Hi Neil, all!
Tompkins Neil wrote:
> So if you have individual indexes for example field_1, field_2 and field_3
> 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 all
> 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 <gto...@ffn.com> 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
>> product_id.
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 are
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 entries
sorted by last name (most significant), then first name, then ... :
If you don't know the last name, you cannot profit from the sorting and
have to scan the wole book.
>>
>> See the manual for full details on how mysql uses indexes:
>> http://dev.mysql.com/doc/refman/5.1/en/mysql-indexes.html
HTH,
Jörg
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