sporadically".
Races in truncate_coverage.test have caused its sporadical
failures.
In the test case we have tried to kill truncate statement
being executed in the first connection which was waiting
for X metadata lock on table being locked by the second
connection. Since we have released metadata lock held by
the second connection right after issuing KILL statement
sometimes TRUNCATE TABLE managed to acquire X lock before
it has noticed that it was killed. In this case TRUNCATE
TABLE was successfully executed till its end and this fact
has caused test failure since this statement didn't return
expected error in such case.
This patch addresses the problem by not releasing metadata
locks in the second connections prematurely.
Bug #22909 Using CREATE ... LIKE is possible to create
field with invalid default value
Bug #35935 CREATE TABLE under LOCK TABLES ignores FLUSH
TABLES WITH READ LOCK
Bug #37371 CREATE TABLE LIKE merge loses UNION parameter
These bugs were originally fixed in the 6.1-fk tree and the fixes
were backported as part of the fix for Bug #42546 "Backup: RESTORE
fails, thinking it finds an existing table". This patch backports
test coverage missing in the original backport. The patch contains
no code changes.
Item*) at opt_sum.cc:305
Queries applying MIN/MAX functions to indexed columns are
optimized to read directly from the index if all key parts
of the index preceding the aggregated key part are bound to
constants by the WHERE clause. A prefix length is also
produced, equal to the total length of the bound key
parts. If the aggregated column itself is bound to a
constant, however, it is also included in the prefix.
Such full search keys are read as closed intervals for
reasons beyond the scope of this bug. However, the procedure
missed one case where a key part meant for use as range
endpoint was being overwritten with a NULL value destined
for equality checking. In this case the key part was
overwritten but the range flag remained, causing open
interval reading to be performed.
Bug was fixed by adding more stringent checking to the
search key building procedure (matching_cond) and never
allow overwrites of range predicates with non-range
predicates.
An assertion was added to make sure open intervals are never
used with full search keys.
Valgrind warning happpens because of uninitialized null bytes.
In row_sel_push_cache_row_for_mysql() function we fill fetch cache
with necessary field values, row_sel_store_mysql_rec() is called
for this and leaves null bytes untouched.
Later row_sel_pop_cached_row_for_mysql() rewrites table record
buffer with uninited null bytes. We can see the problem from the
test case:
At 'SELECT...' we call row_sel_push...->row_sel_store...->row_sel_pop_cached...
chain which rewrites table->record[0] buffer with uninitialized null bytes.
When we call 'UPDATE...' statement, compare_record uses this buffer and
valgrind warning occurs.
The fix is to init null bytes with default values.
mysql-test/suite/innodb/r/innodb_mysql.result:
test case
mysql-test/suite/innodb/t/innodb_mysql.test:
test case
mysql-test/t/ps_3innodb.test:
enable valgrind testing
storage/innobase/row/row0sel.c:
init null bytes with default values as they might be
left uninitialized in some cases and these uninited bytes
might be copied into mysql record buffer that leads to
valgrind warnings on next use of the buffer.
Problem: the server missed the fact that one can read from
2 indexes alternately using HANDLER interface.
Fix: check if the same (initialized) index is involved
reading next/prev values from the index.
mysql-test/r/handler_myisam.result:
Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
- test result.
mysql-test/t/handler_myisam.test:
Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
- test case.
sql/sql_handler.cc:
Fix for bug #54007: assert in ha_myisam::index_next, HANDLER
- check if we use the same (initialized) index
to read next/prev values from the index.
mysql-test/suite/innodb/r/innodb_information_schema.result:
The transaction is in REPEATABLE READ, since the
iso level was changed to SERIALIZABLE only for the
previous trx.
mysql-test/suite/innodb/t/innodb-semi-consistent.test:
'commit' was missing from the original patch
mysql-test/t/parser.test:
Moved the test for Bug#46527 to parser_not_embedded,
since "disconnect default" isn't allowed with
the embedded server.
mysql-test/t/parser_not_embedded.test:
Moved the test for Bug#46527 to parser_not_embedded,
since "disconnect default" isn't allowed with
the embedded server.
