DOING BAD DDL IN PREPARED STATEMENT
Analysis
========
A repeat execution of the prepared statement 'ALTER TABLE v1
CHECK PARTITION' where v1 is a view leads to server exit.
ALTER TABLE ... CHECK PARTITION is not applicable for views
and check for the same check is missing. This leads to
further execution and creation of derived table for the view
(Allocated under temp_table mem_root). Any reference to open
view or related pointers from second execution leads to
server exit as the same was freed at previous execution closure.
Fix:
======
Added check for view in mysql_admin_table() on PARTITION
operation. This will prevent mysql_admin_table() from
going ahead and creating temp table and related issues.
Changed message on admin table view operation error to
be more appropriate.
===========
When doing an upgrade, you execute mysql_upgrade. If
mysql_upgrade fails to connect or it connects with a user
without the proper privileges, it will return the error:
FATAL ERROR: Upgrade failed
which is not very informative.
ANALYSIS
========
In main() and check_version_match(), the condition for
errors are clubbed together and throw the same error msg.
The functions need to be splitted up and the corresponding
error msgs have to be displayed.
FIX
===
Splitted the functions and added the specific error msg.
Issue: main.events_1 will fail after 10-10-2015 due
to hardcoded dates specified with events.
Fix: Replace harcoded dates with current dates and offset.
IMPLEMENTED IN ALL CLIENT PROGRAMS
Description: Option "enable-cleartext-plugin" is not
available for the following client utilities:-
mysqldump
mysqlimport
mysqlshow
mysqlcheck
Analysis: The unavailability of this option limits the
features like PAM authentication from using the above
mentioned utilities.
Fix: Option "enable-cleartext-plugin" is implemented in the
above mentioned client utilities.
Issue:
======
The fulltext predicate is inside a subquery and involves
an outer reference; it thus cannot be used for FT index look-up,
but MySQL does not see it, which causes a illegal access.
Solution:
=========
Solution is backported from bug#21140088. Outer reference can
not be used as argument of the MATCH function. Added check for
outer reference.
FILE
PROBLEM
In 5.5 when doing doing a rename of a column ,we ignore the case between
old and new column names while comparing them,so if the change is just
the case then we don't even mark the field FIELD_IS_RENAMED ,we just update
the frm file ,but don't recreate the table as is the norm when alter is
used.This leads to inconsistency in the innodb data dictionary which causes
index creation to fail.
FIX
According to the documentation any innodb column rename should trigger
rebuild of the table. Therefore for innodb tables we will do a strcmp()
between the column names and if there is case change in column name
we will trigger a rebuild.
PROBLEMS
Description:- Server variable "--lower_case_tables_names"
when set to "0" on windows platform which does not support
case sensitive file operations leads to problems. A warning
message is printed in the error log while starting the
server with "--lower_case_tables_names=0". Also according to
the documentation, seting "lower_case_tables_names" to "0"
on a case-insensitive filesystem might lead to index
corruption.
Analysis:- The problem reported in the bug is:-
Creating an INNODB table 'a' and executing a query, "INSERT
INTO a SELECT a FROM A;" on a server started with
"--lower_case_tables_names=0" and running on a
case-insensitive filesystem leads innodb to flat spin.
Optimizer thinks that "a" and "A" are two different tables
as the variable "lower_case_table_names" is set to "0". As a
result, optimizer comes up with a plan which does not need a
temporary table. If the same table is used in select and
insert, a temporary table is needed. This incorrect
optimizer plan leads to infinite insertions.
Fix:- If the server is started with
"--lower_case_tables_names" set to 0 on a case-insensitive
filesystem, an error, "The server option
'lower_case_table_names'is configured to use case sensitive
table names but the data directory is on a case-insensitive
file system which is an unsupported combination. Please
consider either using a case sensitive file system for your
data directory or switching to a case-insensitive table name
mode.", is printed in the server error log and the server
exits.
DESCRIPTION
===========
Inability of mysql LOAD XML command to handle empty XML
tags i.e. <row><tag/></row>. Also the behaviour is wrong
and (different than above) when there is a space in empty
tag i.e. <row><tag /></row>
ANALYSIS
========
In read_xml() the case where we encounter a close tag ('/')
we're decreasing the 'level' blindly which is wrong.
Actually when its an without-space-empty-tag (succeeding
char is '>'), we need to skip the decrement. In other words
whenever we hit a close tag ('/'), decrease the 'level'
only when (i) It's not an (without space) empty tag i.e.
