Lines below which were added in the patch for Bug#56814 cause this crash:
+ if (table->table)
+ table->table->maybe_null= FALSE;
Consider following test case:
--
CREATE TABLE t1(f1 INT NOT NULL);
INSERT INTO t1 VALUES (16777214),(0);
SELECT COUNT(*) FROM t1 LEFT JOIN t1 t2
ON 1 WHERE t2.f1 > 1 GROUP BY t2.f1;
DROP TABLE t1;
--
We set TABLE::maybe_null to FALSE for t2 table
and in create_tmp_field() we create appropriate tmp table field
using create_tmp_field_from_item() function instead of
create_tmp_field_from_field. As a result we have
LONGLONG field. As we have GROUP BY clause we calculate
group buffer length, see calc_group_buffer().
Item from group list which is used for calculation
refer to the field from real tables and have LONG type.
So group buffer length become insufficient for storing of
LONGLONG value. It leads to overwriting of wrong memory
area in do_field_int() function which is called from
end_update().
After some investigation I found out that
create_tmp_field_from_item() is used only for OLAP
grouping and can not be used for common grouping
as it could be an incompatibility between tmp
table fields and group buffer length.
We can not remove create_tmp_field_from_item() call from
create_tmp_field as OLAP needs it and we can not use this
function for common grouping. So we should remove setting
TABLE::maybe_null to FALSE from simplify_joins().
In this case we'll get wrong behaviour of
list_contains_unique_index() back. To fix it we
could use Field::real_maybe_null() check instead of
Field::maybe_null() and add addition check of
TABLE_LIST::outer_join.
mysql-test/r/group_by.result:
test case
mysql-test/r/join_outer.result:
test case
mysql-test/t/group_by.test:
test case
mysql-test/t/join_outer.test:
test case
sql/sql_select.cc:
--remove wrong code
--use Field::real_maybe_null() check instead of
Field::maybe_null() and add addition check of
TABLE_LIST::outer_join
The problem is caused by bug49487 fix and became visible
after after bug56679 fix.
Items are cleaned up and set to unfixed state after filling derived table.
So we can not rely on item::fixed state in Item_func_group_concat::print
and we can not use 'args' array as items there may be cleaned up.
The fix is always to use orig_args array of items as it
always should contain the correct data.
mysql-test/r/func_gconcat.result:
test case
mysql-test/t/func_gconcat.test:
test case
sql/item_sum.cc:
The fix is always to use orig_args array of items.
The problem is dividing by const value when
the result is out of supported range.
The fix:
-return LONGLONG_MIN if the result is out of supported range for DIV operator.
-return 0 if divisor is -1 for MOD operator.
mysql-test/r/func_math.result:
test case
mysql-test/t/func_math.test:
test case
sql/item_func.cc:
-return LONGLONG_MIN if the result is out of supported range for DIV operator.
-return 0 if divisor is -1 for MOD operator.
"Grantor" columns' data is lost when replicating mysql.tables_priv.
Slave SQL thread used its default user ''@'' as the grantor of GRANT|REVOKE
statements executing on it.
In this patch, current user is put in query log event for all GRANT and REVOKE
statement, SQL thread uses the user in query log event as grantor.
mysql-test/suite/rpl/r/rpl_do_grant.result:
Add test for this bug.
mysql-test/suite/rpl/t/rpl_do_grant.test:
Add test for this bug.
sql/log_event.cc:
Refactoring THD::current_user_used and related functions.
current_user_used is used to judge if current user should be
binlogged in query log event. So it is better to call it m_binlog_invoker.
The related functions are renamed too.
sql/sql_class.cc:
Refactoring THD::current_user_used and related functions.
current_user_used is used to judge if current user should be
binlogged in query log event. So it is better to call it m_binlog_invoker.
The related functions are renamed too.
sql/sql_class.h:
Refactoring THD::current_user_used and related functions.
current_user_used is used to judge if current user should be
binlogged in query log event. So it is better to call it m_binlog_invoker.
The related functions are renamed too.
sql/sql_parse.cc:
Call binlog_invoker() for GRANT and REVOKE statements.
Rows events were applied wrongly on the temporary table with the same name.
But rows events are generated only for base tables. As temporary
table's data never be binlogged on row mode. Normally, base table of the
same name cannot be updated if a temporary table has the same name.
But there are two cases which can generate rows events on
the base table of same name.