Bug#46527 COMMIT AND CHAIN RELEASE does not make sense
Bug#53343 completion_type=1, COMMIT/ROLLBACK AND CHAIN don't
preserve the isolation level
Bug#53346 completion_type has strange effect in a stored
procedure/prepared statement
Added test cases to verify the expected behaviour of :
SET SESSION TRANSACTION ISOLATION LEVEL,
SET TRANSACTION ISOLATION LEVEL,
@@completion_type,
COMMIT AND CHAIN,
ROLLBACK AND CHAIN
..and some combinations of the above
The problem is in the Item_func_isnull::update_used_tables() function,
bracket is at the wrong place. Because of that isnull item erroneously
is treated as const item. The fix is to set brackets in the right place.
mysql-test/r/func_isnull.result:
test case
mysql-test/t/func_isnull.test:
test case
sql/item_cmpfunc.h:
set brackets in the right place.
This crash happened if a table was listed twice in a DROP TABLE statement,
and the statement was executed while in LOCK TABLES mode. Since the two
elements of table list were identical, they were assigned the same TABLE object.
During processing of the first table element, the TABLE instance was destroyed
and the second table list element was left with a dangling reference.
When this reference was later accessed, the server crashed.
Listing the same table twice in DROP TABLES should give an ER_NONUNIQ_TABLE
error. However, this did not happen as the check for unique table names was
skipped due to the lock type for table list elements being set to TL_IGNORE.
Previously TL_UNLOCK was used and the unique check was performed.
This bug was a regression introduced by a pre-requisite patch for
Bug#51263 "Deadlock between transactional SELECT and ALTER TABLE ...
REBUILD PARTITION". The regression only existed in an internal team
tree and never in any released code.
This patch reverts DROP TABLE (and DROP VIEW) to the old behavior of
using TL_UNLOCK locks. Test case added to drop.test.
locks for DML statements and changes the way MDL locks
are acquired/granted in contended case.
Instead of backing-off when a lock conflict is encountered
and waiting for it to go away before restarting open_tables()
process we now wait for lock to be released without releasing
any previously acquired locks. If conflicting lock goes away
we resume opening tables. If waiting leads to a deadlock we
try to resolve it by backing-off and restarting open_tables()
immediately.
As result both waiting for possibility to acquire and
acquiring of a metadata lock now always happen within the
same MDL API call. This has allowed to make release of a lock
and granting it to the most appropriate pending request an
atomic operation.
Thanks to this it became possible to wake up during release
of lock only those waiters which requests can be satisfied
at the moment as well as wake up only one waiter in case
when granting its request would prevent all other requests
from being satisfied. This solves thundering herd problem
which occured in cases when we were releasing some lock and
woke up many waiters for SNRW or X locks (this was the issue
in bug#52289 "performance regression for MyISAM in sysbench
OLTP_RW test".
This also allowed to implement more fair (FIFO) scheduling
among waiters with the same priority.
It also opens the door for introducing new types of requests
for metadata locks such as low-prio SNRW lock which is
necessary in order to support LOCK TABLES LOW_PRIORITY WRITE.
Notice that after this sometimes can report ER_LOCK_DEADLOCK
error in cases in which it has not happened before.
Particularly we will always report this error if waiting for
conflicting lock has happened in the middle of transaction
and resulted in a deadlock. Before this patch the error was
not reported if deadlock could have been resolved by backing
off all metadata locks acquired by the current statement.
mysql-test/r/mdl_sync.result:
Added test coverage for some aspects of deadlock handling in
metadata locking subsystem.
Adjusted test case after removing back-off in general case
when conflicting metadata lock is encountered during
open_tables() (now this happens only if waiting for
conflicting lock to go away leads to a deadlock).
mysql-test/r/sp_sync.result:
Adjusted test case after removing back-off in general case
when conflicting metadata lock is encountered during
open_tables() (now this happens only if waiting for
conflicting lock to go away leads to a deadlock).
mysql-test/suite/perfschema/r/dml_setup_instruments.result:
Adjusted test results after renaming MDL_context::
m_waiting_for_lock rwlock to m_LOCK_waiting_for.
