diff options
Diffstat (limited to 'sql')
| -rw-r--r-- | sql/sql_select.cc | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/sql/sql_select.cc b/sql/sql_select.cc index 62f40eeb99c..2682c06f0cf 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -25929,7 +25929,11 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table, uint tablenr= tab - join->join_tab; read_time= join->best_positions[tablenr].read_time; for (uint i= tablenr+1; i < join->table_count; i++) + { fanout*= join->best_positions[i].records_read; // fanout is always >= 1 + // But selectivity is =< 1 : + fanout*= join->best_positions[i].cond_selectivity; + } } else read_time= table->file->scan_time(); @@ -26067,6 +26071,23 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table, */ select_limit= (ha_rows) (select_limit < fanout ? 1 : select_limit/fanout); + + /* + refkey_rows_estimate is E(#rows) produced by the table access + strategy that was picked without regard to ORDER BY ... LIMIT. + + It will be used as the source of selectivity data. + Use table->cond_selectivity as a better estimate which includes + condition selectivity too. + */ + { + // we use MIN(...), because "Using LooseScan" queries have + // cond_selectivity=1 while refkey_rows_estimate has a better + // estimate. + refkey_rows_estimate= MY_MIN(refkey_rows_estimate, + table_records * table->cond_selectivity); + } + /* We assume that each of the tested indexes is not correlated with ref_key. Thus, to select first N records we have to scan @@ -26077,6 +26098,7 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table, N/(refkey_rows_estimate/table_records) > table_records <=> N > refkey_rows_estimate. */ + if (select_limit > refkey_rows_estimate) select_limit= table_records; else |