        tbl.keypart1=expr1 AND tbl.keypart2=expr2 AND ...
      
      and it will only return rows for which this condition is satisfied.
      Suppose, certain expr{i} is a constant. Since ref access only returns
      rows that satisfy
        
         tbl.keypart{i}=const       (*)

      then selectivity of this equality should not be counted in return value 
      of this function. This function uses the value of 
       
         table->cond_selectivity=selectivity(COND(tbl)) (**)
      
      as a starting point. This value includes selectivity of equality (*). We
      should somehow discount it. 
      
      Looking at calculate_cond_selectivity_for_table(), one can see that
      the value is not necessarily a direct multiplicand in 
      table->cond_selectivity

      There are three possible ways to discount
      1. There is a potential range access on t.keypart{i}=const. 
         (an important special case: the used ref access has a const prefix for
          which a range estimate is available)
      
      2. The field has a histogram. field[x]->cond_selectivity has the data.
      
      3. Use index stats on this index:
         rec_per_key[key_part+1]/rec_per_key[key_part]

      (TODO: more details about the "t.key=othertable.col" case)
    */
    KEYUSE *keyuse= pos->key;
    KEYUSE *prev_ref_keyuse= keyuse;
    uint key= keyuse->key;