mysql-test/suite/rpl/r/rpl_sp.result:
Adjusted test case after implementing new approach to
acquiring metadata locks in open_tables(). We no longer
release all MDL locks acquired by statement before waiting
for conflicting lock to go away. As result DROP FUNCTION
statement has to wait for DML statement which managed to
acquire metadata lock on function being dropped and now
waits for other conflicting metadata lock to go away.
mysql-test/suite/rpl/t/rpl_sp.test:
Adjusted test case after implementing new approach to
acquiring metadata locks in open_tables(). We no longer
release all MDL locks acquired by statement before waiting
for conflicting lock to go away. As result DROP FUNCTION
statement has to wait for DML statement which managed to
acquire metadata lock on function being dropped and now
waits for other conflicting metadata lock to go away.
mysql-test/t/mdl_sync.test:
Added test coverage for some aspects of deadlock handling in
metadata locking subsystem.
Adjusted test case after removing back-off in general case
when conflicting metadata lock is encountered during
open_tables() (now this happens only if waiting for
conflicting lock to go away leads to a deadlock).
mysql-test/t/sp_sync.test:
Adjusted test case after removing back-off in general case
when conflicting metadata lock is encountered during
open_tables() (now this happens only if waiting for
conflicting lock to go away leads to a deadlock).
sql/mdl.cc:
Changed MDL subsystem to support new approach to acquring
metadata locks in open tables and more fair and efficient
scheduling of metadata locks. To implement this:
- Made releasing of the lock and granting it to the most
appropriate pending request atomic operation. As result it
became possible to wake up only those waiters requests from
which can be satisfied at the moment as well as wake-up
only one waiter in case when granting its request would
prevent all other requests from being satisfied.
This solved thundering herd problem which occured in cases
when we were releasing some lock and woke up many waiters
for SNRW or X locks (this was the issue in Bug #52289
"performance regression for MyISAM in sysbench OLTP_RW
test".
To emphasize above changes wake_up_waiters() was renamed
to MDL_context::reschedule_waiters().
- Changed code to add tickets for new requests to the back of
waiters queue and to select tickets to be satisfied from
the head of the queue if possible (this makes scheduling of
requests with the same priority fair). To be able to do
this efficiently we now use for waiting and granted queues
version of I_P_List class which provides fast push_back()
method.
- Members and methods of MDL_context related to sending
and waiting for signal were moved to separate MDL_wait
class.
- Since in order to avoid race conditions we must grant the
lock only to the context which was not chosen as a victim
of deadlock, killed or aborted due to timeout
MDL_wait::set_status() (former awake()) was changed not to
send signal if signal slot is already occupied and to
indicate this fact through its return value. As another
consequence MDL_wait::timed_wait() method was changed to
handle timeout (optionally) and abort due to kill as
signals which make signal slot occupied.
- Renamed MDL_context::acquire_lock_impl() to acquire_lock().
Changed it to be able correctly process requests for shared
locks when there are open HANDLERs, made this method more
optimized for acquisition of shared locks. As part of this
change moved code common between try_acquire_lock() and
acquire_lock() to new try_acquire_lock_impl() method.
Also adjusted acquire_lock()'s code to take into account
the fact that in cases when lock is granted as result of
MDL_context::reschedule_waiters() call (i.e. when it is
granted after waiting for lock conflict to go away)
updating MDL_lock state is responsibility of the thread
calling reschedule_waiters().
- Changed MDL_context::find_deadlock() to send VICTIM
signal even if victim is the context which has initiated
deadlock detection. This is required in order to avoid
races in cases when the same context simultaneously is
chosen as a victim and its request for lock is satisfied.
As result return value of this method became unnecessary
and it was changed to return void.
Adjusted MDL_lock::find_deadlock() method to take into
account that now there can be a discrepancy between
MDL_context::m_waiting_for value being set and real state
of the ticket this member points to.
- Renamed MDL_context::m_waiting_for_lock to m_LOCK_waiting_for
and MDL_context::stop_waiting() to done_waiting_for().
- Finally, removed MDL_context::wait_for_lock() method.
sql/mdl.h:
Changed MDL subsystem to support new approach to acquring
metadata locks in open tables and more fair and efficient
scheduling of metadata locks. To implement this:
- Members and methods of MDL_context related to sending
and waiting for signal were moved to separate MDL_wait
class.