<tag/> or, (ii) It is of format <row col="val" .../>
FIX
===
The switch case for '/' is modified. We've removed the
blind decrement of 'level'. We do it only when its not an
without-space-empty-tag. Also we are setting 'in_tag' to
false to let program know that we're done reading current
tag (required in the case of format <row col="val" .../>)
Analysis :
==========
During JOIN::prepare of sub-query which creates the
derived tables we call setup_procedure. Here we call
fix_fields for parameters of procedure clause. Calling
setup_procedure at this point may cause issue. If
sub-query is one of parameter being fixed it might
lead to complicated dependencies on derived tables
being prepared.
SOLUTION :
==========
In 5.6 with WL#6242, we have made procedure clause
parameters can only be NUM, so sub-queries are not
allowed as parameters. So in 5.5 we can block
sub-queries in procedure clause parameters.
This eliminates above conflicting dependencies.
IS REJECTED.
Analysis
========
View creation with named columns over UNION is rejected.
Consider the following view definition:
CREATE VIEW v1 (fld1, fld2) AS SELECT 1 AS a, 2 AS b
UNION ALL SELECT 1 AS a, 1 AS a;
A 'duplicate column' error was reported due to the duplicate
alias name in the secondary SELECT. The VIEW column names
are either explicitly specified or determined from the
first SELECT (which can be auto generated if not specified).
Since a duplicate column name check was performed even
for the secondary SELECTs, an error was reported.
Fix
====
Check for duplicate column names only for the named
columns if specified or only for the first SELECT.
INCORRECT RESULTS
Issue:
-----
Updating varchar and text fields in the same update
statement can produce incorrect results. When a varchar
field is assigned to the text field and the varchar field
is then set to a different value, the text field's result
contains the varchar field's new value.
SOLUTION:
---------
Currently the blob type does not allocate space for the
string to be stored. Instead it contains a pointer to the
varchar string. So when the varchar field is changed as
part of the update statement, the value contained in the
blob also changes.
The fix would be to actually store the value by allocating
space for the blob's string. We can avoid allocating this
space when the varchar field is not being written into.
MULTIPLE THREADS
Description:- The utility "mysqlimport" does not use
multiple threads for the execution with option
"--use-threads". "mysqlimport" while importing multiple
files and multiple tables, uses a single thread even if the
number of threads are specified with "--use-threads" option.
Analysis:- This utility uses ifdef HAVE_LIBPTHREAD to check
for libpthread library and if defined uses libpthread
library for mutlithreaing. Since HAVE_LIBPTHREAD is not
defined anywhere in the source, "--use-threads" option is
silently ignored.
Fix:- "-DTHREADS" is set to the COMPILE_FLAGS which will
enable pthreads. HAVE_LIBPTHREAD macro is removed.
Problem :
---------
Issue-1: The root cause for the issues is that (col1 > 1) is not a
valid partition function and we should have thrown error at much early
stage [partition_info::check_partition_info]. We are not checking
sub-partition expression when partition expression is NULL.
Issue-2: Potential issue for future if any partition function needs to
change item tree during open/fix_fields. We should release changed
items, if any, before doing closefrm when we open the partitioned table
during creation in create_table_impl.
Solution :
----------
1.check_partition_info() - Check for sub-partition expression even if no
partition expression.
[partition by ... columns(...) subpartition by hash(<expr>)]
2.create_table_impl() - Assert that the change list is empty before doing
closefrm for partitioned table. Currently no supported partition function
seems to be changing item tree during open.
Reviewed-by: Mattias Jonsson <mattias.jonsson@oracle.com>
RB: 9345
AVOID DEADLOCK AFTER RESTORE
Analysis
--------
Accessing the restored NDB table in an active multi-statement
transaction was resulting in deadlock found error.
MySQL Server needs to discover metadata of NDB table from
data nodes after table is restored from backup. Metadata
discovery happens on the first access to restored table.
Current code mandates this statement to be the first one
in the transaction. This is because discover needs exclusive
metadata lock on the table. Lock upgrade at this point can
lead to MDL deadlock and the code was written at the time
when MDL deadlock detector was not present. In case when
discovery attempted in the statement other than the first
one in transaction ER_LOCK_DEADLOCK error is reported
pessimistically.