Case1: 'CREATE TABLE ... SELECT' statement.
In mixed format, it will generate rows events if it is unsafe.
Case2: Drop a transactional temporary table in a transaction
(happens only on 5.5+).
BEGIN;
DROP TEMPORARY TABLE t1; # t1 is a InnoDB table
INSERT INTO t1 VALUES(rand()); # t1 is a MyISAM table
COMMIT;
'DROP TEMPORARY TABLE' will be put in the transaction cache and
binlogged after the rows events generated by the 'INSERT' statement.
After this patch, slave opens only base table when applying a rows event.
Fix assorted warnings that are generated in optimized builds.
Most of it is silencing variables that are set but unused.
This patch also introduces the MY_ASSERT_UNREACHABLE macro
which helps the compiler to deduce that a certain piece of
code is unreachable.
include/my_compiler.h:
Use GCC's __builtin_unreachable if available. It allows
GCC to deduce the unreachability of certain code paths,
thus avoiding warnings that, for example, accused that a
variable could be used without being initialized (due to
unreachable code paths).
data dictionary confusion
On file systems with case insensitive file names, and
lower_case_table_names set to '2', the server could crash
due to a table definition cache inconsistency. This is
the default setting on MacOSX, but may also be set and
used on MS Windows.
The bug is caused by using two different strategies for
creating the hash key for the table definition cache, resulting
in failure to look up an entry which is present in the cache,
or failure to delete an existing entry. One strategy was to
use the real table name (with case preserved), and the other
to use a normalized table name (i.e a lower case version).
This is manifested in two cases. One is during 'DROP DATABASE',
where all known files are removed. The removal from
the table definition cache is done via a generated list of
TABLE_LIST with keys (wrongly) created using the case preserved
name. The other is during CREATE TABLE, where the cache lookup
is also (wrongly) based on the case preserved name.
The fix was to use only the normalized table name when
creating hash keys.
sql/sql_db.cc:
Normalize table name (i.e lower case it)
sql/sql_table.cc:
table_name contains the normalized name
alias contains the real table name
create_sort_index() function overwrites original JOIN_TAB::type field.
At re-execution of subquery overwritten JOIN_TAB::type(JT_ALL) is
used instead of JT_FT. It misleads test_if_skip_sort_order() and
the function tries to find suitable key for the order that should
not be allowed for FULLTEXT(JT_FT) table.
The fix is to restore JOIN_TAB strucures for subselect on re-execution
for EXPLAIN.
Additional fix:
Update TABLE::maybe_null field which
affects list_contains_unique_index() behaviour as it
could have the value(maybe_null==TRUE) based on the
assumption that this join is outer
(see setup_table_map() func).
mysql-test/r/explain.result:
test case
mysql-test/t/explain.test:
test case
sql/item_subselect.cc:
Make subquery uncacheable in case of EXPLAIN. It allows to keep
original JOIN_TAB::type(see JOIN::save_join_tab) and restore it
on re-execution.
sql/sql_select.cc:
-restore JOIN_TAB strucures for subselect on re-execution for EXPLAIN
-Update TABLE::maybe_null field as it could have
the value(maybe_null==TRUE) based on the assumption
that this join is outer(see setup_table_map() func).
This change is not related to the crash problem but
affects EXPLAIN results in the test case.
For crash testing: kill the server without generating core file.
include/my_dbug.h
Use kill(getpid(), SIGKILL) which cannot be caught by signal handlers.
All DBUG_XXX macros should be no-ops in optimized mode, do that for DBUG_ABORT as well.
sql/handler.cc
Kill server without generating core.
sql/log.cc
Kill server without generating core.
Subquery executes twice, at top level JOIN::optimize and ::execute stages.
At first execution create_sort_index() function is called and
FT_SELECT object is created and destroyed. HANDLER::ft_handler is cleaned up
in the object destructor and at second execution FT_SELECT::get_next() method
returns error.
The fix is to reinit HANDLER::ft_handler field before re-execution of subquery.
mysql-test/r/fulltext.result:
test case
mysql-test/t/fulltext.test:
test case
sql/item_func.cc:
reinit ft_handler before re-execution of subquery
sql/item_func.h:
Fixed method name
sql/sql_select.cc:
reinit ft_handler before re-execution of subquery
replication aborts
When recieving a 'SLAVE STOP' command, slave SQL thread will roll back the
transaction and stop immidiately if there is only transactional table updated,
even through 'CREATE|DROP TEMPOARY TABLE' statement are in it. But These
statements can never be rolled back. Because the temporary tables to the user
session mapping remain until 'RESET SLAVE', Therefore it will abort SQL thread
with an error that the table already exists or doesn't exist, when it restarts
and executes the whole transaction again.
