- Make SHOW EXPLAIN code take into account that st_select_lex object without joins can be
a full-featured SELECTs which were already executed and cleaned up.
- The problem was that create_ref_for_key() would act differently, depending on
whether we're running EXPLAIN or the actual query.
- As the first step, fixed the EXPLAIN printout not to depend on actions in create_ref_for_key().
CHEAP SQ: Valgrind warnings "Memory lost" with IN and EXISTS nested subquery, materialization+semijoin
Analysis:
The memory leak was a result of the interaction of semi-join optimization
with early optimization of constant subqueries. The function:
setup_jtbm_semi_joins() created a dummy temporary table "dummy_table"
in order to make some JOIN_TAB objects complete. Normally, such temporary
tables are freed inside JOIN_TAB::cleanup.
However, the inner-most subquery is pre-optimized, which allows the
optimization fo the MAX subquery to determine that its WHERE is TRUE,
and thus to compute the result of the MAX during optimization. This
ultimately allows the optimize phase of the outer query to find that
it WHERE clause is FALSE. Once JOIN::optimize finds that the result
set is empty, it sets zero_result_cause, and returns *before* it ever
reached make_join_statistics(). As a result the query plan has no
JOIN_TABs at all. Since the temporary table is supposed to be cleanup
via JOIN_TAB::cleanup, this never happens because there is no JOIN_TAB
for this table. Hence we get a memory leak.
Solution:
Whenever there are no JOIN_TABs, iterate over all table reference in
JOIN::join_list, and free the ones that contain semi-join temporary
tables.
WHEN KILLING
Suppose there is a query waiting for a lock. If the user kills
this query, then "Got error -1 when reading table" error message
must not be logged in the server log file. Since this is a user
requested interruption, no spurious error message must be logged
in the server log. This patch will remove the error message from
the log.
approved by joh and tatjana
WHEN KILLING
Suppose there is a query waiting for a lock. If the user kills
this query, then "Got error -1 when reading table" error message
must not be logged in the server log file. Since this is a user
requested interruption, no spurious error message must be logged
in the server log. This patch will remove the error message from
the log.
approved by joh and tatjana
WHEN KILLING
Suppose there is a query waiting for a lock. If the user kills
this query, then "Got error -1 when reading table" error message
must not be logged in the server log file. Since this is a user
requested interruption, no spurious error message must be logged
in the server log. This patch will remove the error message from
the log.
approved by joh and tatjana
WHEN KILLING
Suppose there is a query waiting for a lock. If the user kills
this query, then "Got error -1 when reading table" error message
must not be logged in the server log file. Since this is a user
requested interruption, no spurious error message must be logged
in the server log. This patch will remove the error message from
the log.
approved by joh and tatjana
- In JOIN::exec(), make the having->update_used_tables() call before we've
made the JOIN::cleanup(full=true) call. The latter frees SJ-Materialization
structures, which correlated subquery predicate items attempt to walk afterwards.
- Don't try to produce plans after JOIN::cleanup() has been called, because:
= JOIN::cleanup leaves data structures in partially-cleaned state
= Walking them is hazardous (see this bug), and has funny effects
(See previous commits, "Using join cache" may or may not be shown)
= Changing data structures to be persisted may cause unwanted side effects
- The consequence is that SHOW EXPLAIN will show "Query plan already deleted" when e.g.
reading data after filesort.
This is a backport of the (unchaged) fix for MySQL bug #11764372, 57197.
Analysis:
When the outer query finishes its main execution and computes GROUP BY,
it needs to construct a new temporary table (and a corresponding JOIN) to
execute the last DISTINCT operation. At this point JOIN::exec calls
JOIN::join_free, which calls JOIN::cleanup -> TMP_TABLE_PARAM::cleanup
for both the outer and the inner JOINs. The call to the inner
TMP_TABLE_PARAM::cleanup sets copy_field = NULL, but not copy_field_end.
The final execution phase that computes the DISTINCT invokes:
evaluate_join_record -> end_write -> copy_funcs
The last function copies the results of all functions into the temp table.
copy_funcs walks over all functions in join->tmp_table_param.items_to_copy.
In this case items_to_copy contains both assignments to user variables.
The process of copying user variables invokes Item_func_set_user_var::check
which in turn re-evaluates the arguments of the user variable assignment.
This in turn triggers re-evaluation of the subquery, and ultimately
copy_field.
However, the previous call to TMP_TABLE_PARAM::cleanup for the subquery
already set copy_field to NULL but not its copy_field_end. This results
in a null pointer access, and a crash.
Fix:
Set copy_field_end and save_copy_field_end to null when deleting
copy fields in TMP_TABLE_PARAM::cleanup().
- make make_cond_after_sjm() correctly handle OR clauses where one branch refers to the semi-join table
while the other branch refers to the non-semijoin table.
