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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.

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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.