After this patch, SQL thread always waits till the transaction ends and then stops,
if 'CREATE|DROP TEMPOARY TABLE' statement are in it.
mysql-test/extra/rpl_tests/rpl_stop_slave.test:
Auxiliary file which is used to test this bug.
mysql-test/suite/rpl/t/rpl_stop_slave.test:
Test case for this bug.
sql/slave.cc:
Checking if OPTION_KEEP_LOG is set. If it is set, SQL thread should wait
until the transaction ends.
sql/sql_parse.cc:
Add a debug point for testing this bug.
mysql-test/r/grant.result:
It was added result for test case for bug#36742.
mysql-test/t/grant.test:
It was added test case for bug#36742.
sql/sql_yacc.yy:
It was added convertation of host name part of user name to lowercase.
Problem: some call of INET_NTOA() function may lead
to a crash due to missing its character set initialization.
Fix: explicitly set the character set.
mysql-test/r/func_misc.result:
Fix for bug#57283: inet_ntoa() crashes
- test result.
mysql-test/t/func_misc.test:
Fix for bug#57283: inet_ntoa() crashes
- test case.
sql/item_strfunc.cc:
Fix for bug#57283: inet_ntoa() crashes
- explicitly set buffer's character set.
Problem: if multibyte and binary string arguments passed to
RPAD(), LPAD() or INSERT() functions, they might return
wrong results or even lead to a server crash due to missed
character set convertion.
Fix: perform the convertion if necessary.
mysql-test/r/ctype_utf8.result:
Fix for bug#57272: crash in rpad() when using utf8
- test result.
mysql-test/t/ctype_utf8.test:
Fix for bug#57272: crash in rpad() when using utf8
- test case.
sql/item_strfunc.cc:
Fix for bug#57272: crash in rpad() when using utf8
- convert multibyte argument's character set to binary in case of
FUNCTION(MULTIBYTE_ARG, .., BINARY_ARG,..) for RPAD(), LPAD() and
INSERT() functions.
After ALTER TABLE which changed only table's metadata, row-based
binlog sometimes got corrupted since the tablemap was unexpectedly
set to 0 for subsequent updates to the same table.
ALTER TABLE which changed only table's metadata always reset
table_map_id for the table share to 0. Despite the fact that
0 is a valid value for table_map_id, this step caused problems
as it could have created situation in which we had more than
one table share with table_map_id equal 0. If more than one
table with table_map_id are 0 were updated in the same statement,
updates to these different tables were written into the same
rows event. This caused slave server to crash.
This bug happens only on 5.1. It doesn't affect 5.5+.
This patch solves this problem by ensuring that ALTER TABLE
statements which change metadata only never reset table_map_id
to 0. To do this it changes reopen_table() to correctly use
refreshed table_map_id value instead of using the old one/
resetting it.
mysql-test/suite/rpl/r/rpl_alter.result:
Add test for BUG#56226
mysql-test/suite/rpl/t/rpl_alter.test:
Add test for BUG#56226
When slave executes a transaction bigger than slave's max_binlog_cache_size,
slave will crash. It is caused by the assert that server should only roll back
the statement but not the whole transaction if the error ER_TRANS_CACHE_FULL
happens. But slave sql thread always rollbacks the whole transaction when
an error happens.
Ather this patch, we always clear any error set in sql thread(it is different
from the error in 'SHOW SLAVE STATUS') and it is cleared before rolling back
the transaction.
mysql-test/suite/rpl/r/rpl_binlog_max_cache_size.result:
SET binlog_cache_size and max_binlog_cache_size for all test cases.
Add test case for bug#55375.
mysql-test/suite/rpl/t/rpl_binlog_max_cache_size-master.opt:
binlog_cache_size and max_binlog_cache_size can be set in the client connection.
so remove this option file.
mysql-test/suite/rpl/t/rpl_binlog_max_cache_size.test:
SET binlog_cache_size and max_binlog_cache_size for all test cases.
