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Re: MDEV-5719: Wrong result with GROUP BY and LEFT OUTER JOIN


Hi, Sergey!

On Jan 25, Sergey Petrunia wrote:
> Hi Sergei,
> == TL;DR ==
> I have debugged that LEFT JOIN problem. The fix is not pretty, but
> correct. I will now push it into the 10.0 tree.

I've spent few hours in the airport and during the flight debugging it.
Agree with the fix, but it needs a better comment - the one that
can be understood without few hours of debugging.

I suggest:

-      Do not change the order of testing. Fields may be associated
-      with a TABLE object without being part of the current row.
-      For NULL value check to work for these fields, they must
-      have a valid null_ptr, and this pointer must be checked before
-      TABLE::null_row.
       Note that if a table->null_row is set then also all null_bits are
       set for the row.
+      In the case of the 'result_field' for GROUP BY, table->null_row might
+      refer to the *next* row in the table (when the algorithm is: read the
+      next row, see if any of group column values have changed, send the
+      result - grouped - row to the client if yes). So, table->null_row might
+      be wrong, but such a result_field is always nullable (that's defined by
+      original_field->maybe_null()) and we trust its null bit.

If you agree, I'll commit this change (it doesn't have to be in 10.0.16,
of course).


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