MySQL Bugs: #46493: CHECKSUM TABLE EXTENDED returns different value from QUICK if ROW

Bug #46493	CHECKSUM TABLE EXTENDED returns different value from QUICK if ROW_FORMAT=FIXED
Submitted:	31 Jul 2009 13:06	Modified:	12 Mar 2010 12:33
Reporter:	Guilhem Bichot	Email Updates:
Status:	In progress	Impact on me:	None
Category:	MySQL Server: MyISAM storage engine	Severity:	S3 (Non-critical)
Version:	5.1-bzr	OS:	Any
Assigned to:	Assigned Account	CPU Architecture:	Any

Description:
When a MyISAM table is created with ROW_FORMAT=FIXED CHECKSUM=1,
CHECKSUM TABLE QUICK and CHECKSUM TABLE EXTENDED return different values,
which shouldn't be (normally the difference between QUICK and EXTENDED is just that QUICK is faster, but same value).

How to repeat:
Testcase:
create table t1 (a int null, v varchar(100)) engine=myisam checksum=1 row_format=fixed;
insert into t1 values(null, null), (1, "hello");
checksum table t1;
checksum table t1 quick;
checksum table t1 extended;

the last checksum value is different from the two first ones.

likely the bug depends on NULL fields.

Guilhem,

I suggest that QUICK skips all float and double values, as those differ from platform to platform. Then, it makes sense to have different checksums with different methods.

Hello Sinisa,
thanks for the suggestion. Though you know, this would be a behaviour change: QUICK is currently not expected to skip platform-dependent data like floats, it's expected to return the same as EXTENDED but in a quicker manner.
If I had to change CHECKSUM TABLE, I'd modify it to use a platform-independent way of incorporating floats. Like, the computation would round floats to a precision gross enough that the rounded value is platform-independent; if this is technically possible. For example if I store 1.1 in a float, it may be 1.1000000001 on some machine, 1.09999999321 on some other, but, both values rounded to, say, 3 decimals, are the same (1.1000) and could be used to compute the checksum. Just a wild idea. Anyway anything like this should be put into WL for design first, there already exists WL#5020 "improve checksum returned by CHECKSUM TABLE" which lists other known problems. Is there a bug report for the float/double checksum problem?

I don't even know if the float/double problems still exist, reading http://bugs.mysql.com/bug.php?id=30041

the "double/float problem" can not be "solved", just consider it part of http://dev.mysql.com/doc/refman/5.0/en/problems-with-float.html

Is this really still "In progress"?