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Displaying posts with tag: order by (reset)
MySQL 8.0: Descending Indexes Can Speed Up Your Queries

In this blog, we’ll discuss descending indexes in MySQL 8.0.

Summary

The future MySQL 8.0 will (probably) have a great new feature: support for index sort order on disk (i.e., indexes can be physically sorted in descending order). In the MySQL 8.0 Labs release (new optimizer preview), when you create an index you can specify the order “asc” or “desc”, and it will be supported (for B-Tree indexes). That can be especially helpful for queries like “SELECT … ORDER BY event_date DESC, name ASC LIMIT 10″ (ORDER BY clause with ASC and DESC sort).

MySQL 5.6 and 5.7 Index Order

Actually, the support for this syntax ( …

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on ORDER BY optimization | Domas Mituzas

http://dom.as/2015/07/30/on-order-by-optimization/

An insightful exploration by Domas (Facebook) on how some of the MySQL optimiser’s decision logic is sometimes naive, in this case regarding ORDER BY optimisation.

Quite often, “simple” logic can work better than complex logic as chasing all the corner cases can just make things worse – but sometimes, logic can be too simple.

Everything must be made as simple as possible, but no simpler.
— Albert Einstein / Roger Sessions

Bulk insert into tables in sorted order to avoid deadlocks

Shard-Query inserts data into a “coordinator” table when answering queries.   When there is a GROUP BY on the original query, the coordinator table contains a UNIQUE KEY over the GROUP BY attributes.   Shard-Query uses INSERT .. ON DUPLICATE KEY UPDATE in combination with bulk insert (insert into … values (),(),() ) when inserting into the table.

For what would normally be efficiency sake, Shard-Query sends queries to the shards using ORDER BY NULL which disables the filesort operation. Of course, this often results in the rows being sent back from the shards in random order.

Because the results are in random order, the bulk insertion that the worker does into the coordinator table can deadlock with other worker threads when using InnoDB or TokuDB as the coordinator table. Right now I’ve just been using MyISAM for the coordinator table, which serializes queries at the bulk insert stage.  Having to insert the …

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Non-Deterministic Query in Replication Stream

You might find a warning like the below in your error log:

130522 17:54:18 [Warning] Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statements writing to a table with an auto-increment column after selecting from another table are unsafe because the order in which rows are retrieved determines what (if any) rows will be written. This order cannot be predicted and may differ on master and the slave.
Statement: INSERT INTO tbl2 SELECT * FROM tbl1 WHERE col IN (417,523)

What do MariaDB and MySQL mean with this warning? The server can’t guarantee that this exact query, with STATEMENT based replication, will always yield identical results on the slave.

Does that mean that you have to use ROW based (or MIXED) replication? Possibly, but not necessarily.

For this type of query, it primarily refers to the fact that without …

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How to select random rows in MySQL

The easiest way to generate random rows in MySQL is to use the ORDER BY RAND() clause. SELECT col1 FROM tbl ORDER BY RAND() LIMIT 10; This can work fine for small tables. However, for big table, it will have a serious performance problem as in order to generate the list of random rows, MySQL […]

Multi Direction Sorts and avoiding a file sort

There are two PRIMARY directions to sort data in SQL: Ascending (ASC) and Descending DESC.
When these two sort definitions are put together in a single statement a filesort is produced.

Why do we want to avoid filesorts?

Filesorts are bad. 1st they tickle a thread based buffer called sort_buffer_size. Additionally filesorts reads the data twice, unless max_length_for_sort_data limit is reached and as a result the Filesort runs slower to reduce disk I/O. If you want filesorts to run faster at the expense of the disk increase the default max_length_for_sort_data. You can read the filesort algorithm here.

So, here is an example


CREATE TABLE `ABCD` (
`A` int(10) unsigned NOT NULL default '0',
`B` int(10) unsigned NOT NULL default '0',
`C` int(10) unsigned NOT NULL …
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Old challenges, new synthax ...

I've blogged before about the new SQL synthax which is becoming available in databases and how it helps solving questions which are increasingly common.Now it's time for another example, something which doesn't come up often in a reporting environment as most tools have this feature, but can be a problem if you're building your output with a scipting language.Adding a "Total" row at the end of

How to pick indexes for order by and group by queries

First some of the things that you need to use and understand

Explain Syntax

Order by Optimization

Group by Optimization

Update: Updated errors.

Now some details that are usually missed. GROUP BY does sorting unless you tell mysql not to. GROUP BY has two optimization methods, loose index scan, and tight index scan.

Loose index scan, scans the entire table index, while tight index scan uses some sort of constraint. For large datasets that are accessed often and require some sort of group by, tight index scans are better.


So how to pick columns to create …

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Showing entries 1 to 8