Fix:
---
Removed the constraint as any potential deadlock will be
handled by deadlock detector. Also changed code in discover
to keep metadata locks of active transaction.
Same issue was present in table auto repair scenario. Same
fix is added in repair path also.
PROBLEM
Create time is calculated as last status change time of .frm file.
The first problem was that innodb was passing file name as
"table_name#po#p0.frm" to the stat() call which calculates the create time.
Since there is no frm file with this name create_time will be stored as NULL.
The second problem is ha_partition::info() updates stats for create time
when HA_STATUS_CONST flag was set ,where as innodb calculates this statistic
when HA_STATUS_TIME is set,which causes create_time to be set as NULL.
Fix
Pass proper .frm name to stat() call and calculate create time when
HA_STATUS_CONST flag is set.
Description: SSL tests are failing in mysql-5.1 pb2
Analysis: The SSL certificates are ended by jan 2015.
Hence the SSL tests are failing.
Fix: We have generated new certificates with SHA1 algorithm.
Description: When querying a subset of columns from the information_schema.TABLES
Analysis: When information about tables is collected for statements like
"SELECT ENGINE FROM I_S.TABLES" we do not perform full-blown table opens
in SE, instead we only use information from table shares from the Table
Definition Cache or .FRMs. Still in order to simplify I_S implementation
mock TABLE objects are created from TABLE_SHARE during this process.
This is done by calling open_table_from_share() function with special
arguments. Since this function always increments "Opened_tables" counter,
calls to it can be mistakingly interpreted as full-blown table opens in SE.
Note that claim that "'SELECT ENGINE FROM I_S.TABLES' statement doesn't
use Table Cache" is nevertheless factually correct. But it misses the
point, since such statements a) don't use full-blown TABLE objects and
therefore don't do table opens b) still use Table Definition Cache.
Fix: We are now incrementing the counter when db_stat(i.e open flags for ha_open(
we have considered an optimization which would use TABLE objects from
Table Cache when available instead of constructing mock TABLE objects,
but found it too intrusive for stable releases.
Analysis:
--------
Certain queries using intrinsic temporary tables may fail due to
name clashes in the file name for the temporary table when the
'temp-pool' enabled.
'temp-pool' tries to reduce the number of different filenames used for
temp tables by allocating them from small pool in order to avoid
problems in the Linux kernel by using a three part filename:
<tmp_file_prefix>_<pid>_<temp_pool_slot_num>.
The bit corresponding to the temp_pool_slot_num is set in the bit
map maintained for the temp-pool when it used for the file name.
It is cleared after the temp table is deleted for re-use.
The 'create_tmp_table()' function call under error condition
tries to clear the same bit twice by calling 'free_tmp_table()'
and 'bitmap_lock_clear_bit()'. 'free_tmp_table()' does a delete
of the table/file and clears the bit by calling the same function
'bitmap_lock_clear_bit()'.
The issue reported can be triggered under the timing window mentioned
below for an error condition while creating the temp table:
a) THD1: Due to an error clears the temp pool slot number used by it
by calling 'free_tmp_table'.
b) THD2: In the process of creating the temp table by using an unused
slot number in the bit map.
c) THD1: Clears the slot number used THD2 by calling
'bitmap_lock_clear_bit()' after completing the call 'free_tmp_table'.
d) THD3: Uses the slot number used the THD2 since it is freed by THD1.
When it tries to create the temp file using that slot number,
an error is reported since it is currently in use by THD2.
[The error: Error 'Can't create/write to file
'/tmp/#sql_277e_0.MYD' (Errcode: 17)']
Another issue which may occur in 5.6 and trunk is that:
When the open temporary table fails after its creation(due to ulimit
or OOM error), the file is not deleted. Thus further attempts to use
the same slot number in the 'temp-pool' results in failure.
Fix:
---
a) Under the error condition calling the 'bitmap_lock_clear_bit()'
function to clear the bit is unnecessary since 'free_tmp_table()'
deletes the table/file and clears the bit. Hence removed the
redundant call 'bitmap_lock_clear_bit()' in 'create_tmp_table()'
This prevents the timing window under which the issue reported
can be seen.
b) If open of the temporary table fails, then the file is deleted
thus allowing the temp-pool slot number to be utilized for the
subsequent temporary table creation.
c) Also if the attempt to create temp table fails since it already
exists, the temp-pool slot for it is marked as used, to avoid
the problem from re-appearing.
