David,
it can't because this field is not the first one in your concat index!
Regards,
Frank.
David BORDAS wrote:
>
> I'm using mysql 3.23.49 on Linux redhat dedicated server.
>
> Here is my query :
>
> mysql> show index from MyTable;
> ############################
> Table : MyTable
> Non_unique : 0
> Key_name : PRIMARY
> Seq_in_index : 1
> Column_name : Field0
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> #############################
> Table : MyTable
> Non_unique : 0
> Key_name : Index1
> Seq_in_index : 1
> Column_name : Field3
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> Table : MyTable
> Non_unique : 0
> Key_name : Index1
> Seq_in_index : 2
> Column_name : Field0
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> #############################
> Table : MyTable
> Non_unique : 0
> Key_name : questions
> Seq_in_index : 1
> Column_name : Field2
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> Table : MyTable
> Non_unique : 0
> Key_name : questions
> Seq_in_index : 2
> Column_name : Field3
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> Table : MyTable
> Non_unique : 0
> Key_name : questions
> Seq_in_index : 3
> Column_name : Field4
> Collation : A
> Cardinality : NULL
> Sub_part : NULL
> Packed : NULL
> Comment : NULL
>
> MyTable info :
> Field0 : int(10) unsigned primary key
> Field1 : int(10) unsigned
> Field2 : int(10) unsigned
> Field3 : int(10) unsigned
> Field4 : datetime Null: Yes default 0000-00-00 00:00:00 ( but this field
> don't have null values )
> Field5 : varchar(50);
>
> mysql> explain SELECT * FROM Forums WHERE Field2=5020 AND Field3=0 ORDER BY
> Field4 DESC LIMIT 0,20;
> **************
> table : MyTable
> type : ref
> possible_keys : Index1,questions
> key : questions
> key_len : 1
> ref : const,const
> rows : 390
> Extra : where used; Using filesort
> 1 row in set (0.00 sec)
>
> We can see that index questions which have Fields 2, 3 and 4 isn't use for
> the order by.
> Why ?
>
> Thanks in advance
> David
>
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