The cause for this bug is that the method JOIN::get_examined_rows iterates over all
JOIN_TABs of the join assuming they are just a sequence. In the query above, the
innermost subquery is merged into its parent query. When we call
JOIN::get_examined_rows for the second-level subquery, the iteration that
assumes sequential order of join tabs goes outside the join_tab array and calls
the method JOIN_TAB::get_examined_rows on uninitialized memory.
The fix is to iterate over JOIN_TABs in a way that takes into account the nested
semi-join structure of JOIN_TABs. In particular iterate as select_describe.
The patch enables back constant subquery execution during
query optimization after it was disabled during the development
of MWL#89 (cost-based choice of IN-TO-EXISTS vs MATERIALIZATION).
The main idea is that constant subqueries are allowed to be executed
during optimization if their execution is not expensive.
The approach is as follows:
- Constant subqueries are recursively optimized in the beginning of
JOIN::optimize of the outer query. This is done by the new method
JOIN::optimize_constant_subqueries(). This is done so that the cost
of executing these queries can be estimated.
- Optimization of the outer query proceeds normally. During this phase
the optimizer may request execution of non-expensive constant subqueries.
Each place where the optimizer may potentially execute an expensive
expression is guarded with the predicate Item::is_expensive().
- The implementation of Item_subselect::is_expensive has been extended
to use the number of examined rows (estimated by the optimizer) as a
way to determine whether the subquery is expensive or not.
- The new system variable "expensive_subquery_limit" controls how many
examined rows are considered to be not expensive. The default is 100.
In addition, multiple changes were needed to make this solution work
in the light of the changes made by MWL#89. These changes were needed
to fix various crashes and wrong results, and legacy bugs discovered
during development.
The optimizer chose a less efficient execution plan due to the following
defects of the code:
1. the generic handler function handler::keyread_time did not take into account
that in clustered primary keys record data is included into each index entry
2. the function make_join_readinfo erroneously decided that index only scan
could not be used if join cache was empoyed.
Added no additional test case.
Adjusted some of the test results.
RESULTS ON IN() & NOT IN() COMP #3
This bug causes a wrong result in mysql-trunk when ICP is used
and bad performance in mysql-5.5 and mysql-trunk.
Using the query from bug report to explain what happens and causes
the wrong result from the query when ICP is enabled:
1. The t3 table contains four records. The outer query will read
these and for each of these it will execute the subquery.
2. Before the first execution of the subquery it will be optimized. In
this case the important is what happens to the first table t1:
-make_join_select() will call the range optimizer which decides
that t1 should be accessed using a range scan on the k1 index
It creates a QUICK_RANGE_SELECT object for this.
-As the last part of optimization the ICP code pushes the
condition down to the storage engine for table t1 on the k1 index.
This produces the following information in the explain for this table:
2 DEPENDENT SUBQUERY t1 range k1 k1 5 NULL 3 Using index condition; Using filesort
Note the use of filesort.
3. The first execution of the subquery does (among other things) due
to the need for sorting:
a. Call create_sort_index() which again will call find_all_keys():
b. find_all_keys() will read the required keys for all qualifying
rows from the storage engine. To do this it checks if it has a
quick-select for the table. It will use the quick-select for
reading records. In this case it will read four records from the
storage engine (based on the range criteria). The storage engine
will evaluate the pushed index condition for each record.
c. At the end of create_sort_index() there is code that cleans up a
lot of stuff on the join tab. One of the things that is cleaned
is the select object. The result of this is that the
quick-select object created in make_join_select is deleted.
4. The second execution of the subquery does the same as the first but
the result is different:
a. Call create_sort_index() which again will call find_all_keys()
(same as for the first execution)
b. find_all_keys() will read the keys from the storage engine. To
do this it checks if it has a quick-select for the table. Now
there is NO quick-select object(!) (since it was deleted in
step 3c). So find_all_keys defaults to read the table using a
table scan instead. So instead of reading the four relevant records
in the range it reads the entire table (6 records). It then
evaluates the table's condition (and here it goes wrong). Since
the entire condition has been pushed down to the storage engine
using ICP all 6 records qualify. (Note that the storage engine
will not evaluate the pushed index condition in this case since
it was pushed for the k1 index and now we do a table scan
without any index being used).
The result is that here we return six qualifying key values
instead of four due to not evaluating the table's condition.
c. As above.
5. The two last execution of the subquery will also produce wrong results
for the same reason.
Summary: The problem occurs due to all but the first executions of the
subquery is done as a table scan without evaluating the table's
condition (which is pushed to the storage engine on a different
index). This is caused by the create_sort_index() function deleting
the quick-select object that should have been used for executing the
subquery as a range scan.
Note that this bug in addition to causing wrong results also can
result in bad performance due to executing the subquery using a table
scan instead of a range scan. This is an issue in MySQL 5.5.
The fix for this problem is to avoid that the Quick-select-object that
the optimizer created is deleted when create_sort_index() is doing
clean-up of the join-tab. This will ensure that the quick-select
object and the corresponding pushed index condition will be available
and used by all following executions of the subquery.
sql/sql_select.cc:
Fix for Bug#12667154: Change how create_sort_index() cleans up the
join_tab's select and quick-select objects in order to avoid that a
quick-select object created outside of create_sort_index() is deleted.