Add test case for bug#55375.
sql/log_event.cc:
Some functions don't return the error code, so it is a wrong error code.
The error should always be set into thd->main_da. So we use
slave_rows_error_report to report the right error.
sql/slave.cc:
exec_relay_log_event() need call cleanup_context() to clear context.
clearup_context() will call end_trans().
Clear thd's error before cleanup_context. It avoid to trigger the assert
which cause this bug.
This is a regression from the fix for bug no 38999. A storage engine capable
of reading only a subset of a table's columns updates corresponding bits in
the read buffer to signal that it has read NULL values for the corresponding
columns. It cannot, and should not, update any other bits. Bug no 38999
occurred because the implementation of UPDATE statements compare the NULL bits
using memcmp, inadvertently comparing bits that were never requested from the
storage engine. The regression was caused by the storage engine trying to
alleviate the situation by writing to all NULL bits, even those that it had no
knowledge of. This has devastating effects for the index merge algorithm,
which relies on all NULL bits, except those explicitly requested, being left
unchanged.
The fix reverts the fix for bug no 38999 in both InnoDB and InnoDB plugin and
changes the server's method of comparing records. For engines that always read
entire rows, we proceed as usual. For engines capable of reading only select
columns, the record buffers are now compared on a column by column basis. An
assertion was also added so that non comparable buffers are never read. Some
relevant copy-pasted code was also consolidated in a new function.
Suprisingly, a Slave_log_event would show up in the binary
log. This event is never used and should not appear in the
logs. As such, when the slave (or the mysqlbinlog tool) reads the
event, it will hit an invalid pointer (reference to the
descriptor event when deserializing the Slave_log_event was
purposodely set to NULL).
The presence of the Slave_log_event denotes a corrupted log, but
we cannot tell how the log got corrupted in the first
place. However, we can make the server cope with such events when
it reads them - in case of log corruption - and fail gracefully.
This patch makes the server/mysqlbinlog to report that it has
found an invalid log event when Slave_log_event is read.
LOAD DATA into partitioned MyISAM table
Problem was that both partitioning and myisam
used the same table_share->mutex for different protections
(auto inc and repair).
Solved by adding a specific mutex for the partitioning
auto_increment.
Also adding destroying the ha_data structure in
free_table_share (which is to be propagated
into 5.5).
This is a 5.1 ONLY patch, already fixed in 5.5+.
The crash happens because original join table is replaced with temporary table
at execution stage and later we attempt to use this temporary table in
select_describe. It might happen that
Item_subselect::update_used_tables() method which sets const_item flag
is not called by some reasons (no where/having conditon in subquery for example).
It prevents JOIN::join_tmp creation and breaks original join.
The fix is to call ::update_used_tables() before ::const_item() check.
mysql-test/r/ps.result:
test case
mysql-test/t/ps.test:
test case
sql/item_subselect.cc:
call ::update_used_tables() before ::const_item() check.
Bug#57113: ha_partition::extra(ha_extra_function):
Assertion `m_extra_cache' failed
Fix for bug#55458 included DBUG_ASSERTS causing
debug builds of the server to crash on
another multi-table update.
Removed the asserts since they where wrong.
(updated after testing the patch in 5.5).
mysql-test/r/partition.result:
updated result
mysql-test/t/partition.test:
Added test for bug#57113
sql/ha_partition.cc:
Removed the assert for m_extra_cache when
::extra(HA_PREPARE_FOR_UPDATE) was called.
The procedure for setting up a fake binary log, by changing the
relay log files manually, is run twice when we issue mtr with
--repeat=2. However, when setting it up for the second time,
neither the sql thread is reset nor the server is restarted. This
means that previous stale relay log IO_CACHE metadata - from
first run - is left around. As a consequence, when the test is
run for the second time, the IO_CACHE for the relay log has
position in file different than zero, triggering the assertion.
We fix this by deploying a call to my_b_seek before calling
check_binlog_magic in next_event. This prevents the server
from asserting, in the cases that the SQL thread was reads
from a hot log (relay.NNNNN), then is stopped, then is restarted
from a previous cold log (relay.MMMMM), and then it reaches
the hot log relay.NNNNN again, in which case, the read
coordinates are not set to zero, but to the old values.
Additionally, some comments to the source code were added.
In case of outer join and emtpy WHERE conditon
'always true' condition is created for WHERE clasue.
Later in mysql_select() original SELECT_LEX WHERE
condition is overwritten with created cond.
