Dear MariaDB users, (I've asked the same question at https://dba.stackexchange.com/q/332557, so feel free to ignore this one if you've already seen the other.) Our scientific application needs to store and query fundamental parameters for a number of molecules. There are 2 to 28 million rows per molecule, but the number of molecules is expected to stay small (currently 4). Here's our schema: CREATE TABLE `mol_trans` ( `species_id` int(11) DEFAULT NULL, `wl_vac` double DEFAULT NULL, `upper_id` int(11) DEFAULT NULL, `lower_id` int(11) DEFAULT NULL, `prob` double DEFAULT NULL, `flag` tinyint(4) DEFAULT NULL, KEY `spid_flag_wl` (`species_id`,`flag`,`wl_vac`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_general_ci PARTITION BY LIST (`species_id`) (PARTITION `CaO` VALUES IN (6115) ENGINE = InnoDB, PARTITION `CN3` VALUES IN (6121) ENGINE = InnoDB, PARTITION `CN2` VALUES IN (6119) ENGINE = InnoDB, PARTITION `AlO` VALUES IN (6109) ENGINE = InnoDB) (The partitions are here to make it easier to drop a whole molecule if needed, which would otherwise be a painful large DELETE.) I'm using the admittedly old MariaDB 10.3.39 from Debian Buster for tests (connecting via the UNIX domain socket using the command line client), but we've been seeing the same problem on MySQL 5.6 and MariaDB 10.11 on Windows 10. The following query currently takes 1m 7s on my computer, as measured by running `time echo "$QUERY" | mysql "$DATABASE" >/dev/null`: select mtr.prob, mtr.lower_id, mtr.upper_id from mol_trans mtr where ( mtr.species_id=6115 and mtr.wl_vac > 766.0 and mtr.wl_vac < 883.0 and mtr.flag = 1 ) order by mtr.wl_vac; It needs to return 3024559 rows. The query seems to be using the right index, judging by the EXPLAIN output: id: 1 select_type: SIMPLE table: mtr type: ref possible_keys: spid_flag_wl key: spid_flag_wl key_len: 7 ref: const,const rows: 14158123 Extra: Using where With SET PROFILING=ON and the query cache disabled, the single operation with a duration of >1 second in the 23-row profile is "Sending data": Status: Sending data Duration: 66.712277 CPU_user: 66.436978 CPU_system: 0.279932 Context_voluntary: 2156 Context_involuntary: 6964 Block_ops_out: 0 Messages_sent: 0 Messages_received: 0 Page_faults_major: 0 Page_faults_minor: 0 Swaps: 0 Source_function: <unknown> Source_file: sql_parse.cc Source_line: 6086 Is there a way to spend less time sending data? I know the query can in theory run faster because it takes less than 10 seconds on SQLite and PostgreSQL, but if we switched our application from database engine to database engine every time we started having performance problems, we would be very far from done by now. I also know that MariaDB can be very fast at transferring data because it takes me only a few minutes more to download the whole database dump. -- Best regards, Ivan