Max JSON column length in MySQL

Question

What\'s the max number of characters I can store in a JSON column in MySQL? I don\'t see this mentioned in the MySQL manual.

Answer 1

Here's a demo of what @JorgeLondoño is talking about.

Set the server's max allowed packet size:

mysql> set global max_allowed_packet=1024*1024*1024;

Exit and open the mysql client again, this time setting the client max packet size to match:

$ mysql --max-allowed-packet=$((1024*1024*1024*))

Create a test table with a JSON column and fill it with the longest JSON document you can:

mysql> create table test.jtest ( j json );

mysql> insert into test.jtest 
  set j = concat('[', repeat('"word",', 100000000), '"word"]');
Query OK, 1 row affected (1 min 49.67 sec)

mysql> select length(j) from test.jtest;
+-----------+
| length(j) |
+-----------+
| 800000008 |
+-----------+

This shows that I was able to create a single JSON document with 100 million elements, and MySQL stores this in approximately 800MB.

I didn't try a longer document. I assume it maxes out at 1 GB, which is the largest value you can set for max_allowed_packet.

Answer 2

The size of JSON documents stored in JSON columns is limited to the value of the max_allowed_packet system variable. (While the server manipulates a JSON value internally in memory, it can be larger; the limit applies when the server stores it.)

Command-Line Format --max_allowed_packet=#
System Variable Name    max_allowed_packet
Variable Scope  Global, Session
Dynamic Variable    Yes
Permitted Values    Type    integer
Default 4194304
Min Value   1024
Max Value   1073741824