- Since now in order to avoid race conditions we must grant
the lock only to the context which was not chosen as a
victim of deadlock, killed or aborted due to timeout
MDL_wait::set_status (former awake()) was changed not to
send signal if signal slot is already occupied and to
indicate this fact through its return value.
Also NORMAL_WAKE_UP signal became GRANTED, and timeouts
and aborts due to kill became full blown signals rather
than simple return values.
- MDL_wait::timed_wait() now takes extra parameter that
indicates whether signal should be set if timeout is
reached.
- Enabled fast push_back() operation in MDL_context::m_tickets
list to make move_ticket_after_trans_sentinel() method more
efficient.
- Removed MDL_context::wait_for_lock() method.
- Renamed MDL_context::m_waiting_for_lock to m_LOCK_waiting_for
and MDL_context::stop_waiting() to done_waiting_for().
- MDL_context::acquire_lock_impl() became acquire_lock().
- Introduced MDL_context::try_acquire_lock_impl() as a
place for code shared by try_acquire_lock and
acquire_lock().
- Due to fact that now VICTIM signal is sent even if victim
is the context which has initiated deadlock detection
find_deadlock() no longer needs a return value.
sql/sql_base.cc:
Implemented new approach to acquiring metadata locks in
open_tables(). We no longer perform back-off when conflicting
metadata lock is encountered. Instead we wait for this lock
to go away while holding all locks which were acquired so
far. Back-off is only used in situation when further waiting
will cause a deadlock which could be avoided by performing
back-off and restarting open_tables() process. Absence of
waiting between back-off and restart of acquiring metadata
locks can't lead to livelocks as MDL subsystem was changed
to make release of lock and granting it to waiting lock
an atomic action, so back-off will automatically give way
to other participants of deadlock loop.
Accordingly:
- open_table_get_mdl_lock() and open_and_process_routine()
were changed to wait for conflicting metadata lock to
go away without back-off. Only if such wait leads to a
deadlock back-off is requested. As part of this change
new error handler class was introduced which converts,
if possible, ER_LOCK_DEADLOCK error to a request for
back-off and re-start of open_tables() process.
- Open_table_context::recover_from_failed_open() was changed
not to wait in case of metadata lock conflict. Instead we
immediately proceed to re-acquiring locks.
- Open_table_context::request_backoff_action() now always
emits error if back-off is requested in the middle of
transaction as we can't be sure that releasing lock
which were acquired only by current statement will
resolve a deadlock. Before this patch such situations were
successfully detected thanks to the fact that we called
MDL_context::wait_for_lock() method in
recover_from_failed_open().
- In order to avoid deadlocks open_tables() code was adjusted
to flush open HANDLERs for which there are pending requests
for X locks before restarting the process of acquiring
metadata locks.
- Changed close_tables_for_reopen() not to reset MDL_request
for tables belonging to the tail of prelocking list. It is
no longer necessary as these MDL_request objects won't be
used for any waiting.
- Adjusted comment in tdc_wait_for_old_version() to avoid
mentioning removed MDL_context::wait_for_lock() method.
sql/sql_base.h:
As we no longer wait for conflicting metadata lock away in
Open_table_context::recover_from_failed_open() method,
Open_table_context::OT_WAIT_MDL_LOCK action was renamed to
OT_MDL_CONFLICT.
Also Open_table_context::m_failed_mdl_request became
unnecessary and was removed.
sql/sql_plist.h:
Extended I_P_List template to support efficient push_back()
operation if it is parameterized with an appropriate policy
class.
sql/sql_show.cc:
Adjusted code after removal of MDL_context::wait_for_lock()
method. Now if one needs to acquire metadata lock with waiting
one has to use a variant of MDL_context::acquire_lock() method.
Conflicts:
Text conflict in mysql-test/r/archive.result
Contents conflict in mysql-test/r/innodb_bug38231.result
Text conflict in mysql-test/r/mdl_sync.result
Text conflict in mysql-test/suite/binlog/t/disabled.def
Text conflict in mysql-test/suite/rpl_ndb/r/rpl_ndb_binlog_format_errors.result
Text conflict in mysql-test/t/archive.test
Contents conflict in mysql-test/t/innodb_bug38231.test
Text conflict in mysql-test/t/mdl_sync.test
Text conflict in sql/sp_head.cc
Text conflict in sql/sql_show.cc
Text conflict in sql/table.cc
Text conflict in sql/table.h
Some of the server implementations don't support dates later
than 2038 due to the internal time type being 32 bit.
