Looking Inside the MySQL 5.7 Document Store

Document Store

MySQL 5.7.12 is a major new release, as it contains quite a number of new features:

1. Document store and "MongoDB" like NoSQL interface to JSON storage
2. Protocol X / X Plugin, which can be used for asynchronous queries (I will write about it as well)
3. New MySQL shell

Peter already wrote the document store overview; in this post, I will look deeper into the document store implementation. In my next post, I will demonstrate how to use document store for Internet of Things (IoT) and event logging.

Older MySQL 5.7 versions already have a JSON data type, and an ability to create virtual columns that can be indexed. The new document store feature is based on the JSON datatype.

So, what is the document store anyway? It is an add-on to a normal MySQL table with a JSON field. Let’s take a deep dive into it and see how it works.

First of all: one can interface with the document store’s collections using the X Plugin (default port: 33060). To do that:

1. Enable X Plugin and install MySQL shell.

2. Login to a shell:

mysqlsh--uri root@localhost

3. Run commands (JavaScript mode, can be switched to SQL or Python):

mysqlsh--uri root@localhost

Creating anXSession toroot@localhost:33060

Enter password:

No defaultschema selected.

Welcome toMySQL Shell1.0.3Development Preview

Copyright(c)2016,Oracle and/orits affiliates.All rights reserved.

Oracle isaregistered trademark of Oracle Corporation and/orits

affiliates.Other names may be trademarks of their respective

owners.

Type'help','h'or'?'forhelp.

Currently inJavaScript mode.Usesql toswitchtoSQL mode andexecute queries.

mysql-js>db=session.getSchema('world_x')<Schema:world_x>

mysql-js>db.getCollections()

"CountryInfo":<Collection:CountryInfo>

Now, how is the document store’s collection different from a normal table? To find out, I’ve connected to a normal MySQL shell:

mysqlworld_x

Readingtableinformationforcompletionoftableandcolumnnames

Youcanturnoffthisfeaturetogetaquickerstartupwith-A

WelcometotheMySQLmonitor.Commandsendwith;org.

YourMySQLconnectionidis2396

Serverversion:5.7.12MySQLCommunityServer(GPL)

Copyright(c)2000,2016,Oracleand/oritsaffiliates.Allrightsreserved.

OracleisaregisteredtrademarkofOracleCorporationand/orits

affiliates.Othernamesmaybetrademarksoftheirrespective

owners.

Type'help;'or'h'forhelp.Type'c'toclearthecurrentinputstatement.

mysql>showcreatetableCountryInfo

***************************1.row***************************

Table:CountryInfo

CreateTable:CREATETABLE`CountryInfo`(

`doc`jsonDEFAULTNULL,

`_id`varchar(32)GENERATEDALWAYSAS(json_unquote(json_extract(`doc`,'$._id')))STOREDNOT NULL,

PRIMARY KEY(`_id`)

)ENGINE=InnoDBDEFAULTCHARSET=utf8

1rowinset(0.00sec)

mysql>showtables;

+-------------------+

|Tables_in_world_x|

+-------------------+

|City|

|Country|

|CountryInfo|

|CountryLanguage|

+-------------------+

4rowsinset(0.00sec)

So the document store is actually an InnoDB table with one field: doc json + Primary key, which is a generated column.

As we can also see, there are four tables in the world_x database, but db.getCollections() only shows one. So how does MySQL distinguish between a “normal” table and a “document store” table? To find out, we can enable the general query log and see which query is being executed:

$mysql-e'set global general_log=1'

$tail/var/log/general.log

2016-05-17T20:53:12.772114Z186Query  SELECT table_name,COUNT(table_name)cFROM information_schema.columnsWHERE((column_name='doc'anddata_type='json')OR(column_name='_id'andgeneration_expression='json_unquote(json_extract(`doc`,''$._id''))'))ANDtable_schema='world_x'GROUP BY table_name HAVINGc=2

2016-05-17T20:53:12.773834Z186Query  SHOW FULL TABLES FROM`world_x`

As you can see, every table that has a specific structure (doc JSON or specific generation_expression) is considered to be a JSON store. Now, how does MySQL translate the .find or .add constructs to actual MySQL queries? Let’s run a sample query:

mysql-js>db.getCollection("CountryInfo").find('Name= "United States"').limit(1)

"GNP":8510700,

"IndepYear":1776,

"Name":"United States",

"_id":"USA",

"demographics":{

"LifeExpectancy":77.0999984741211,

"Population":278357000

"geography":{

"Continent":"North America",

"Region":"North America",

"SurfaceArea":9363520

"government":{

"GovernmentForm":"Federal Republic",

"HeadOfState":"George W. Bush",

"HeadOfState_title":"President"

1documentinset(0.02sec)

And now, look at the slow query log again:

2016-05-17T21:02:21.213899Z186Query  SELECT doc FROM`world_x`.`CountryInfo`WHERE(JSON_EXTRACT(doc,'$.Name')='United States')LIMIT1

