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Displaying posts with tag: MySQL Optimizer (reset)
Why Optimization derived_merge can Break Your Queries

Lately, I worked on several queries which started returning wrong results after upgrading MySQL Server to version 5.7 The reason for the failure was derived merge optimization which is one of the default


  options. Issues were solved, though at the price of performance, when we turned it


 . But, more importantly, we could not predict if any other query would start returning incorrect data, to allow us to fix the application before it was too late. Therefore I tried to find reasons why


  can fail. Analyzing the problem

In the first run, we turned SQL Mode


on, and this removed most of the problematic queries. That said, few of the queries that were successfully working with


  were affected.

A quick search in the …

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Introduction into storage engine troubleshooting: Q & A

In this blog, I will provide answers to the Q & A for the “Introduction into storage engine troubleshooting” webinar.

First, I want to thank everybody for attending the July 14 webinar. The recording and slides for the webinar are available here. Below is the list of your questions that I wasn’t able to answer during the webinar, with responses:

Q: At which isolation level do 




  copy data from a table?

A: Both tools do not change the server’s default transaction isolation level. Use either


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Showing entries 1 to 2