MySQL Support Blogs

This is an aggregated feed of various blogs of Oracle support staff who work and support MySQL customers and users. There is an RSS feed to which you can subscribe.

JSON in MySQL: Keys which do NOT match a value (the more elegant way)

Given a JSON object:
SET @j := '{"BOOSTER": 1, "RETRO": 2, "GUIDANCE": 0, "SURGEON": 1, "RECOVERY": 0}';

How do you find the keys which do NOT contain a value of 0?

A second approach to finding the non-0 values from a JSON object is to turn the keys and values into separate columns of a table. This is cleaner, but a little wordier.

We’ll get the keys in one table:

SELECT ord, keyname 
    keyname VARCHAR(100) PATH '$')
) AS keyTable;
 | ord  | keyname  |
 |    1 | RETRO    |
 |    2 | BOOSTER  |
 |    3 | SURGEON  |
 |    4 | GUIDANCE |
 |    5 | RECOVERY |

And the values in a second table:

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JSON in MySQL: Keys which do NOT match a value

Given a JSON object:
SET @j := '{"BOOSTER": 1, "RETRO": 2, "GUIDANCE": 0, "SURGEON": 1, "RECOVERY": 0}';

How do you find the keys which do NOT contain a value of 0?

One approach is to find the keys which do match 0, and then remove them.

Sounds like JSON_SEARCH and JSON_REMOVE? But there’s a gotcha: JSON_SEARCH works only with strings. Ok, so we REPLACE(@j, ‘0’, ‘”0″‘) – but that doesn’t help, because JSON_REMOVE can’t accept an array of paths like JSON_SEARCH would return; it requires each path as a separate parameter.

Instead, JSON_MERGE_PATCH will suffice. It has the feature that values from the second parameter are preserved only if they are not NULL. So, the approach becomes turning the 0s into NULLs, and then merging that document into an empty one.

SELECT REPLACE(@j, 0, 'null') AS j; -- case matters! null, not NULL …

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How To Monitor MySQL With dim_STAT

Monitoring a database is a critical mission: collecting information over time allows tracking how's behaving certain instance in response to a determined workload. There are several solution on the market, and MySQL offers its own solution in the Enterprise Edition, MySQL Enterprise Monitor (MEM). Today I'd like to remember that there's also another nice solution that is free, and designed by MySQL performance architect, Dimitri Kravtchuk (hence not to be considered as an Oracle MySQL solution, not supported and "as is"). I'm talking about dim_STAT. dim_STAT has a slightly different purpose than MEM as it is specialized to make some low level analysis of the impact of the workload on our MySQL Server instance (e.g. can profile mysqld process), and above all, permits offline metrics captures.

Extensive instructions to setup, …

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Aggregate JSON arrays

Given: a table with JSON arrays

CREATE TABLE t (id int auto_increment primary key, d json);
INSERT INTO t VALUES (1, '["apple", "apple", "orange"]');
INSERT INTO t VALUES (2, '["apple", "banana", "orange", "orange", "orange"]');

The desired output is each row with a count of the unique objects:

| id   | fruitCount                             |
|    1 | {"apple": 2, "orange": 1}              |
|    2 | {"apple": 1, "banana": 1, "orange": 3} |

JSON_TABLE() can transform the array into rows.

SELECT id, fruit
    "$[*]" COLUMNS (
        fruit VARCHAR(100) PATH "$"
) AS dt;

| id | fruit  |
|  1 | apple  |
|  1 | apple  |
|  1 | orange |
|  2 | apple  |
|  2 | banana |
|  2 | orange |
|  2 | orange |
|  2 | orange | …

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Manage InnoDB Cluster using MySQL Shell Extensions

At times, when playing with different InnoDB Clusters for testing (I usually deploy all Group Replication instances on the same host on different ports) I find myself stopping the group and doing operations on every instance (e.g. a static reconfiguration). Or I may need to shutdown all instances at once. Scripting is the usual approach, but in addition, MySQL Shell offers a very nice (and powerful) way to integrate custom scripts into the Shell itself to manage an InnoDB Cluster. This is the purpose of MySQL Shell extensions, to create new custom reports and functions and have the flexibility to manage one or more instances at once. I found particularly practical the new plugin feature, introduced in MySQL Shell 8.0.17, that can aggregate reports and functions under the same umbrella: the …

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MySQL Server Performance Tuning: The Perfect Scalability

When data and concurrency grow, together with queries complexity, a standard configuration does not fit anymore. This content walks through the ways MySQL has to adapt to an ever-increasing amount of data in context of tuning, under the heading “multiplication”:
—Scaling up through partitioning and how partitioning can help manage/archive data
—How to relocate binlog, undo, redo, tablespaces, or disk temporary tables and more on different mount points to take advantage of multiple storages and tackle I/O bottlenecks
—All improvements to parallel slave replication
—A quick OS approach, to verify swapping and affinity tuning take the most out of the machine.

