Problem: When building the condition for JOIN::outer_ref_cond the optimizer forgot to take into account
that this condition could depend on constant tables as well.
- Create/use do_copy_nullable_row_to_notnull() function for ref access, which is used
when copying from not-NULL field in table that can be NULL-complemented to not-NULL field.
fixed several defects in the greedy optimization:
1) The greedy optimizer calculated the 'compare-cost' (CPU-cost)
for iterating over the partial plan result at each level in
the query plan as 'record_count / (double) TIME_FOR_COMPARE'
This cost was only used locally for 'best' calculation at each
level, and *not* accumulated into the total cost for the query plan.
This fix added the 'CPU-cost' of processing 'current_record_count'
records at each level to 'current_read_time' *before* it is used as
'accumulated cost' argument to recursive
best_extension_by_limited_search() calls. This ensured that the
cost of a huge join-fanout early in the QEP was correctly
reflected in the cost of the final QEP.
To get identical cost for a 'best' optimized query and a
straight_join with the same join order, the same change was also
applied to optimize_straight_join() and get_partial_join_cost()
2) Furthermore to get equal cost for 'best' optimized query and a
straight_join the new code substrcated the same '0.001' in
optimize_straight_join() as it had been already done in
best_extension_by_limited_search()
3) When best_extension_by_limited_search() aggregated the 'best' plan a
plan was 'best' by the check :
'if ((search_depth == 1) || (current_read_time < join->best_read))'
The term '(search_depth == 1' incorrectly caused a new best plan to be
collected whenever the specified 'search_depth' was reached - even if
this partial query plan was more expensive than what we had already
found.
If the sorted table belongs to a dependent subquery then the function
create_sort_index() should not clear TABLE:: select and TABLE::select
for this table after the sort of the table has been performed, because
these members are needed for the second execution of the subquery.
The patch differs from the original MySQL patch as follows:
- All test case differences have been reviewed one by one, and
care has been taken to restore the original plan so that each
test case executes the code path it was designed for.
- A bug was found and fixed in MariaDB 5.3 in
Item_allany_subselect::cleanup().
- ORDER BY is not removed because we are unsure of all effects,
and it would prevent enabling ORDER BY ... LIMIT subqueries.
- ref_pointer_array.m_size is not adjusted because we don't do
array bounds checking, and because it looks risky.
Original comment by Jorgen Loland:
-------------------------------------------------------------
WL#5953 - Optimize away useless subquery clauses
For IN/ALL/ANY/SOME/EXISTS subqueries, the following clauses are
meaningless:
* ORDER BY (since we don't support LIMIT in these subqueries)
* DISTINCT
* GROUP BY if there is no HAVING clause and no aggregate
functions
This WL detects and optimizes away these useless parts of the
query during JOIN::prepare()
- Correctly handle plan refinement stage for LooseScan plans: run create_ref_for_key() if LooseScan
plan includes a ref access, and if we don't have any fixed key components, switch to a full index scan.
- Let JTBM optimization code handle the case where the subquery is degenerate and doesn't have a
join query plan. Regular materialization would fall back to IN->EXISTS for such cases. Semi-Join
materialization does not have such option, instead we introduce and use "constant JTBM join tabs".
- Do a "more thorough" cleanup of SJ-Materialization join tab in JOIN_TAB::cleanup. The bug
was due to the fact that JOIN_TAB::cleanup() may be called multiple times for the same tab
if the join has grouping.
If the duplicate elimination strategy is used for a semi-join and potentially
one of the block-based join algorithms can be employed to join the inner
tables of the semi-join then sorting of the head (first non-constant) table
for a query with ORDER BY / GROUP BY cannot be used.
The execution plan cannot use sorting on the first table from the
sequence of the joined tables if it plans to employ the block-based
hash join algorithm.
- Part 1 of the fix: for semi-join merged subqueries, calling child_join->optimize() until we're done with all
PS-lifetime optimizations in the parent.
