On 07-01-18 19:09, x wrote:
>> Because reading the whole record (all 3 fields) is more expensive than
>> just reading the index which has all the info you need to give a correct
>> answer on 'SELECT ID FROM TBL WHERE ID IS NULL;'
> Yes, but the covering index has 2 fields (X & ID). The pk has only 1 field 
> (ID) so why not use that, particularly in the case where ‘ORDER BY ID’ was 
> included in the query?
>
>
sorry, i do not know
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