However SELECT_LEX condition is also used as inital
condition in mysql_select()->JOIN::prepare().
On second execution of PS modified SELECT_LEX condition
is taken and it leads to crash.
The fix is to restore original SELECT_LEX condition
(set to NULL if original cond is NULL) in
reinit_stmt_before_use().
HAVING clause is fixed too for safety reason
(no test case as I did not manage to think out
appropriate example).
mysql-test/r/ps.result:
test case
mysql-test/t/ps.test:
test case
sql/sql_prepare.cc:
restore original SELECT_LEX condition
(set to NULL if original cond is NULL) in
reinit_stmt_before_use()
Fixed a number of memory leaks discovered by valgrind.
dbug/dbug.c:
This is actually an addendum to the fix for bug #52629:
- there is no point in limiting the fix to just global
variables, session ones are also affected.
- zero all fields when allocating a new 'state' structure so
that FreeState() does not deal with unitialized data later.
- add a check for a NULL pointer in DBUGCloseFile()
mysql-test/r/partition_error.result:
Added a test case for bug #56709.
mysql-test/r/variables_debug.result:
Added a test case for bug #56709.
mysql-test/t/partition_error.test:
Added a test case for bug #56709.
mysql-test/t/variables_debug.test:
Added a test case for bug #56709.
sql/item_timefunc.cc:
There is no point in declaring 'value' as a member of
Item_extract and dynamically allocating memory for it in
Item_extract::fix_length_and_dec(), since this string is only
used as a temporary storage in Item_extract::val_int().
sql/item_timefunc.h:
Removed 'value' from the Item_extract class definition.
sql/sql_load.cc:
- we may need to deallocate 'buffer' even when 'error' is
non-zero in some cases, since 'error' is public, and there is
external code modifying it.
- assign NULL to buffer when deallocating it so that we don't
do it twice in the destructor
- there is no point in changing 'error' in the destructor.
Subselect executes twice, at JOIN::optimize stage
and at JOIN::execute stage. At optimize stage
Innodb prebuilt struct which is used for the
retrieval of column values is initialized in.
ha_innobase::index_read(), prebuilt->sql_stat_start is true.
After QUICK_ROR_INTERSECT_SELECT finished his job it
restores read_set/write_set bitmaps with initial values
and deactivates one of the handlers used by
QUICK_ROR_INTERSECT_SELECT in JOIN::cleanup
(it's the case when we reuse original handler as one of
handlers required by QUICK_ROR_INTERSECT_SELECT object).
On second subselect execution inactive handler is activated
in QUICK_RANGE_SELECT::reset, file->ha_index_init().
In ha_index_init Innodb prebuilt struct is reinitialized
with inappropriate read_set/write_set bitmaps. Further
reinitialization in ha_innobase::index_read() does not
happen as prebuilt->sql_stat_start is false.
It leads to partial retrieval of required field values
and we get a mix of field values from different records
in the record buffer.
The fix is to reset
read_set/write_set bitmaps as these values
are required for proper intialization of
internal InnoDB struct which is used for
the retrieval of column values
(see build_template(), ha_innodb.cc)
mysql-test/include/index_merge_ror_cpk.inc:
test case
mysql-test/r/index_merge_innodb.result:
test case
mysql-test/r/index_merge_myisam.result:
test case
sql/opt_range.cc:
if ROR merge scan is used we need to reset
read_set/write_set bitmaps as these values
are required for proper intialization of
internal InnoDB struct which is used for
the retrieval of column values
(see build_template(), ha_innodb.cc)
adding new indexes
A fast alter table requires that the existing (old) table
and indices are unchanged (i.e only new indices can be
added). To verify this, the layout and flags of the old
table/indices are compared for equality with the new.
The PACK_KEYS option is a no-op in InnoDB, but the flag
exists, and is used in the table compare. We need to
check this (table) option flag before deciding whether an
index should be packed or not. If the table has
explicitly set PACK_KEYS to 0, the created indices should
not be marked as packed/packable.
compression protocol.
The loss of connection was caused by a malformed packet
sent by the server in case when query cache was in use.
When storing data in the query cache, the query cache
memory allocation algorithm had a tendency to reduce
the amount of memory block necessary to store a result
set, up to finally storing the entire result set in a single
block. With a significant result set, this memory block
could turn out to be quite large - 30, 40 MB and on.