RESULTS ON IN() & NOT IN() COMP #3
This bug causes a wrong result in mysql-trunk when ICP is used
and bad performance in mysql-5.5 and mysql-trunk.
Using the query from bug report to explain what happens and causes
the wrong result from the query when ICP is enabled:
1. The t3 table contains four records. The outer query will read
these and for each of these it will execute the subquery.
2. Before the first execution of the subquery it will be optimized. In
this case the important is what happens to the first table t1:
-make_join_select() will call the range optimizer which decides
that t1 should be accessed using a range scan on the k1 index
It creates a QUICK_RANGE_SELECT object for this.
-As the last part of optimization the ICP code pushes the
condition down to the storage engine for table t1 on the k1 index.
This produces the following information in the explain for this table:
2 DEPENDENT SUBQUERY t1 range k1 k1 5 NULL 3 Using index condition; Using filesort
Note the use of filesort.
3. The first execution of the subquery does (among other things) due
to the need for sorting:
a. Call create_sort_index() which again will call find_all_keys():
b. find_all_keys() will read the required keys for all qualifying
rows from the storage engine. To do this it checks if it has a
quick-select for the table. It will use the quick-select for
reading records. In this case it will read four records from the
storage engine (based on the range criteria). The storage engine
will evaluate the pushed index condition for each record.
c. At the end of create_sort_index() there is code that cleans up a
lot of stuff on the join tab. One of the things that is cleaned
is the select object. The result of this is that the
quick-select object created in make_join_select is deleted.
4. The second execution of the subquery does the same as the first but
the result is different:
a. Call create_sort_index() which again will call find_all_keys()
(same as for the first execution)
b. find_all_keys() will read the keys from the storage engine. To
do this it checks if it has a quick-select for the table. Now
there is NO quick-select object(!) (since it was deleted in
step 3c). So find_all_keys defaults to read the table using a
table scan instead. So instead of reading the four relevant records
in the range it reads the entire table (6 records). It then
evaluates the table's condition (and here it goes wrong). Since
the entire condition has been pushed down to the storage engine
using ICP all 6 records qualify. (Note that the storage engine
will not evaluate the pushed index condition in this case since
it was pushed for the k1 index and now we do a table scan
without any index being used).
The result is that here we return six qualifying key values
instead of four due to not evaluating the table's condition.
c. As above.
5. The two last execution of the subquery will also produce wrong results
for the same reason.
Summary: The problem occurs due to all but the first executions of the
subquery is done as a table scan without evaluating the table's
condition (which is pushed to the storage engine on a different
index). This is caused by the create_sort_index() function deleting
the quick-select object that should have been used for executing the
subquery as a range scan.
Note that this bug in addition to causing wrong results also can
result in bad performance due to executing the subquery using a table
scan instead of a range scan. This is an issue in MySQL 5.5.
The fix for this problem is to avoid that the Quick-select-object that
the optimizer created is deleted when create_sort_index() is doing
clean-up of the join-tab. This will ensure that the quick-select
object and the corresponding pushed index condition will be available
and used by all following executions of the subquery.
The patch backports two patches from mysql 5.6:
- BUG#12640437: USING SQL_BUFFER_RESULT RESULTS IN A DIFFERENT QUERY OUTPUT
- Bug#12578908: SELECT SQL_BUFFER_RESULT OUTPUTS TOO MANY ROWS WHEN GROUP IS OPTIMIZED AWAY
Original comment:
-----------------
3714 Jorgen Loland 2012-03-01
BUG#12640437 - USING SQL_BUFFER_RESULT RESULTS IN A DIFFERENT
QUERY OUTPUT
For all but simple grouped queries, temporary tables are used to
resolve grouping. In these cases, the list of grouping fields is
stored in the temporary table and grouping is resolved
there (e.g. by adding a unique constraint on the involved
fields). Because of this, grouping is already done when the rows
are read from the temporary table.
In the case where a group clause may be optimized away, grouping
does not have to be resolved using a temporary table. However, if
a temporary table is explicitly requested (e.g. because the
SQL_BUFFER_RESULT hint is used, or the statement is
INSERT...SELECT), a temporary table is used anyway. In this case,
the temporary table is created with an empty group list (because
the group clause was optimized away) and it will therefore not
create groups. Since the temporary table does not take care of
grouping, JOIN::group shall not be set to false in
make_simple_join(). This was fixed in bug 12578908.
However, there is an exception where make_simple_join() should
set JOIN::group to false even if the query uses a temporary table
that was explicitly requested but is not strictly needed. That
exception is if the loose index scan access method (explain
says "Using index for group-by") is used to read into the
temporary table. With loose index scan, grouping is resolved
by the access method. This is exactly what happens in this bug.
