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The speed of JSON_MERGE_PATCH and synonyms depends significantly on the length of values in the merged JSON document, even if the values are not merged with anything. I suppose it is worth checking whether it is an expected effect.
drop table if exists t;
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create table t (f_1 JSON, f_100 JSON, f_10000 JSON) engine=MyISAM;
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insert into t values ('{"x":"a"}',CONCAT('{"x":"',REPEAT('a',100),'"}'),CONCAT('{"x":"',REPEAT('a',10000),'"}'));
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SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_1, '{}' )) from t;
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SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_100, '{}' )) from t;
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SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_10000, '{}' )) from t;
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10.2 067465cd
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MariaDB [test]> SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_1, '{}' )) from t;
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+-------------------------------------------------+
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| BENCHMARK(100000,JSON_MERGE_PATCH( f_1, '{}' )) |
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+-------------------------------------------------+
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1 row in set (0.030 sec)
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MariaDB [test]> SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_100, '{}' )) from t;
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+---------------------------------------------------+
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| BENCHMARK(100000,JSON_MERGE_PATCH( f_100, '{}' )) |
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+---------------------------------------------------+
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1 row in set (0.062 sec)
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MariaDB [test]> SELECT BENCHMARK(100000,JSON_MERGE_PATCH( f_10000, '{}' )) from t;
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+-----------------------------------------------------+
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| BENCHMARK(100000,JSON_MERGE_PATCH( f_10000, '{}' )) |
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+-----------------------------------------------------+
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1 row in set (3.778 sec)
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Reproducible on 10.2-10.6.
MySQL 5.7, 8.0 have the similar effect.
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