diff options
Diffstat (limited to 'sql/sql_select.cc')
| -rw-r--r-- | sql/sql_select.cc | 55 |
1 files changed, 33 insertions, 22 deletions
diff --git a/sql/sql_select.cc b/sql/sql_select.cc index d537dccc5a6..53c41482a36 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -1898,7 +1898,7 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count, best=best_time=records=DBL_MAX; KEYUSE *best_key=0; uint best_max_key_part=0; - my_bool found_constrain= 0; + my_bool found_constraint= 0; if (s->keyuse) { /* Use key if possible */ @@ -1979,7 +1979,7 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count, } else { - found_constrain= 1; + found_constraint= 1; /* Check if we found full key */ @@ -2133,12 +2133,28 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count, s->table->used_keys && best_key) && !(s->table->force_index && best_key)) { // Check full join + ha_rows rnd_records= s->found_records; /* Estimate cost of reading table. */ - tmp= (double) s->read_time; + tmp= s->table->file->scan_time(); + /* + If there is a restriction on the table, assume that 25% of the + rows can be skipped on next part. + This is to force tables that this table depends on before this + table + */ + if (found_constraint) + rnd_records-= rnd_records/4; + if (s->on_expr) // Can't use join cache { - /* We have to read the whole table for each record */ - tmp*= record_count; + tmp= record_count * + /* We have to read the whole table for each record */ + (tmp + + /* + And we have to skip rows which does not satisfy join + condition for each record. + */ + (s->records - rnd_records)/(double) TIME_FOR_COMPARE); } else { @@ -2146,30 +2162,25 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count, tmp*= (1.0 + floor((double) cache_record_length(join,idx) * record_count / (double) thd->variables.join_buff_size)); + /* + We don't make full cartesian product between rows in the scanned + table and existing records because we skip all rows from the + scanned table, which does not satisfy join condition when + we read the table (see flush_cached_records for details). Here we + take into account cost to read and skip these records. + */ + tmp+= (s->records - rnd_records)/(double) TIME_FOR_COMPARE; } /* - We estimate the cost of making full cortesian product between - rows in the scanned table and generated records as - record_count*s->records/TIME_FOR_COMPARE. Taking into account - cost of evaluating WHERE clause for s->found_records is not - necessary because it costs much less than the cost mentioned - above. + We estimate the cost of evaluating WHERE clause for found records + as record_count * rnd_records + TIME_FOR_COMPARE. This cost plus + tmp give us total cost of using TABLE SCAN */ if (best == DBL_MAX || - (tmp + record_count/(double) TIME_FOR_COMPARE*s->records < + (tmp + record_count/(double) TIME_FOR_COMPARE*rnd_records < best + record_count/(double) TIME_FOR_COMPARE*records)) { - /* - If there is a restriction on the table, assume that 25% of the - rows can be skipped on next part. - This is to force tables that this table depends on before this - table - */ - ha_rows rnd_records= s->found_records; - if (found_constrain) - rnd_records-= rnd_records/4; - /* If the table has a range (s->quick is set) make_join_select() will ensure that this will be used |