We have a table called Feedback as follows :
id - int(10) unsigned NO PRI auto_increment vendor_Id - int(10) unsigned YES MUL created_At - datetime CURRENT_TIMESTAMP updated_At - datetime(3) code - int(11) YES feedback - varchar(300) YES
and we need to create index for our Feedback Table.
So we added 3 indexes based on fields as part of where clause :
index1 (vendor_Id, updated_At); codeIndex (vendor_Id, updated_At, code) feedbackIndex(vendor_Id, code, feedback, updated_At)
Now when we run the following query :
explain select * from Feedback where vendor_Id = 1 and updated_At < '2017-11-15 12:58:12.387' and updated_At >= '2017-11-01 00:00:00.000' and code is not null and code >=0 and code <= 6 order by updated_At DESC limit 10
the mysql output says :
possible_keys : index1,codeIndex,feedbackIndex keys : feedbackIndex
Am curious to know why the explain statement shows ‘feedbackIndex’ as the key selected. We were expecting it to select ‘codeIndex’
There are other 2 queries :
explain select code, count(*) as count from Feedback where vendor_Id = 1 and updated_At < '2017-11-15 12:58:12.387' and updated_At >= '2017-11-01 00:00:00.000' and code is not null group by code
===> shows ‘codeIndex’ as key in output of explain statement. which is as expected
explain select code, count(*) as count from Feedback where vendor_Id = 1 and updated_At < '2017-11-15 12:58:12.387' and updated_At >= '2017-11-01 00:00:00.000' and code is not null and feedback is not null group by code`
===> shows ‘feedbackIndex’ as key in output of explain statement. which is as expected
I can not figure out why our first query is showing feedbackIndex as the keys.
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