Following on from my previous email I have columns containing numbers
which are then used in SUM and MIN/ MAX functions should these be
indexed too ?
On 3 Oct 2010, at 16:44, Joerg Bruehe <joerg.bruehe@stripped> wrote:
> Hi Neil, all!
> Tompkins Neil wrote:
>> So if you have individual indexes for example field_1, field_2 and
>> etc and then perform a search like
>> WHERE field_1 = 10
>> AND field_3 = 'abc'
>> This wouldn't improve the search ? You have to create a index for
>> possible combined field searches ?
> No - you didn't read Gavin's mail exact enough:
>> On Fri, Oct 1, 2010 at 9:35 PM, Gavin Towey <gtowey@stripped> wrote:
>>> Additionally indexes are always read left to right. So an index on
>>> ('user_id', 'product_id') will help when doing WHERE user_id=N AND
>>> product_id IN (1,2,3), but wouldn't help for just the condtion on
> What Gavin calls "left to right" is what I call "most significant
> first", the result is the same:
> In a multi-column index, the columns are listed in the order of their
> significance. Any DBMS (this is not limited to MySQL) can use such an
> index only if a condition for the first (= most significant) field
> (s) is
> (are) specified.
> Example: Assume the index is on fields A, B, and C in that order.
> A statement "... where A = x and B = y and C = z" can use the index.
> A statement "... where A = x and B = y" can use the index, limited to
> the first two fields.
> A statement "... where A = x" can use the index. the first field only.
> A statement "... where A = x and C = z" can also use the index for A,
> but will have to evaluate the condition on C by scanning all records
> matching A.
> A statement "... where B = y and C = z" cannot use the index, because
> there is no condition on A.
> If there are many searches based on A and C only (not B), and there
> many records matching A with different values of C, then an additional
> index on these two columns may be helpful.
> Compare the index with a phone book, which (typically) lists the
> sorted by last name (most significant), then first name, then ... :
> If you don't know the last name, you cannot profit from the sorting
> have to scan the wole book.
>>> See the manual for full details on how mysql uses indexes:
> Joerg Bruehe, MySQL Build Team, joerg.bruehe@stripped
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