Added checks so that the server will refuse dates that cannot
be handled by either throwing an error when setting date at
runtime or by refusing to start or shutting down the server if
the system date cannot be stored in my_time_t.
Problems:
- regression (compating to version 5.1) in metadata for BLOB types
- inconsistency between length metadata in server and embedded for BLOB types
- wrong max_length calculation in items derived from BLOB columns
@ libmysqld/lib_sql.cc
Calculating length metadata in embedded similary to server version,
using new function char_to_byte_length_safe().
@ mysql-test/r/ctype_utf16.result
Adding tests
@ mysql-test/r/ctype_utf32.result
Adding tests
@ mysql-test/r/ctype_utf8.result
Adding tests
@ mysql-test/r/ctype_utf8mb4.result
Adding tests
@ mysql-test/t/ctype_utf16.test
Adding tests
@ mysql-test/t/ctype_utf32.test
Adding tests
@ mysql-test/t/ctype_utf8.test
Adding tests
@ mysql-test/t/ctype_utf8mb4.test
Adding tests
@ sql/field.cc
Overriding char_length() for Field_blob:
unlike in generic Item::char_length() we don't
divide to mbmaxlen for BLOBs.
@ sql/field.h
- Making Field::char_length() virtual
- Adding prototype for Field_blob::char_length()
@ sql/item.h
- Adding new helper function char_to_byte_length_safe()
- Using new function
@ sql/protocol.cc
Using new function char_to_byte_length_safe().
modified:
libmysqld/lib_sql.cc
mysql-test/r/ctype_utf16.result
mysql-test/r/ctype_utf32.result
mysql-test/r/ctype_utf8.result
mysql-test/r/ctype_utf8mb4.result
mysql-test/t/ctype_utf16.test
mysql-test/t/ctype_utf32.test
mysql-test/t/ctype_utf8.test
mysql-test/t/ctype_utf8mb4.test
sql/field.cc
sql/field.h
sql/item.h
sql/protocol.cc
Due to a BZR bug, that merge was done by the following command:
bzr merge -r 'revid:tor.didriksen@sun.com-20100527074248-6qtv0p1ugy6o1hjo..' <mysql-trunk-bugfixing path>
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080324111246-00461
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080414125521-40866
BUG#35274 - merge table doesn't need any base tables, gives
error 124 when key accessed
SELECT queries that use index against a merge table with empty
underlying tables list may return with error "Got error 124 from
storage engine".
The problem was that wrong error being returned.
mysql-test/r/merge.result:
Backport of
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080324111246-00461
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080414125521-40866
A test case for BUG#35274.
Modified a test case according to fix for BUG#35274. Key based
reads are now allowed for merge tables with no underlying tables
defined.
mysql-test/t/merge.test:
Backport of
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080324111246-00461
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080414125521-40866
A test case for BUG#35274.
Modified a test case according to fix for BUG#35274. Key based
reads are now allowed for merge tables with no underlying tables
defined.
storage/myisammrg/myrg_queue.c:
Backport of
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080324111246-00461
- revid:sp1r-svoj@mysql.com/june.mysql.com-20080414125521-40866
Return "end of file" error instead of "wrong index" error when
we got a merge table with empty underlying tables list.
In 5.1 we cannot rely on info->open_tables value when checking
if a merge table has no underlying tables defined.
Use info->tables instead.
when it should use index
Sometimes the LEFT/RIGHT JOIN with an empty table caused an
unnecessary filesort.
Sample query, where t1.i1 is indexed and t3 is empty:
SELECT t1.*, t2.* FROM t1 JOIN t2 ON t1.i1 = t2.i2
LEFT JOIN t3 ON t2.i2 = t3.i3
ORDER BY t1.i1 LIMIT 5;
The server erroneously used an item of empty outer-joined
table as a common constant of a Item_equal (multi-equivalence
expression).
