MySQL Manual | D.6.8 Changes in release 3.21.26

FROM_DAYS(0) now returns "0000-00-00".
In DATE_FORMAT(), PM and AM were swapped for hours 00 and 12.
Extended the default maximum key size to 256.
Fixed bug when using BLOB/TEXT in GROUP BY with many tables.
An ENUM field that is not declared NOT NULL has NULL as the default value. (Previously, the default value was the first enumeration value.)
Fixed bug in the join optimiser code when using many part keys on the same key: INDEX (Organisation,Surname(35),Initials(35)).
Added some tests to the table order optimiser to get some cases with SELECT ... FROM many_tables much faster.
Added a retry loop around accept() to possibly fix some problems on some Linux machines.

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