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Hello,
We spot a bug with group by & group_concat, here is the simplified test case :
– Step 1 create a temp table with 2 different rows of 1025 char (datatype is text)
drop temporary table if exists _test; create temporary table _test as select lpad('a', 1025, '0') as chaine union select lpad('b', 1025, '0');
– This result is OK
select * from _test group by chaine;
– In this query, all is grouped, it should not !
select group_concat(chaine) from _test group by chaine;
– we found an ugly workaround...
select group_concat(chaine) from _test group by crc32(chaine);
This is an upstream bug, i reported it here :
http://bugs.mysql.com/bug.php?id=78383
Tested on MySQL 5.5.39, MySQL 5.6.26, MariaDB 10.0.19
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Upstream closed it as not-a-bug.
Here is Hartmut's comment from the upstream bug report, for future reference:
It turned out the limiting factor is max_sort_length, which is documented behavior after all, but it's strange that this documented limitation only kicks in in the TEXT/GROUP_CONCAT()/GROUP BY scenario while in the other cases comparison was not limited by this ...
OK, this is not a bug, but this seem so odd !
As Hartmut commented in the upstream bug report, it is actually a documented behavior, there are still some questions about it, so lets see what MySQL responds.