By the fix for the bug 16590 the constant status of such
an item has been propagated to st_table::const_key_parts
map bits related to other Item_equal argument-related
key parts (those are obviously not constant in our case).
As far as test_if_skip_sort_order function skips constant
prefixes of testing keys, this caused an ignorance of
available indices, since some prefixes were marked as
constant by mistake.
mysql-test/r/order_by.result:
Test case for bug #38745.
mysql-test/t/order_by.test:
Test case for bug #38745.
sql/item.h:
Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY
when it should use index
Item::is_outer_field() has been added and overloaded for
Item_field and Item_ref classes.
sql/item_cmpfunc.cc:
Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY
when it should use index
Item_equal::update_const() and Item_equal::update_used_tables()
have been updated to not take into account the constantness
of outer-joined table items.
Field_time::get_date method does not initialize MYSQL_TIME::time_type field.
The fix is to init this field.
mysql-test/r/type_time.result:
test case
mysql-test/t/type_time.test:
test case
sql/field.cc:
--use Field_time::get_time in Field_time::get_date
--removed duplicated code in Field_time::get_date method
bug #46947 "Embedded SELECT without FOR UPDATE is causing
a lock".
Fixed comments in tests. Improved comments and performance of
auxiliary scripts.
mysql-test/include/check_concurrent_insert.inc:
Changed script to use temporary table for backing up data in
order to make this operation less expensive. Made script more
a bit generic by allowing to use multi-column tables with it.
Improved comments.
mysql-test/include/check_no_concurrent_insert.inc:
Changed script to use temporary table for backing up data in
order to make this operation less expensive. Made script more
a bit generic by allowing to use multi-column tables with it.
Improved comments.
mysql-test/include/check_no_row_lock.inc:
Improved comments in auxiliary script.
mysql-test/r/innodb_mysql_lock2.result:
Fixed errors in comments for test.
mysql-test/r/lock_sync.result:
Fixed typo in comments for test.
mysql-test/t/innodb_mysql_lock2.test:
Fixed errors in comments for test.
mysql-test/t/lock_sync.test:
Fixed typo in comments for test.
and .tar.gz, windows vs linux..
On Intel x86 machines index selection by the MySQL query
optimizer could sometimes depend on the compiler version and
optimization flags used to build the server binary.
The problem was a result of a known issue with floating point
calculations on x86: since internal FPU precision (80 bit)
differs from precision used by programs (32-bit float or 64-bit
double), the result of calculating a complex expression may
depend on how FPU registers are allocated by the compiler and
whether intermediate values are spilled from FPU to memory. In
this particular case compiler versions and optimization flags
had an effect on cost calculation when choosing the best index
in best_access_path().
A possible solution to this problem which has already been
implemented in mysql-trunk is to limit FPU internal precision
to 64 bits. So the fix is a backport of the relevant code to
5.1 from mysql-trunk.
configure.in:
Configure check for fpu_control.h
mysql-test/r/explain.result:
Test case for bug #48537.
mysql-test/t/explain.test:
Test case for bug #48537.
sql/mysqld.cc:
Backport of the code to switch FPU on x86 to 64-bit precision.
The problem is that if a NULL is stored in an Item_cache_decimal object,
the associated my_decimal object is not initialized. However, it is still
accessed when val_int() is called. The fix is to check for null_value
within val_int(), and return without accessing the my_decimal object when
the cached value is NULL.
Bug#52122 reports the same issue for val_real(), and this patch also includes
fixes for val_real() and val_str() and corresponding test cases from that
bug report.
Also, NULL is returned from val_decimal() when value is null. This will
avoid that callers access an uninitialized my_decimal object.
Made similar changes to all other Item_cache classes. Now all val_*
methods should return a well defined value when actual value is NULL.
mysql-test/r/type_decimal.result:
Updated result file with test cases for Bug#52168 and Bug#52122.
mysql-test/t/type_decimal.test:
Added test cases for Bug#52168 and Bug#52122.
sql/item.cc:
In Item_cache_*::val_* methods, return a well defined value
when actual value is NULL.
This is especially important for Item_cache_decimal since
otherwise one risk accessing an uninitialized my_decimal object.
sql/item.h:
Added method Item_cache::has_value() which returns TRUE if cache
object contains a non-null value.