Description:- Replace, the string replacement utility fails
on the windows platform.
Analysis:- The utility takes files and multiple string
pairs as input. In the windows platform, the utility maps
the standard input incorrectly (Considers 0 as the fd for
standard input instead of 2048 on windows). Hence when the
input file is supplied to the utility due to the incorrect
mapping, a bad file descriptor error is generated causing
the utility to exit with an error.
Fix:- "my_fileno()" is used for getting the correct file
despscriptor for standard input.
AND IS_USED_LOCK().
Analysis:
-----------
In functions Item_func_is_free_lock::val_int() and
Item_func_is_used_lock::val_int(), for the specified user lock
name, pointer to its "User_level_lock" object is obtained from hash
"hash_user_locks". Mutex "LOCK_user_locks" is acquired for this
and released immediately. And we are accessing members of
User_level_lock after releasing the mutex. If same user lock is
deleted(released) from concurrent thread then accessing members
results in invalid(freed) memory access issue.
Deleting of user lock is also protected from the mutex
"LOCK_user_locks". Since this mutex is released in "val_int"
functions mentioned above, delete operation proceeds while concurrent
thread tries to access its members.
With the test case, valgrind reports invalid read issues in val_int
functions.
Fix:
-----------
To fix this issue, in "val_int" function of classes
"Item_func_is_free_lock" and "Item_func_is_used_lock", now releasing
mutex "LOCK_user_locks" after accessing User_level_lock members.
COLLATIONS ARE USED.
ISSUE :
-------
Code points of HALF WIDTH KATAKANA in SJIS/CP932 range from
A1 to DF. In function my_wildcmp_mb_bin_impl while comparing
such single byte code points, there is a code which compares
signed character with unsigned character. Because of this,
comparisons of two same code points representing a HALF
WIDTH KATAKANA character always fails.
Solution:
---------
A code point of HALF WIDTH KATAKANA at-least need 8 bits.
Promoting the variable from uchar to int will fix the issue.
GOES AWAY, MYSQL QUITS WORKING.
Analysis:
-----------------
Issue in this bug and in bug 11907705 is, the socket file or
fifo file is set for general log at command line while starting
the server. But currently, only regular file can be set for the
general log. Instead of reporting any error, the provided files
are opened for writing and continued. Because of this issues
mentioned in the bug reports are seen.
As mentioned, only when any non-regular file is set for general
log at command line while starting the server, these issues are
seen. If general log file is set to non-regular file from CLI
using system variable general_log_file then error is reported.
These issues can also be faced with slow query log file, if it is
set to non-regular file.
Fix:
-----------------
Currently while starting the server if we fail to open log file
then we report an error, disable logging to file and continue.
To fix issue reported code is modified to check whether file
is regular file or not before opening it. If file is not a
regular file then error is logged to error log and logging to
file is disabled.
Bug#16415173 CRLF INSTEAD OF LF IN SQL-BENCH SCRIPTS
Correct perms and converts from Windows style to UNIX style line endings on some files.
Fix perms on installed ini files.
(MySQL 5.5 version)
CORRUPTS FRM
Analysis:
---------
ALTER TABLE on a partitioned table resulted in the wrong
engine being written into the table's FRM file and displayed
in SHOW CREATE TABLE.
The prep_alter_part_table() modifies the partition_info object
for TABLE instance representing the old version of table.
If the ALTER TABLE ENGINE statement fails, the partition_info
object for the TABLE contains the altered storage engine name.
The SHOW CREATE TABLE uses the TABLE object to display the table
information, hence displays incorrect storage engine for the table.
Also a subsequent successful ALTER TABLE operation will write the
incorrect engine information into the FRM file.
Fix:
---
A copy of the partition_info object is created before modification so
that any changes would not cause the the original partition_info object
to be modified if the ALTER TABLE fails.(Backported part of the code
provided as fix for bug#14156617 in mysql-5.6.6).
"HAVING SUM(DISTINCT)": WRONG RESULTS.
ISSUE:
------
If a query uses loose index scan and it has both
AGG(DISTINCT) and MIN()/MAX()functions. Then, result values
of MIN/MAX() is set improperly.
When query has AGG(DISTINCT) then end_select is set to
end_send_group. "end_send_group" keeps doing aggregation
until it sees a record from next group. And, then it will
send out the result row of that group.
