- 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.
Analysis:
The fix for lp:944706 introduces early subquery optimization.
While a subquery is being optimized some of its predicates may be
removed. In the test case, the EXISTS subquery is constant, and is
evaluated to TRUE. As a result the whole OR is TRUE, and thus the
correlated condition "b = alias1.b" is optimized away. The subquery
becomes non-correlated.
The subquery cache is designed to work only for correlated subqueries.
If constant subquery optimization is disallowed, then the constant
subquery is not evaluated, the subquery remains correlated, and its
execution is cached. As a result execution is fast.
However, when the constant subquery was optimized away, it was neither
cached by the subquery cache, nor it was cached by the internal subquery
caching. The latter was due to the fact that the subquery still appeared
as correlated to the subselect_XYZ_engine::exec methods, and they
re-executed the subquery on each call to Item_subselect::exec.
Solution:
The solution is to update the correlated status of the subquery after it has
been optimized. This status consists of:
- st_select_lex::is_correlated
- Item_subselect::is_correlated
- SELECT_LEX::uncacheable
- SELECT_LEX_UNIT::uncacheable
The status is updated by st_select_lex::update_correlated_cache(), and its
caller st_select_lex::optimize_unflattened_subqueries. The solution relies
on the fact that the optimizer already called
st_select_lex::update_used_tables() for each subquery. This allows to
efficiently update the correlated status of each subquery without walking
the whole subquery tree.
Notice that his patch is an improvement over MySQL 5.6 and older, where
subqueries are not pre-optimized, and the above analysis is not possible.
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.
Description: When the table has more than one unique or primary key,
INSERT... ON DUP KEY UPDATE statement is sensitive to the order in which
the storage engines checks the keys. Depending on this order, the storage
engine may determine different rows to mysql, and hence mysql can update
different rows on master and slave.
Solution: We mark INSERT...ON DUP KEY UPDATE on a table with more than on unique
key as unsafe therefore the event will be logged in row format if it is available
(ROW/MIXED). If only STATEMENT format is available, a warning will be thrown.
mysql-test/suite/binlog/r/binlog_unsafe.result:
Updated result file
mysql-test/suite/binlog/t/binlog_unsafe.test:
Added test to check for warning being thrown when the unsafe statement is executed
mysql-test/suite/rpl/r/rpl_known_bugs_detection.result:
Updated result file
sql/share/errmsg-utf8.txt:
Added new warning message
sql/sql_base.cc:
check for tables in the query with more than one UNIQUE KEY and INSERT ON DUPLICATE KEY UPDATE, and mark such statements unsafe.
Description: When the table has more than one unique or primary key,
INSERT... ON DUP KEY UPDATE statement is sensitive to the order in which
the storage engines checks the keys. Depending on this order, the storage
engine may determine different rows to mysql, and hence mysql can update
different rows on master and slave.
Solution: We mark INSERT...ON DUP KEY UPDATE on a table with more than on unique
key as unsafe therefore the event will be logged in row format if it is available
(ROW/MIXED). If only STATEMENT format is available, a warning will be thrown.
Background:
- as described in MySQL Internals Prepared Stored
(http://forge.mysql.com/wiki/MySQL_Internals_Prepared_Stored),
the Optimizer sometimes does destructive changes to the parsed
LEX-object (Item-tree), which makes it impossible to re-use
that tree for PS/SP re-execution.
- in order to be able to re-use the Item-tree, the destructive
changes are remembered and rolled back after the statement execution.
The problem, discovered by this bug, was that the objects representing
GROUP-BY clause did not restored after query execution. So, the GROUP-BY
part of the statement could not be properly re-initialized for re-execution
after destructive changes.
Those objects do not take part in the Item-tree, so they can not be saved
using the approach for Item-tree.
The fix is as follows:
- introduce a new array in st_select_lex to store the original
ORDER pointers, representing the GROUP-BY clause;
- Initialize this array in fix_prepare_information().
- restore the list of GROUP-BY items in reinit_stmt_before_use().
Background:
- as described in MySQL Internals Prepared Stored
(http://forge.mysql.com/wiki/MySQL_Internals_Prepared_Stored),
the Optimizer sometimes does destructive changes to the parsed
LEX-object (Item-tree), which makes it impossible to re-use
that tree for PS/SP re-execution.
- in order to be able to re-use the Item-tree, the destructive
changes are remembered and rolled back after the statement execution.
The problem, discovered by this bug, was that the objects representing
GROUP-BY clause did not restored after query execution. So, the GROUP-BY
part of the statement could not be properly re-initialized for re-execution
after destructive changes.
Those objects do not take part in the Item-tree, so they can not be saved
using the approach for Item-tree.
The fix is as follows:
- introduce a new array in st_select_lex to store the original
ORDER pointers, representing the GROUP-BY clause;
- Initialize this array in fix_prepare_information().
- restore the list of GROUP-BY items in reinit_stmt_before_use().
mysql-test/suite/innodb/t/group_commit_crash.test:
remove autoincrement to avoid rbr being used for insert ... select
mysql-test/suite/innodb/t/group_commit_crash_no_optimize_thread.test:
remove autoincrement to avoid rbr being used for insert ... select
mysys/my_addr_resolve.c:
a pointer to a buffer is returned to the caller -> the buffer cannot be on the stack
mysys/stacktrace.c:
my_vsnprintf() is ok here, in 5.5
https://mariadb.atlassian.net/browse/MDEV-28
This task implements a new clause LIMIT ROWS EXAMINED <num>
as an extention to the ANSI LIMIT clause. This extension
allows to limit the number of rows and/or keys a query
would access (read and/or write) during query execution.
