- This is a regession introduced by fix for BUG#951937
- The problem was that there were scenarios where check_simple_equality() would create an
Item_equal object but would not call item_equal->set_context_field() on it.
- The fix was to add the missing calls.
- The problem was with execution strategy for cases where FirstMatch's inner tables
were interleaved with outer-uncorrelated tables.
- I was unable to find any cases where such join orders would be practically useful,
so fixed it by disabling them.
- Fix equality propagation to work with SJM nests and OR clauses (full descirption of problem and
solution in the comment in the patch)
(The second commit with post-review fixes)
- The problem was that
= we've picked a LooseScan that used full index scan (tab->type==JT_ALL) on certain index.
= there was also a quick select (tab->quick!=NULL), that used other indexes.
= some old code assumes that (tab->type==JT_ALL && tab->quick) -> means that the
quick select should be used, which is not true.
Fixed by discarding the quick select as soon as we know we're using LooseScan
without using the quick select.
The result of materialization of the right part of an IN subquery predicate
is placed into a temporary table. Each row of the materialized table is
distinct. A unique key over all fields of the temporary table is defined and
created. It allows to perform key look-ups into the table.
The table created for a materialized subquery can be accessed by key as
any other table. The function best_access-path search for the best access
to join a table to a given partial join. With some where conditions this
function considers a possibility of a ref_or_null access. If such access
employs the unique key on the temporary table then when estimating
the cost this access the function tries to use the array rec_per_key. Yet,
such array is not built for this unique key. This causes a crash of the server.
Rows returned by the subquery that contain nulls don't have to be placed
into temporary table, as they cannot be match any row produced by the
left part of the subquery predicate. So all fields of the temporary table
can be defined as non-nullable. In this case any ref_or_null access
to the temporary table does not make any sense and it does not make sense
to estimate such an access.
The fix makes sure that the temporary table for a materialized IN subquery
is defined with columns that are all non-nullable. The also ensures that
any row with nulls returned by the subquery is not placed into the
temporary table.
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.
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()
- if we're considering FirstMatch access with one inner table, and
@@optimizer_switch has semijoin_with_cache flag, calculate costs
as if we used join cache (because we will be able to do so)
- 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.
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.
- 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)
If a materialized derived table / view is empty then for this table
the value of file->ref is 0. This was not taken into account by
the function JOIN_CACHE::write_record_data. As a result a query
using an empty materialized derived tables as inner tables of outer
joins and IN subqueries in WHERE conditions could cause server crashes
when the optimizer employed join caches and duplicate elimination for
semi-joins.
- in make_join_select(), use the correct condition to check whether the current table is a SJM nest (the previous
condition used to be correct before, but then sj-materialization temp table creation was moved to happen before
make_join_select was called)
of the 5.3 code line after a merge with 5.2 on 2010-10-28
in order not to allow the cost to access a joined table to be equal
to 0 ever.
Expanded data sets for many test cases to get the same execution plans
as before.
- The problem was that Item_direct_view_ref and its embedded Item_field were getting incorrect
value of item->used_tables() after fix_fields() in the second and subsequent EXECUTE.
- Made relevant fixes in Item_field::fix_fields() and find_field_in_tables(), so that the
Item_field gets the correct attributes.
(This is not a real fix for this bug, even though it makes it to no longer repeat)
- Semi-join subquery predicates, i.e. ... WHERE outer_expr IN (SELECT ...) may have null-rejecting properties,
may allow to convert outer joins into inner.
- When convert_subq_to_sj() injected IN-equality into parent's WHERE/ON clause, it didn't call
$new_cond->top_level_item(), which would cause null-rejecting properties to be lost.
- Fixed, now the mentioned outer-to-inner conversion will really take place.
- Set the default
- Adjust the testcases so that 'new' tests are run with optimizations turned on.
- Pull out relevant tests from "irrelevant" tests and run them with optimizations on.
- Run range.test and innodb.test with both mrr=on and mrr=off
semijoin=on,firstmatch=on,loosescan=on
to
semijoin=off,firstmatch=off,loosescan=off
Adjust the testcases:
- Modify subselect*.test and join_cache.test so that all tests
use the same execution paths as before (i.e. optimizations that
are being tested are enabled)
- Let all other test files run with the new default settings (i.e.
with new optimizations disabled)
- Copy subquery testcases from these files into t/subselect_extra.test
which will run them with new optimizations enabled.
- in advance_sj_state(), remember join->cur_dups_producing_tables in
pos->prefix_dups_producing_tables *before* we modify it, so that
restore_prev_sj_state() restores cur_dups_producing_tables in all cases.
- Updated test results in subselect_sj2[_jcl6].result (the original EXPLAIN
was invalid there)
- "Using MRR" is no longer shown with range access.
- Instead, both range and BKA accesses will show one of the following:
= "Rowid-ordered scan"
= "Key-ordered scan"
= "Key-ordered Rowid-ordered scan"
depending on whether DS-MRR implementation will do scan keys in order, rowids in order,
or both.
- The patch also introduces a way for other storage engines/MRR implementations to
pass information to EXPLAIN output about the properties of employed MRR scans.
even in the cases when there existed range/index-merge scans that
were cheaper than the full table scan.
This was a defect/bug of the implementation of mwl #128.
Now hash join can work not only with full table scan of the joined
table, but also with full index scan, range and index-merge scans.
Accordingly, in the cases when hash join is used the column 'type'
in the EXPLAINs can contain now 'hash_ALL', 'hash_index', 'hash_range'
and 'hash_index_merge'. If hash join is coupled with a range/index_merge
scan then the columns 'key' and 'key_len' contain info not only on
the used hash index, but also on the indexes used for the scan.
