Indexing needs to be sped up - mysql

Question

I have a table with the following details:
CREATE TABLE `test` (
`seenDate` datetime NOT NULL DEFAULT '0001-01-01 00:00:00',
`corrected_test` varchar(45) DEFAULT NULL,
`corrected_timestamp` timestamp NULL DEFAULT NULL,
`unable_to_correct` tinyint(1) DEFAULT '0',
`fk_zone_for_correction` int(11) DEFAULT NULL,
PRIMARY KEY (`sightinguid`),
KEY `corrected_test` (`corrected_test`),
KEY `idx_seenDate` (`seenDate`),
KEY `idx_corrected_test_seenDate` (`corrected_test`,`seenDate`),
KEY `zone_for_correction_fk_idx` (`fk_zone_for_correction`),
KEY `idx_corrected_test_zone` (`fk_zone_for_correction`,`corrected_test`,`seenDate`),
CONSTRAINT `zone_for_correction_fk` FOREIGN KEY (`fk_zone_for_correction`) REFERENCES `zone_test` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I am then using the following query:
SELECT
*
FROM
test
WHERE
fk_zone_for_correction = 1
AND (unable_to_correct = 0
OR unable_to_correct IS NULL)
AND (corrected_test = ''
OR corrected_test IS NULL)
AND (last_accessed_timestamp IS NULL
OR last_accessed_timestamp < (NOW() - INTERVAL 30 MINUTE))
ORDER BY seenDate ASC
LIMIT 1
Here is a screenshot of the optimiser - the ORDER BY is slowing things down, and in my opinion seems to be indexed properly, and the correct index (idx_corrected_test_zone) is being selected. What can be done to improve it?

Answer 1

There is no INDEX that will help much.
This might help:
INDEX(fk_zone_for_correction, seenDate)
Both columns can perhaps be used -- the first for filtering, the second for avoiding having to sort. But, it could backfire if it can't find the 1 row quickly.
The killer is OR. If you could avoid ever populating any of those 3 columns with NULL, then this might be better:
INDEX(fk_zone_for_correction, unable_to_correct, corrected_test, last_accessed_timestamp)
-- the range thing needs to be last
-- this index would do the filtering, but fail to help with `ORDER` and `LIMIT`.
Even though it is using idx_corrected_test_zone, it is probably not using more than the first two columns -- because of OR.
You have two cases of redundant indexes. For example, the first of these is the left part of the second; so the first is redundant and can be DROPped:
KEY `corrected_test` (`corrected_test`),
KEY `idx_corrected_test_seenDate` (`corrected_test`,`seenDate`),