Re: Optimize fails due to duplicate rows error but no duplicates found

Hello Machiel,

On 2/13/2018 3:02 AM, Machiel Richards wrote:
> Good day guys,
>
>
>      I am hoping this mail finds you well.
>
>
>     I am at a bit of a loss here...
>
>
>      We are trying to run optimize against a table in order to reclaim disk space from archived data which has been removed.
>
>
>      However, after running for over an hour , the optimize fails stating there is a duplicate entry in the table.
>
>
>
>       We have now spent 2 days using various methods but we are unable to find any duplicates in the primary key and also nothing on the unique key fields.
>
>
> Any idea on why optimize would still be failing ?
>
>
>
> Regards
>

Is it possible that the duplicate keys were the result of 
re-partitioning your data where one of the "older" copies was in the 
wrong partition as part of an upgrade from an earlier version?

See the entry in 
https://dev.mysql.com/doc/relnotes/mysql/5.5/en/news-5-5-31.html that 
start with...
> Incompatible Change; Partitioning: Changes in the KEY partitioning hashing functions used with numeric, date and time, ENUM, and SET columns in MySQL 5.5 makes tables using partitioning or subpartitioning by KEY on any of the affected column types and created on a MySQL 5.5 or later server incompatible with a MySQL 5.1 server. This is because the partition IDs as calculated by a MySQL 5.5 or later server almost certainly differ from those calculated by a MySQL 5.1 server for the same table definition and data as a result of the changes in these functions.

A normal indexed lookup against a partitioned table will use 
(particularly for a PK value) "partition pruning" .  To see all of your 
PK values regardless of which partition they are in, you need to scan 
the table and avoid all indexes.

example:
# for a numeric PK column
CREATE TABLE myPK_list SELECT pk from sourcetable WHERE pk+0 > 0;

Then you can check the list in the generated table to find any duplicate 
values.

Then you can modify a SELECT command to search each partition or 
subpartition individually until you find the rows that are in the wrong 
spots.
https://dev.mysql.com/doc/refman/5.7/en/partitioning-selection.html


Yours,
-- 
Shawn Green
MySQL Senior Principal Technical Support Engineer
Oracle USA, Inc. - Integrated Cloud Applications & Platform Services
Office: Blountville, TN

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