What's the fastest way to check if a value exists in a MySQL JSON array?

Question

I have a posts table with a JSON column of an array with IDs. This column's data looks exactly like:

[1, 3, 17, 19] These values are not quoted.

To check if a user should see a post, I simply use JSON_CONTAINS(userlist, '(the user ID)', '$') . However, after many thousands of posts, this is starting to get too slow. I'd like to avoid normalizing this to another table with proper relations for now , so I'm wondering what's the best way to see if a user ID exists in a field like what I have.

Note: this is not exactly a duplicate. My values are straight integers, which is why I seemingly can't use JSON_SEARCH() ?

Answer 1

You really should bite the bullet and normalise, as this operation is only going to get slower. In the meantime, there are a couple of ways you can do this with string operations using LIKE and REGEXP :

select userlist regexp '\\b3\\b' AS got_3,
       userlist regexp '\\b7\\b' AS got_7
from test
;
select userlist like '[3,%' or userlist like '% 3,%' or userlist like '%,3]' AS got_3,
       userlist like '[7,%' or userlist like '% 7,%' or userlist like '%,7]' AS got_7
from test

In both cases for your sample data the output is:

got_3   got_7
1       0

Using LIKE will probably be faster than JSON_CONTAINS , but using REGEXP probably won't. You'd need to benchmark on your server.

If you're using MySQL 8+, then you can use JSON_TABLE :

select *
from test
join json_table(userlist,
                '$[*]' columns (user int path '$')
               ) ul
where ul.user = 3

Again, performance will be dependent on your server.

Demo on db-fiddle

Answer 2

Demo of the MySQL 8.0 multi-valued index:

mysql> create table mytable (id serial primary key, data json);

mysql> insert into mytable set data = '[1, 3, 17, 19]';

mysql> create index i on mytable ((cast(data as unsigned array)));

mysql> explain select * from mytable where 17 member of (data)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: mytable
   partitions: NULL
         type: ref
possible_keys: i
          key: i    <-- see, it is using the index `i`
      key_len: 9
          ref: const
         rows: 1
     filtered: 100.00
        Extra: Using where

It's true that JSON_SEARCH() doesn't work with integer values, but the multi-valued index I defined in the example is indexing integer values, so this works.

Of course the whole task would be simpler if you normalized your table instead of using a JSON array. JSON generally makes queries and optimization harder.