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Re: Block Nested Loop
Posted by: Peter Brawley
Date: June 15, 2018 11:43AM

Consider reading the docs for Explain at https://dev.mysql.com/doc/refman/5.7/en/explain-extended.html

The opening para says ...

For SELECT statements, the EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. The Message value in SHOW WARNINGS output displays how the optimizer qualifies table and column names in the SELECT statement, what the SELECT looks like after the application of rewriting and optimization rules, and possibly other notes about the optimization process.

Using block nested loop is not a warning, it's just extended query analysis.

Is this problem query running on the system you sought help with in Sept 2017 in the thread https://forums.mysql.com/read.php?22,660420,660420#msg-660420 ? At that time, your system had grossly insufficient RAM for its MySQL buffer settings, insufficient also for the database load, so the system was swapping, thus killing performance.

If that is still the case, that is the most likely reason for the poor query performance you observe.

Otherwise for help with improving the performance of the present problem query, post the query, the complete Explain result inside BBCode code tags, and the result of SHow Create Table on each table referenced by the query.

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