Since query also has MIN()/MAX() and loose index scan is
used, values of MIN/MAX() are set as part of loose index
scan itself. Setting MIN()/MAX() values as part of loose
index scan overwrites values computed in end_send_group.
This caused invalid result.
For such queries to work loose index scan should stop
performing MIN/MAX() aggregation. And, let end_send_group to
do the same. But according to current design loose index
scan can produce only one row per group key. If we have both
MIN() and MAX() then it has to give two records out. This is
not possible as interface has to use common buffer
record[0]! for both records at a time.
SOLUTIONS:
----------
For such queries to work we need a new interface for loose
index scan. Hence, do not choose loose_index_scan for such
cases. So a new rule SA7 is introduced to take care of the
same.
SA7: "If Q has both AGG_FUNC(DISTINCT ...) and
MIN/MAX() functions then loose index scan access
method is not used."
SHOW PROCESSLIST, SHOW BINLOGS
Fixing post push test failure (MTR does not like giving
127.0.0.1 for localhost incase of --embedded run, it thinks
it is an external ip address)
SHOW PROCESSLIST, SHOW BINLOGS
Problem: A deadlock was occurring when 4 threads were
involved in acquiring locks in the following way
Thread 1: Dump thread ( Slave is reconnecting, so on
Master, a new dump thread is trying kill
zombie dump threads. It acquired thread's
LOCK_thd_data and it is about to acquire
mysys_var->current_mutex ( which LOCK_log)
Thread 2: Application thread is executing show binlogs and
acquired LOCK_log and it is about to acquire
LOCK_index.
Thread 3: Application thread is executing Purge binary logs
and acquired LOCK_index and it is about to
acquire LOCK_thread_count.
Thread 4: Application thread is executing show processlist
and acquired LOCK_thread_count and it is
about to acquire zombie dump thread's
LOCK_thd_data.
Deadlock Cycle:
Thread 1 -> Thread 2 -> Thread 3-> Thread 4 ->Thread 1
The same above deadlock was observed even when thread 4 is
executing 'SELECT * FROM information_schema.processlist' command and
acquired LOCK_thread_count and it is about to acquire zombie
dump thread's LOCK_thd_data.
Analysis:
There are four locks involved in the deadlock. LOCK_log,
LOCK_thread_count, LOCK_index and LOCK_thd_data.
LOCK_log, LOCK_thread_count, LOCK_index are global mutexes
where as LOCK_thd_data is local to a thread.
We can divide these four locks in two groups.
Group 1 consists of LOCK_log and LOCK_index and the order
should be LOCK_log followed by LOCK_index.
Group 2 consists of other two mutexes
LOCK_thread_count, LOCK_thd_data and the order should
be LOCK_thread_count followed by LOCK_thd_data.
Unfortunately, there is no specific predefined lock order defined
to follow in the MySQL system when it comes to locks across these
two groups. In the above problematic example,
there is no problem in the way we are acquiring the locks
if you see each thread individually.
But If you combine all 4 threads, they end up in a deadlock.
Fix:
Since everything seems to be fine in the way threads are taking locks,
In this patch We are changing the duration of the locks in Thread 4
to break the deadlock. i.e., before the patch, Thread 4
('show processlist' command) mysqld_list_processes()
function acquires LOCK_thread_count for the complete duration
of the function and it also acquires/releases
each thread's LOCK_thd_data.
LOCK_thread_count is used to protect addition and
deletion of threads in global threads list. While show
process list is looping through all the existing threads,
it will be a problem if a thread is exited but there is no problem
if a new thread is added to the system. Hence a new mutex is
introduced "LOCK_thd_remove" which will protect deletion
of a thread from global threads list. All threads which are
getting exited should acquire LOCK_thd_remove
followed by LOCK_thread_count. (It should take LOCK_thread_count
also because other places of the code still thinks that exit thread
is protected with LOCK_thread_count. In this fix, we are changing
only 'show process list' query logic )
(Eg: unlink_thd logic will be protected with
LOCK_thd_remove).
Logic of mysqld_list_processes(or file_schema_processlist)
will now be protected with 'LOCK_thd_remove' instead of
'LOCK_thread_count'.
Now the new locking order after this patch is:
LOCK_thd_remove -> LOCK_thd_data -> LOCK_log ->
LOCK_index -> LOCK_thread_count
ISSUE:
------
For UNION of selects, rows examined by the query will be sum
of rows examined by individual select operations and rows
examined for union operation. The value of session level
global counter that is used to count the rows examined by a
select statement should be accumulated and reset before it
is used for next select statement. But we have missed to
reset the same. Because of this examined row count of a
select query is accounted more than once.
