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Displaying posts with tag: 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

optimizer_switch

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

OFF

 . 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

derived_merge

  can fail. Analyzing the problem

In the first run, we turned SQL Mode

ONLY_FULL_GROUP_BY

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

ONLY_FULL_GROUP_BY

  were affected.

A quick search in the …

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ANALYZE TABLE Is No Longer a Blocking Operation

In this post, I’ll discuss the fix for lp:1704195 (migrated to PS-2503), which prevents

ANALYZE TABLE

 from blocking all subsequent queries on the same table.

In November 2017, Percona released a fix for lp:1704195 (migrated to PS-2503), created by Laurynas Biveinis. The fix, included with Percona Server for MySQL since versions 5.6.38-83.0 and 5.7.20-18, stops

ANALYZE TABLE …
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Histogram statistics in MySQL

As of MySQL 8.0.3, you now have the ability to create histogram statistics in order to provide more statistics to the optimizer. In this blog post, we will have a look at how you can create histogram statistics, and we will explain when it might be useful to have histogram statistics.…

When order of appearance of indexes matters in MySQL

Sometimes MySQL surprises you in ways you would have never imagined.

Would you think that the order in which the indexes appear in a table matters?
It does. Mind you, not the order of the columns - the order of the indexes.
MySQL optimizer can, in specific circumstances, take different paths, sometimes with nefarious effects.


Please consider the following table:

CREATE TABLE `mypartitionedtable ` (
  `HASH_ID` char(64) NOT NULL,
  `RAW_DATA` mediumblob NOT NULL,
  `EXPIRE_DATE` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  KEY `EXPIRE_DATE_IX` (`EXPIRE_DATE`),
  KEY `HASH_ID_IX` (`HASH_ID`)
) ENGINE=TokuDB DEFAULT CHARSET=latin1 ROW_FORMAT=TOKUDB_UNCOMPRESSED
/*!50100 PARTITION BY RANGE (UNIX_TIMESTAMP(EXPIRE_DATE))
(PARTITION p2005 VALUES LESS THAN (1487847600) ENGINE = …

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Percona Live Featured Tutorial with Morgan Tocker — MySQL 8.0 Optimizer Guide

Welcome to another post in the series of Percona Live featured tutorial speakers blogs! In these blogs, we’ll highlight some of the tutorial speakers that will be at this year’s Percona Live conference. We’ll also discuss how these tutorials can help you improve your database environment. Make sure to read to the end to get a special Percona Live 2017 registration bonus!

In this Percona Live featured tutorial, we’ll meet Morgan Tocker, MySQL Product Manager at Oracle. His tutorial is a MySQL 8.0 Optimizer Guide. Many users who follow MySQL development are aware that recent versions introduced a number of improvements to query execution (via the addition of …

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MySQL 8.0 Labs: [Recursive] Common Table Expressions in MySQL (CTEs), Part Three – hierarchies

Here is the third in a series of posts about CTEs, a new feature of MySQL 8.0, available in this Labs release. In the first post we had explored the new SQL syntax, and in the second we had applied it to generating series.…

MySQL 8.0 Labs – Descending Indexes in MySQL

Starting with the 8.0 optimizer labs release the MySQL server now supports descending indexes. As I will detail in this post, this new feature can be used to eliminate the need for sorting results, and lead to performance improvements in a number of queries.…

Percona Live Europe 2016: “MySQL 8.0: what’s new in Optimizer” with Manyi Lu

Today was the first day of sessions at Percona Live Europe 2016, and it was packed with exciting talks and important information on open source databases. Some of the most anticipated talks are those that cover what to expect in MySQL 8.0.

One of those talks was given by Oracle’s Manyi Lu, Director of Software Development. She discussed MySQL 8.0: what’s new in Optimizer.

In her talk, Manyi discussed what we could look forward to in MySQL 8.0’s optimizer. There are substantial improvements in the optimizer in MySQL 5.7 and MySQL 8.0. Most noticeably, users can now combine relational data with NoSQL using the new JSON features. MySQL also now supports functional indexes through generated columns. …

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MySQL 8.0: Faster batch record retrieval

One of the things we have been working on for MySQL 8, is speeding up scans that read multiple records from a table or an index. Such scans benefit from reading the records in batches, so that they don’t get the overhead of latching data pages and navigating in the B-tree for every single record that is read.…

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 

pt-online-schema-change

 and 

pt-archive

  copy data from a table?

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

REPEATABLE READ

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