All of this with different approaches to monitor the instance to spot what parts of the system and what queries need to be considered, mainly using:

Sys Schema
— …

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How To Bulk Import Data Into InnoDB Cluster?

If you need to do bulk importing into InnoDB Cluster, it is certainly possible to do so by using any of:

Unfortunately both imports will add load to instances and to channels interconnecting instances: data imported on the primary instance needs to be replicated to the rest of instances. And the bigger the data to import, the higher the load (and this could end up affecting performance). The import operation could be batched to reduce load, and Group Replication allows at least to throttle workload with …

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Window Functions with Unusual Boundaries

Somebody on Freenode wanted this:

   Source            Result
+----+------+    +----+------+
| id | x    |    | id | c    |
+----+------+    +----+------+
|  1 |    1 |    |  1 |    2 |
|  2 |    1 |    |  2 |    2 |
|  3 | NULL |    |  3 | NULL |
|  4 | NULL | -> |  4 | NULL |
|  5 |    1 |    |  5 |    1 |
|  6 | NULL |    |  6 | NULL |
|  7 |    1 |    |  7 |    3 |
|  9 |    1 |    |  9 |    3 |
| 10 |    1 |    | 10 |    3 |
+----+------+    +----+------+

The result uses the NULL values in x as boundaries of windows, and counts the number of rows within each window. I don’t know why anyone wants such a thing; it is not ours to reason why…

Anyway, the point is that you can use arbitrary expressions, even subqueries, to define your window partitions.

    -- Count of rows in windows bound by NULL values in x
      x IS NULL, 
        -- …

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High Availability with MySQL Cluster, Setup From Command Line (II)

In the first delivery of this series of posts, delivered for whom who are interested to understand the basics of MySQL Cluster "by examples", I wrote about installing MySQL Cluster with a python utility called ndb_setup-py, which offers a nice web graphical interface to define and start our brand new cluster.

In this post I will share an example to do everything from scratch and manually, instead. Doing things manually is always the best recommendation to learn everything about processes life cycle through their:

  • Initialization
  • Administration (start/stop/reconfigure)
  • Monitoring (logs/counters/status)
  • Troubleshooting 

The resulting topology I'd like to setup is composed of 2 data nodes, 1 management node and …

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Top 5 Things to Consider to Get Started with MySQL Performance Tuning

Today I'll share a few slides I prepared last year for a presentation delivered at Oracle Open World. This is a quick and easy hands-on lab for fresh MySQL Server DBAs. I chose 5 among the most relevant topics when tuning and scaling a MySQL Server using InnoDB tables.

In particular, in this hands-on, I will talk about:

  • Scaling connections
  • The threads model
  • InnoDB REDO log
  • InnoDB Buffer Pool
  • The Execution plan

MySQL Performance Tuning 101 from Mirko Ortensi

Rate at which MySQL is delivering new features and improvements is impressive, in fact MySQL Server 8 boosts performance in many fields, especially regarding InnoDB REDO logging. Hence while the rest of …

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High Availability with MySQL Cluster, a Quick How-To Guide for Dummies (I)

I have been playing with MySQL Cluster for some years now, and today I'd like to start writing a bit about it, how to set it up, configure, backup and also how to use it, as there's plenty of ways to drive operations towards the Cluster for brutal speed  and concurrency. MySQL Cluster, the open source in memory database from Oracle MySQL, is available for free from MySQL Cluster download page (Community version has GPL license).
But before starting with an overview of installation and setup, if you're new to MySQL Cluster, I would strongly recommend to have a look at this video.

What I find more interesting about MySQL Cluster, is that it is possible to have a setup running on commodity hardware (the bare laptop), as it can be configured to have a minimum footprint in terms of memory and storage requirements. About high availability, MySQL Cluster offers unique …

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There’s a GTID_SUBTRACT function, and the manual shows how to write your own cheap GTID_UNION, but not a GTID_INTERSECT. Fortunately it’s easy to write your own, as it’s just subtracting twice.


What use is it?

