- Fix the crash by making get_column_range_cardinality()
to handle the special case where Column_stats objects
is an all-zeros object (the question of what is the point
of having Field::read_stats point to such object remains a
mystery)
- Added a few comments. Learning the code still.
- Histogram::find_bucket() should not walk off the end of the value range.
- Address review feedback in Histogram::point_selectivity(): different handling
for zero-width buckets, and explanations.
- Fix Histogram::point_selectivity() to work in the case where the
passed value_pos=0 (or 1) and the first (or the last) bucket in the
histogram has zero value-range (i.e one value).
[Attempt #2]
- Use a new selectivity calculation formula in Histogram::point_selectivity.
The formula is different from the old one because it was developed from scratch.
it doesn't have any possible division-by-zero problems.
After constant table row substitution the where condition may be converted
to always true. The function calculate_cond_selectivity_for_table() should
take into account this possibility.
The function calculate_cond_selectivity_for_table() must consider
the case when the key range tree returned by the call of get_mm_tree()
is of the type SEL_TREE::ALWAYS.
When calculating the selectivity of a range in the function
get_column_range_cardinality a check whether NULL values are
included into into the range must be done.
Don't try to a histogram if it is not read into the cache for statistical data.
It may happen so if optimizer_use_condition_selectivity is set to 3. This
setting orders the optimizer not use histograms to calculate selectivity.
When performing the range analysis for a conjunction the function
calculate_cond_selectivity_for_table should take in to account that
the analysis of some conjuncts may return SEL_ARG::IMPOSSIBLE.
One of them is quite serious: the function table_cond_selectivity used
the TABLE_REF structure for ref/eq_ref access methods as if they had been
filled. In fact these structure are filled after the best execution plan
has been chosen.
The other bugs happened due to:
- an erroneous attempt at get statistics on the result of materialization
of a view
- incorrect handling of ranges with no left/right limits when calculating
selectivity of range conditions on non-indexed columns
- lack of cleanup for some newly introduced fields
