The original code was there to favor index search over table scan.
This is not needed anymore as the cost calculations for table scans
and index lookups are now more exact.
calculate_cond_selectivity_for_table() is largely rewritten:
- Process keys in the order of rows found, smaller ranges first. If two
ranges has equal number of rows, use the one with more key parts.
This helps us to mark more used fields to not be used for further
selectivity calculations. See cmp_quick_ranges().
- Ignore keys with fields that where used by previous keys
- Don't use rec_per_key[] to calculate selectivity for smaller
secondary key parts. This does not work as rec_per_key[] value
is calculated in the context of the previous key parts, not for the
key part itself. The one exception is if the previous key parts
are all constants.
Other things:
- Ensure that select->cond_selectivity is always between 0 and 1.
- Ensure that select->opt_range_condition_rows is never updated to
a higher value. It is initially set to the number of rows in table.
- We now store in table->opt_range_condition_rows the lowest number of
rows that any row-read-method has found so far. Before it was only done
for QUICK_SELECT_I::QS_TYPE_ROR_UNION and
QUICK_SELECT_I::QS_TYPE_INDEX_MERGE.
Now it is done for a lot more methods. See
calculate_cond_selectivity_for_table() for details.
- Calculate and use selectivity for the first key part of a multiple key
part if the first key part is a constant.
WHERE key1_part1=5 and key2_part1=5. IF key1 is used, then we can still
use selectivity for key2
Changes in test results:
- 'filtered' is slightly changed, usually to something slightly smaller.
- A few cases where for group by queries the table order changed. This was
because the number of resulting rows from a group by query with MIN/MAX
is now set to be smaller.
- A few index was changed as we now prefer index with more key parts if
the number of resulting rows is the same.
The issue was that calc_cond_selectivity_for_table prefered ranges with
many parts and when deciding on which selectivity to use.
Fixed by going through ranges according to the number of rows in the range.
This ensures that selectivity from ranges with few rows will be prefered
over ranges with many rows for indexes that uses the same columns.
