mysql: index on json array not used when joining another table - mysql

Question

I have 2 tables:
CREATE TABLE `directory` (
`id` bigint NOT NULL,
`datasets` json DEFAULT NULL
PRIMARY KEY (`id`) USING BTREE,
KEY `idx_datasets` ((cast(`datasets` as unsigned array)))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
CREATE TABLE `dataset` (
`id` bigint NOT NULL,
`name` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE,
KEY `idx_name` (`name`,`id`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
The query below uses indexes on the two tables as expected:
explain
SELECT * FROM dataset d inner join `directory` dir
on JSON_CONTAINS(dir.datasets, cast(d.id as json))
where d.id = 111;
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE d const PRIMARY PRIMARY 8 const 1 100.00
1 SIMPLE dir range idx_datasets idx_datasets 9 2 100.00 Using where
However, this query uses index only on the left table
explain
SELECT * FROM dataset d inner join `directory` dir
on JSON_CONTAINS(dir.datasets, cast(d.id as json))
where d.name like '111';
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE d range idx_name idx_name 259 1 100.00 Using index condition
1 SIMPLE dir ALL 1000 100.00 Using where; Using join buffer (hash join)
Could someone explain the difference between the two queries?
I change the condition "like" to "=", the result is the same:
explain
SELECT * FROM dataset d inner join catalog dir
on JSON_CONTAINS(dir.datasets, cast(d.id as json))
where d.name = '111';
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE d ref idx_name idx_name 259 const 1 100.00
1 SIMPLE dir ALL 1000 100.00 Using where; Using join buffer (hash join)

Answer 1

this is caused by the like expression in the WHERE clause in the 2nd query
here is why, explained in some threads:
1- Equals(=) vs. LIKE
2- SQL 'like' vs '=' performance
EDIT looks like the issue here is caused by the fact that in the first query you are searching against the Primary Key, while in the second you dont
More details in this question:
Behavior of WHERE clause on a primary key field