SET @slave_executed = '33738f8c-c1a5-11e7-8fc3-0a002700000f:1-681577,

SET @master_executed = '33738f8c-c1a5-11e7-8fc3-0a002700000f:1-730294,

SET @master_purged = '33738f8c-c1a5-11e7-8fc3-0a002700000f:1-681582,

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Raspberry Surveillance System, The Complete Solution

I have shown how to build a basic system based on a Raspberry to capture a picture through a PiCamera and share it on a web page. Now I'd like to share a bit more of my experiments by describing an end-to-end approach to detect movements in the monitored area and alert of a possible intrusion. To the purpose, I'll share what I did setup with basic pieces of code found browsing around the internet. This minimal system is made of three simple elements:

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Raspberry surveillance system, your house goes live on the internet

After leaving my brand new Raspberry 3 on the shelf for months (I admit being an impulsive buyer) I decided to give it some use by setting up a surveillance system with it. Raspberry 3 has more than enough power for easy image processing, and the camera developed for its GPU has nice quality at a very accessible price.

So, in order to start with this project, I got this PiCamera v2. I got that one with InfraRed (IR) filter, as I don't plan to take nightly shots with added IR lighting …

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ERROR 3037 (22023): Invalid GIS data provided to function st_geometryfromtext.

1. Watch the parentheses. It’s:

ST_GeomFromText('POLYGON((outerRing), (innerRing), (innerRing), ...)')

The inner rings are optional. If you have just the outer ring, then it’s still:


and not:


2. Polygons have to start and end at the same point.

3. Watch the commas. Rings are comma-delimited sets of of whitespace-delimited coordinate pairs:

x1 y1, x2 y2, x3 y3, x4 y4

x1 y1 x2 y2 x3 y3
x1, y1, x2, y2, x3, y3, x4, y4
(x1, y1), (x2, y2), (x3, y3)
or other variations on that theme.

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On slave_parallel_workers and the logical clock

How can you tell if a given workload on the master could be replicated with many parallel workers on the slave?

The slave_parallel_type=LOGICAL_CLOCK is an implementation of a Lamport clock. The implementation is described in WL #7165 – including a neat little ASCII-art graphic.

Each event in the binary log is stamped with two values:
– The sequence_number increments for each commit
– The last_committed is the sequence_number which was in effect when this transaction entered the prepare phase.

A slave worker cannot begin doing a transaction until the last_committed value is smaller than the sequence_number of all other running threads.

mysqlbinlog mysql-bin.0000x | grep last_committed

#160118 15:31:34 server id 3  end_log_pos 1527 CRC32 0xcdf6bd8d         GTID    last_committed=0 …

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BIT values cannot be loaded using binary notation (for example, b’011010′).

$ cat test.txt



CREATE TABLE loadTest (b BIT(6));


| Level   | Code | Message                               |
| Warning | 1406 | Data too long for column 'b' at row 1 |
| Warning | 1406 | Data too long for column 'b' at row 2 |

-- Note the wrong values:

SELECT BIN(b) FROM loadTest;
| BIN(b) |
| 111111 |
| 111111 |

TRUNCATE loadTest;

LOAD DATA LOCAL INFILE 'test.txt' INTO TABLE loadTest (@b) 

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Quiz: Drop non-global users

Somebody asked on Freenode. I don’t know why they wanted it. How would you drop all MySQL users who do not have “GRANT ALL ON *.* … WITH GRANT OPTION”? That is, drop any users who have ‘N’ in any of the privilege columns in `mysql`.`user`.

My solution shown below. Did you think of a different approach?

My solution ▼

I used SQL to build SQL which built more SQL. Get the list of ‘priv’ columns from information_schema.columns. Build a query that looks for ‘N’ in any of those columns (it feels funny to search for a constant in a list of fields instead of a field in a list of constants, but it’s perfectly legal). Use the results to build the DROP USER statement.

mysql -BNe "SELECT CONCAT('SELECT CONCAT(''DROP USER '', QUOTE(user), ''@'', QUOTE(Host), '';'') FROM mysql.user WHERE ''N'' IN (', GROUP_CONCAT(column_name), ')') FROM information_schema.columns WHERE table_schema = 'mysql' AND table_name …

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Debugging Large Data with Rewriter

A customer showed that a particular client reported a less-than-helpful error message when it tried to display some meta-data about a table.

I couldn’t repeat the behavior with just a copy of the schema, so I suspected it was because of the size of data in the customer’s server – somebody had used an int where they needed a long.

The customer’s data was quite large – many hundreds of GB – more than …

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Aggregate JSON function in MySQL

There is not yet an equivalent to GROUP_CONCAT that produces a JSON array. (There is in MySQL 8, but that's not GA yet.) Until then, you can hack it together with string functions:

| id   | data   |
|    1 | First  |
|    2 | Second |

SELECT CONCAT('[', GROUP_CONCAT(JSON_OBJECT('id', id, 'value', data) SEPARATOR ', '), ']') AS j FROM t;
| j                                                           |
| [{"id": 1, "value": "First"}, {"id": 2, "value": "Second"}] |

Or you can use all JSON functions but hack the grouping:

         @c := @c + 1 AS c, …

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