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InnoDB not doing Index Merge
CREATE TABLE values (
    id int(10) auto_increment NOT NULL,
    molecule_id int(5) NOT NULL,
    descriptor_id int(5) NOT NULL,
    T double DEFAULT NULL,
    value double NOT NULL,
    PRIMARY KEY (id),
    KEY index1 (molecule_id, T),
    KEY index2 (descriptor_id, T)
) ENGINE=InnoDB;

SELECT T, value FROM values WHERE molecule_id = X AND descriptor_id = Y