Comments - Query in a MariaDB cluster 10.3.13 of 3 nodes get slow responses in nodes 2 and 3
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You don't give details, but if you have default settings, the server is using storage engines statistics to determine the query plan, which can be unreliable, especially if there are many NULLs. See innodb_stats_method. Also see Histogram-Based Statistics and Engine-independent Table Statistics as alternatives that may help in your use-case.
The problem is why the first node makes a correct query plan and nodes 2 and 3 not, all with the same settings