We can verify that MySQL translates all document store commands to SQL. That also means that it is 100% transparent to the existing MySQL storage level and will work with other storage engines. Let’s verify that, just for fun:

mysql>alter table CountryInfo engine=MyISAM;

Query OK,239rows affected(0.06sec)

Records:239Duplicates:0Warnings:0

mysql-js>db.getCollection("CountryInfo").find('Name= "United States"').limit(1)

"GNP":8510700,

"IndepYear":1776,

"Name":"United States",

"_id":"USA",

"demographics":{

"LifeExpectancy":77.0999984741211,

"Population":278357000

"geography":{

"Continent":"North America",

"Region":"North America",

"SurfaceArea":9363520

"government":{

"GovernmentForm":"Federal Republic",

"HeadOfState":"George W. Bush",

"HeadOfState_title":"President"

1document inset(0.00sec)

2016-05-17T21:09:21.074726Z2399Query  alter table CountryInfo engine=MyISAM

2016-05-17T21:09:41.037575Z2399Quit

2016-05-17T21:09:43.014209Z186Query  SELECT doc FROM`world_x`.`CountryInfo`WHERE(JSON_EXTRACT(doc,'$.Name')='United States')LIMIT1

Worked fine!

Now, how about the performance? We can simply take the SQL query and run explain:

mysql>explain SELECT doc FROM`world_x`.`CountryInfo`WHERE(JSON_EXTRACT(doc,'$.Name')='United States')LIMIT1

***************************1.row***************************

id:1

select_type:SIMPLE

table:CountryInfo

partitions:NULL

type:ALL

possible_keys:NULL

key:NULL

key_len:NULL

ref:NULL

rows:239

filtered:100.00

Extra:Using where

1row inset,1warning(0.00sec)

Hmm, it looks like it is not using an index. That’s because there is no index on Name. Can we add one? Sure, we can add a virtual column and then index it:

mysql>altertableCountryInfoaddcolumnNamevarchar(255)

->GENERATEDALWAYSAS(json_unquote(json_extract(`doc`,'$.Name')))VIRTUAL;

QueryOK,0rowsaffected(0.12sec)

Records:0Duplicates:0Warnings:0

mysql>altertableCountryInfoaddkey(Name);

QueryOK,0rowsaffected(0.02sec)

Records:0Duplicates:0Warnings:0

mysql>explainSELECTdocFROM`world_x`.`CountryInfo`WHERE(JSON_EXTRACT(doc,'$.Name')='United States')LIMIT1

***************************1.row***************************

id:1

select_type:SIMPLE

table:CountryInfo

partitions:NULL

type:ref

possible_keys:name

key:name

key_len:768

ref:const

rows:1

filtered:100.00

Extra:NULL

1rowinset,1warning(0.00sec)

That is really cool! We have added an index, and now the original query starts using it. Note that we do not have to reference the new field, the MySQL optimizer is smart enough to translate the (JSON_EXTRACT(doc,'$.Name')='United States' to an index scan on the virtual column.

But please note: JSON attributes are case-sensitive. If you will use (doc,'$.name') instead of (doc,'$.Name') it will not generate an error, but will simply break the search and all queries looking for “Name” will return 0 rows.

Finally, if you looked closely at the output of db.getCollection("CountryInfo").find('Name= "United States"').limit(1) , you noticed that the database has outdated info:

"government":{

"GovernmentForm":"Federal Republic",

"HeadOfState":"George W. Bush",

"HeadOfState_title":"President"

Let’s change “George W. Bush” to “Barack Obama” using the .modify clause:

mysql-js>db.CountryInfo.modify("Name = 'United States'").set("government.HeadOfState","Barack Obama");

Query OK,1item affected(0.02sec)

mysql-js>db.CountryInfo.find('Name= "United States"')

"GNP":8510700,

"IndepYear":1776,

"Name":"United States",

"_id":"USA",

"demographics":{

"LifeExpectancy":77.0999984741211,

"Population":278357000

"geography":{

"Continent":"North America",

"Region":"North America",

"SurfaceArea":9363520

"government":{

"GovernmentForm":"Federal Republic",

"HeadOfState":"Barack Obama",

"HeadOfState_title":"President"

1documentinset(0.00sec)

Conclusion

Document store is an interesting concept and a good add-on on top of the existing MySQL JSON feature. Using the new .find/.add/.modify methods instead of the original SQL statements can be convenient in some cases.

Some might ask, “why do you want to use document store and store information in JSON inside the database if it is relational anyway?” Storing data in JSON can be quite useful in some cases, for example:

• You already have a JSON (i.e., from external feeds) and need to store it anyway. Using the JSON datatype will be more convenient and more efficient.
• You have a flexible schema, typical for the Internet of Things for example, where some sensors might only send temperature data, some might send temperature/humidity/light (but light information is only recorded during the day), etc. Storing it in the JSON format can be more convenient so that you do not have to declare all possible fields in advance, and do not have to run “alter table” if a new sensor starts sending new types of data.

In the next two blog posts, I will show how to use document store for Internet of Things / event streaming, and how to use X Protocol for asynchronous queries in MySQL.