MySQL JSON – How to Remove String Quotes in MySQL 5.7 for JSON_EXTRACT

This isn't a full answer but you might try looking over the 5.5 change list starting at http://dev.mysql.com/doc/refman/5.5/en/news-5-5-0.html

Note the section nav on the right that goes subversion by subversion. Skim through each looking at "incompatible changes". If you find something describing what you are seeing it will at least give you better context of what is happening and why.

Create a Subquery that Gathers Keys from the the log_table with Maximum Date Per Entity. Then, perform an INNER JOIN of that Subquery back to the log_table.

SELECT
    B.entity_id,B.last_log_date,B.last_comment
FROM
(
    SELECT entity_id,MAX(last_log_date) last_log_date
    FROM log_table GROUP BY entity_id
) A INNER JOIN B USING (entity_id,last_log_date);

Give it a Try !!!

You can really speed this up if you have a compound index like this

ALTER TABLE log_table ADD INDEX entity_date_ndx (entity_id,last_log_date);

Indexes on each separate column may yield index merges. This compound index will bypass that.

Please try LEFT JOIN instead of INNER JOIN

SELECT
    B.entity_id,B.last_log_date,B.last_comment
FROM
(
    SELECT entity_id,MAX(last_log_date) last_log_date
    FROM log_table GROUP BY entity_id
) A LEFT JOIN B USING (entity_id,last_log_date);