- Make create_tmp_table() set KEY_PART_INFO attributes for the keys it creates.
This wasn't needed before but is needed now, when temp. tables that are
results of SJ-Materialization are being used for joins.
This particular bug depended on HA_VAR_LENGTH_PART being set,
but also added code to set HA_BLOB_PART and HA_NULL_PART when appropriate.
KEYUSE elements for a possible hash join key are not sorted by field
numbers of the second table T of the hash join operation. Besides
some of these KEYUSE elements cannot be used to build any key as their
key expressions depend on the tables that are planned to be accessed
after the table T.
The code before the patch did not take this into account and, as a result,
execition of a query the employing block-based hash join algorithm could
cause a crash or return a wrong result set.
in EXPLAIN as select_type==MATERIALIZED.
Before, we had select_type==SUBQUERY and it was difficult to tell materialized
subqueries from uncorrelated scalar-context subqueries.
If has been decided that the first match strategy is to be used to join table T
from a semi-join nest while no buffer can be employed to join this table
then no join buffer can be used to join any table in the join sequence between
the first one belonging to the semi-join nest and table T.
The tables from the same semi-join or outer join nest cannot use
join buffers if in the join sequence of the query execution plan
they are separated by a table that is planned to be joined without
usage of a join buffer.
Analysis:
lp:894397 was a consequence of a prior incorrect fix of lp:833777
which didn't take into account that even when all tables are
constant there may be correlated conditions, and the where clause
is not equivalent to the constant conditions.
Solution:
When there are constant tables only, evaluate only the conditions
that reference outer fields, because the constant conditions are
already checked, and the where clause doesn't have other conditions
than constant ones, and outer referencing ones. The fix for
lp:894397 also fixes lp:833777.
- Make EXPLAIN display "Start temporary" at the start of the fanout (it used to display
at the first table whose rowid gets into temp. table which is not that useful for
the user)
- Updated test results (all checked)
- Break down POSITION/advance_sj_state() into four classes
representing potential semi-join strategies.
- Treat all strategies uniformly (before, DuplicateWeedout
was special as it was the catch-all strategy. Now, we're
still relying on it to be the catch-all, but are able to
function,e.g. with firstmatch=on,duplicate_weedout=off.
- Update test results (checked)
Analysis:
The optimizer distinguishes two kinds of 'constant' conditions:
expensive ones, and non-expensive ones. The non-expensive conditions
are evaluated inside make_join_select(), and if false, already the
optimizer detects empty query results.
In order to avoid arbitrarily expensive optimization, the evaluation of
expensive constant conditions is delayed until execution. These conditions
are attached to JOIN::exec_const_cond and evaluated in the beginning of
JOIN::exec. The relevant execution logic is:
JOIN::exec()
{
if (! join->exec_const_cond->val_int())
{
produce an empty result;
stop execution
}
continue execution
execute the original WHERE clause (that contains exec_const_cond)
...
}
As a result, when an expensive constant condition is
TRUE, it is evaluated twice - once through
JOIN::exec_const_cond, and once through JOIN::cond.
When the expensive constant condition is a subquery,
predicate, the subquery is evaluated twice. If we have
many levels of subqueries, this logic results in a chain
of recursive subquery executions that walk a perfect
binary tree. The result is that for subquries with depth N,
JOIN::exec is executed O(2^N) times.
Solution:
Notice that the second execution of the constant conditions
happens inside do_select(), in the branch:
if (join->table_count == join->const_tables) { ... }
In this case exec_const_cond is equivalent to the whole WHERE
clause, therefore the WHERE clause has already been checked in
the beginnig of JOIN::exec, and has been found to be true.
The bug is addressed by not evaluating the WHERE clause if there
was exec_const_conds, and it was TRUE.
If the optimizer switch 'semijoin_with_cache' is set to 'off' then
join cache cannot be used to join inner tables of a semijoin.
Also fixed a bug in the function check_join_cache_usage() that led
to wrong output of the EXPLAIN commands for some test cases.