SOLUTION:
---------
In union reset the session level global counter used to
accumulate count of examined rows after its value is saved.
Problem:
If there is a predicate on a column referenced by MIN/MAX and
that predicate is not present in all the disjunctions on
keyparts earlier in the compound index, Loose Index Scan will
not return correct result.
Analysis:
When loose index scan is chosen, range optimizer currently
groups all the predicates that contain group parts separately
and minmax parts separately. It therefore applies all the
conditions on the group parts first to the fetched row.
Then in the call to next_max, it processes the conditions
which have min/max keypart.
For ex in the following query:
Select f1, max(f2) from t1 where (f1 = 10 and f2 = 13) or
(f1 = 3) group by f1;
Condition (f2 = 13) would be applied even for rows that
satisfy (f1 = 3) thereby giving wrong results.
Solution:
Do not choose loose_index_scan for such cases. So a new rule
WA2 is introduced to take care of the same.
WA2: "If there are predicates on C, these predicates must
be in conjuction to all predicates on all earlier keyparts
in I."
Todo the same, fix reuses the function get_constant_key_infix().
Since this funciton will fail for all multi-range conditions, it
is re-written to recognize that if the sub-conditions are
equivalent across the disjuncts: it will now succeed.
And to achieve this a new helper function is introduced called
all_same().
The fix also moves the test of NGA3 up to the former only
caller, get_constant_key_infix().
Typo leading to not including the last list values (partition).
Also improved pruning to skip last partition if not used.
rb#4762 approved by Aditya and Marko.
ARE PERMANENTLY SKIPPED IN 5.5/5.6).
The problem was that some result files were not updated,
so the tests were skipped.
The fix is to record updated result files.
Bug#18396916 MAIN.OUTFILE_LOADDATA TEST FAILS ON ARM, AARCH64, PPC/PPC64
The recorded results for the failing tests were wrong.
They were introduced by the patch for
Bug#30946 mysqldump silently ignores --default-character-set when used with --tab
Correct results were returned for platforms where 'char' is implemented as unsigned.
This was reported as
Bug#46895 Test "outfile_loaddata" fails (reproducible)
Bug#11755168 46895: TEST "OUTFILE_LOADDATA" FAILS (REPRODUCIBLE)
The patch for that bug fixed only parts of the problem,
leaving the incorrect results in the .result file.
Solution: use 'uchar' for field_terminator and line_terminator on all platforms.
Also: remove some un-necessary casts, leaving the ones we actually need.
LOCAL AND IMPORT ERRORS
Description:
-----------
This bug happens due to the fact that current algorithm is designed
that in the case of LOCAL load of data, in case of the error, the
remaining part of the file is read in order to return the proper
error message to the client side.
But, the problem with current implementation is that data stream
for the client side is cleared only in the case where line delimiters
exist, which is not a case with, for example fixed width
fields.
Fix:
----
Ported patch provided by Sinisa Milivojevic n bug report for this
issue to 5.5+ versions.
As part of this patch code is changed to clear the data stream
by calling new member function "READ_INFO::skip_data_till_eof".
ACCEPTED BUT PARSED INCORRECTLY
When we are setting the value in a system variable,
We can set it like
set sys_var="Iden1.Iden2"; //1
set sys_var='Iden1.Iden2'; //2
set sys_var=Iden1.Iden2; //3
set sys_var=.ident1.ident2; //4
set sys_var=`Iden1.Iden2`; //5
While parsing, for case 1(when ANSI_QUOTES is enable) and 2,
we will take as string literal(we will make item of type Item_string).
for case 3 & 4, taken as Item_field, where Iden1 is a table name and
iden2 is a field name.
for case 5, again Item_field type, where iden1.iden2 is taken as
field name.
Now in case 1, when we are assigning some value to system variable
(which can take string or enumerate type data), we are setting only
field part.
This means only iden2 value will be set for system variable. This
result in wrong result.
Solution:
(for string type) We need to Document that we are not allowed to set
system variable which takes string as identifier, otherwise result
in unexpected behaviour.
(for enumerate type)
if we pass iden1.iden2, we will give an error ER_WRONG_TYPE_FOR_VAR
(Incorrect argument type to variable).
