If when extracting a range condition for an index from the WHERE condition
Range Optimizer sees that the range condition covers the whole index then
such condition should be discarded because it cannot be used in any range
scan. In some cases Range Optimizer really does it, but there remained some
conditions for which it was not done. As a result the optimizer could
produce index merge plans with the full index scan for one of the indexes
participating in the index merge.
This could be observed in one of the test cases from index_merge1.inc
where a plan with index_merge_sort_union was produced and in the test case
reported for this bug where a plan with index_merge_sort_intersect was
produced. In both cases one of two index scans participating in index merge
ran over the whole index.
The patch slightly changes the original above mentioned test case from
index_merge1.inc to be able to produce an intended plan employing
index_merge_sort_union. The original query was left to show that index
merge is not used for it anymore.
It should be noted that for the plan with index_merge_sort_intersect could
be chosen for execution only due to a defect in the InnoDB code that
returns wrong estimates for the cardinality of big ranges.
This bug led to serious problems in 10.4+ where the optimization using
Rowid filters is employed (see mdev-26446).
Approved by Sergey Petrunia <sergey@mariadb.com>
This bug could manifest itself in a very rare cases when the optimizer
chose an execution plan by which a joined table was accessed by a table
scan and the optimizer was checking whether ranges checked for each record
could improve this plan. In such cases the optimizer evaluates range
conditions over a table that depend on other tables. For such conditions
the constructed SEL_ARG trees are marked as MAYBE_KEY. If a SEL_ARG object
constructed for a sargable condition marked as RANGE_KEY had the same
first key part as a MAYBE_KEY SEL_ARG object and the key_and() function
was called for this pair of SEL_ARG objects then an invalid SEL_ARG
object could be constructed that ultimately could lead to a crash before
the execution phase.
In the function QUICK_RANGE_SELECT::init_ror_merged_scan we create a seperate handler if the handler in
head->file cannot be reused. The flag free_file tells us if we have a seperate handler or not.
There are cases where you might create a handler and then there might be a failure(running ALTER)
and then we have to revert the handler back to the original one. The code does that
but it does not reset the flag 'free_file' in this case.
Also backported f2c418079d.
"Range Checked for Each Record" should be only employed when the other
option would be cross-product join (i.e. the other option is so bad that
we hardly risk anything).
Previous logic was: use RCfER if there are no possible quick selects, or
quick select would read > 100 rows. Also, it didn't always work as
expected due to range optimizer changing table->quick_keys and us
looking at sel->quick_keys.
Another angle is that recent versions have enabled use of Join Buffering
in e.g. outer joins. This further reduces the range of cases where RCfER
should be used.
We are still unable to estimate the cost of RCfER with any precision, so
now changing the condition of "no quick select or quick->records> 100"
to a hopefully better condition "no quick select or quick would cost more
than full table scan".