When such a result set was sent to the client, the entire
memory block was compressed and written to network as a
single network packet. However, the length of the
network packet is limited by 0xFFFFFF (16MB), since
the packet format only allows 3 bytes for packet length.
As a result, a malformed, overly large packet
with truncated length would be sent to the client
and break the client/server protocol.
The solution is, when sending result sets from the query
cache, to ensure that the data is chopped into
network packets of size <= 16MB, so that there
is no corruption of packet length. This solution,
however, has a shortcoming: since the result set
is still stored in the query cache as a single block,
at the time of sending, we've lost boundaries of individual
logical packets (one logical packet = one row of the result
set) and thus can end up sending a truncated logical
packet in a compressed network packet.
As a result, on the client we may require more memory than
max_allowed_packet to keep, both, the truncated
last logical packet, and the compressed next packet.
This never (or in practice never) happens without compression,
since without compression it's very unlikely that
a) a truncated logical packet would remain on the client
when it's time to read the next packet
b) a subsequent logical packet that is being read would be
so large that size-of-new-packet + size-of-old-packet-tail >
max_allowed_packet.
To remedy this issue, we send data in 1MB sized packets,
that's below the current client default of 16MB for
max_allowed_packet, but large enough to ensure there is no
unnecessary overhead from too many syscalls per result set.
sql/net_serv.cc:
net_realloc() modified: consider already used memory
when compare packet buffer length
sql/sql_cache.cc:
modified Query_cache::send_result_to_client: send result to client
in chunks limited by 1 megabyte.
When having a sub query in partitioned innodb one could
make the partitioning engine to search for a 'index_next_same'
on a partition that had not been initialized.
Problem was that the subselect function looks at table->status
which was not set in the partitioning handler when it skipped
scanning due to no matching partitions found.
Fixed by setting table->status = STATUS_NOT_FOUND when
there was no partitions to scan. (If there are partitions to
scan, it will be set in the partitions handler.)
mysql-test/r/partition_innodb.result:
added result
mysql-test/t/partition_innodb.test:
added test
sql/ha_partition.cc:
set table status to not found, if there ar no partitions to scan.
ORDER BY computed col
GROUP BY implies ORDER BY in the MySQL dialect of SQL. Therefore, when an
index on the first table in the query is used, and that index satisfies
ordering according to the GROUP BY clause, the query optimizer estimates the
number of tuples that need to be read from this index. If there is a LIMIT
clause, table statistics on tables following this 'sort table' are employed.
There may be a separate ORDER BY clause however, which mandates reading the
whole 'sort table' anyway. But the previous estimate was left untouched.
Fixed by removing the estimate from EXPLAIN output if GROUP BY is used in
conjunction with an ORDER BY clause that mandates using a temporary table.
Version "5.1.42 SUSE MySQL RPM"
When a query was using a DATE or DATETIME value formatted
using different formatting than "yyyy-mm-dd HH:MM:SS", a
query with a greater-or-equal '>=' condition matched only
greater values in an indexed TIMESTAMP column.
The problem was introduced by the fix for the bug 46362
and partially solved (for DATE and DATETIME columns only)
by the fix for the bug 47925.
The stored_field_cmp_to_item function has been modified
to take into account TIMESTAMP columns like we do for
DATE and DATETIME columns.
mysql-test/r/type_timestamp.result:
Test case for bug #55779.
mysql-test/t/type_timestamp.test:
Test case for bug #55779.
sql/item.cc:
Bug #55779: select does not work properly in mysql server
Version "5.1.42 SUSE MySQL RPM"
The stored_field_cmp_to_item function has been modified
to take into account TIMESTAMP columns like we do for
DATE and DATETIME.
The patch caused some test failures when merged to 5.5 because,
unlike 5.1, it utilizes Item_cache_row to actually cache row
values. The problem was that Item_cache_row::bring_value()
essentially did nothing. In particular, it did not update its
null_value, so all Item_cache_row objects were always having
their null_values set to TRUE. This went unnoticed previously,
but now when Arg_comparator::compare_row() actually depends on
the row's null_value to evaluate the comparison, the problem
has surfaced.
Fixed by calling the underlying item's bring_value() and
updating null_value in Item_cache_row::bring_value().
Since the problem also exists in 5.1 code (albeit hidden, since
the relevant code is not used anywhere), the addendum patch is
against 5.1.