Problem: Statements that write to tables with auto_increment columns
based on the selection from another table, may lead to master
and slave going out of sync, as the order in which the rows
are retrieved from the table may differ on master and slave.
Solution: We mark writing to a table with auto_increment table
based on the rows selected from another table as unsafe. This
will cause the execution of such statements to throw a warning
and forces the statement to be logged in ROW if the logging
format is mixed.
Changes:
1. All the statements that writes to a table with auto_increment
column(s) based on the rows fetched from another table, will now
be unsafe.
2. CREATE TABLE with SELECT will now be unsafe.
sql/share/errmsg-utf8.txt:
Added new warning messages.
sql/sql_base.cc:
-Created function to check statements that write to
tables with auto_increment column and has select.
-Marked all the statements that write to a table
with auto_increment column based on rows fetched
from other table(s) as unsafe.
sql/sql_table.cc:
mark CREATE TABLE[with auto_increment column] as unsafe.
Problem: Statements that write to tables with auto_increment columns
based on the selection from another table, may lead to master
and slave going out of sync, as the order in which the rows
are retrieved from the table may differ on master and slave.
Solution: We mark writing to a table with auto_increment table
based on the rows selected from another table as unsafe. This
will cause the execution of such statements to throw a warning
and forces the statement to be logged in ROW if the logging
format is mixed.
Changes:
1. All the statements that writes to a table with auto_increment
column(s) based on the rows fetched from another table, will now
be unsafe.
2. CREATE TABLE with SELECT will now be unsafe.
Problem: Statements that write to tables with auto_increment columns
based on the selection from another table, may lead to master
and slave going out of sync, as the order in which the rows
are retrived from the table may differ on master and slave.
Solution: We mark writing to a table with auto_increment table
as unsafe. This will cause the execution of such statements to
throw a warning and forces the statement to be logged in ROW if
the logging format is mixed.
Changes:
1. All the statements that writes to a table with auto_increment
column(s) based on the rows fetched from another table, will now
be unsafe.
2. CREATE TABLE with SELECT will now be unsafe.
sql/share/errmsg-utf8.txt:
Added new Warning messages
sql/sql_base.cc:
created a new function that checks for select + write on a autoinc table
made all such statements to be unsafe.
sql/sql_parse.cc:
made create autoincremnet tabble + select unsafe
Problem: Statements that write to tables with auto_increment columns
based on the selection from another table, may lead to master
and slave going out of sync, as the order in which the rows
are retrived from the table may differ on master and slave.
Solution: We mark writing to a table with auto_increment table
as unsafe. This will cause the execution of such statements to
throw a warning and forces the statement to be logged in ROW if
the logging format is mixed.
Changes:
1. All the statements that writes to a table with auto_increment
column(s) based on the rows fetched from another table, will now
be unsafe.
2. CREATE TABLE with SELECT will now be unsafe.
There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.
Actually the same bug58915 was fixed in trunk with relocating the dyn-array
destruction into THD::cleanup_after_query() which can't be bypassed.
The current patch backports magne.mahre@oracle.com-20110203101306-q8auashb3d7icxho
and adds two optimizations: were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.
mysql-test/suite/rpl/t/rpl_empty_master_host.test:
the test is binlog-format insensitive so it will be run with MIXED mode only.
mysql-test/suite/rpl/t/rpl_server_id_ignore.test:
the test is binlog-format insensitive so it will be run with MIXED mode only.
sql/sql_class.cc:
relocating the dyn-array
destruction into THD::cleanup_after_query().
sql/sql_lex.cc:
LEX.mi zero initialization is done in LEX().
sql/sql_lex.h:
Optimization for repl_ignore_server_ids to base on a static buffer
which size is chosen to fit to most common use cases.
sql/sql_repl.cc:
dyn-array destruction is relocated to THD::cleanup_after_query().
sql/sql_yacc.yy:
Refining logics of Lex->mi.repl_ignore_server_ids initialization.
The array is initialized once a corresponding option in CHANGE MASTER token sequence
is found.
There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.
Actually the same bug58915 was fixed in trunk with relocating the dyn-array
destruction into THD::cleanup_after_query() which can't be bypassed.
The current patch backports magne.mahre@oracle.com-20110203101306-q8auashb3d7icxho
and adds two optimizations: were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.
There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.
Fixed with relocating the dyn-array's destructor at ~LEX() that is
the end of the session, per Gleb's patch idea.
Two optimizations were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.
mysql-test/suite/rpl/t/rpl_empty_master_host.test:
the test is binlog-format insensitive so it will be run with MIXED mode only.
sql/sql_lex.cc:
the new flag is initialized.
sql/sql_lex.h:
A new bool flag new member to LEX.mi is added to stay UP since after
LEX.mi.repl_ignore_server_ids dynarray initialization was called
for the first time on the session. So it is set once and its life time
is session.
The array is destroyed at the end of the session.
sql/sql_repl.cc:
dyn-array destruction is relocated to ~LEX.
sql/sql_yacc.yy:
Refining logics of Lex->mi.repl_ignore_server_ids initialization.
The array is initialized once a corresponding option in CHANGE MASTER token sequence
is found.
The fact of initialization is memorized into the new flag.
There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.
Fixed with relocating the dyn-array's destructor at ~LEX() that is
the end of the session, per Gleb's patch idea.
Two optimizations were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.
