Neil Tompkins wrote:
> Thanks for your reply. So should we create individual indexes on each
> field or a multiple column index ??
This question cannot be answered without checking and measuring your
installation. The decision whether to create an index is always an act
- If there is an index, the database server can use it to find data
records by looking up the index, not scanning the base data.
This results in load reduction (both CPU and disk IO) and speeds up
- If there is an index, the database server must maintain it whenever
data are altered (insert/update/delete), in addition to the base data.
This is increased load (both CPU and disk IO) and slows down data
So obviously you want to create only those indexes that are helpful for
query execution: you will never (voluntarily) create an index on a
column which isn't used in search conditions, or whose use is already
provided by other indexes.
Of the remaining candidate indexes, you will never (voluntarily) create
one that provides less gain in searches than it costs in data changes.
With MySQL, AFAIK there is the limitation that on one table only one
index can be used. As a result, the choice of indexes to create depends
on the searches executed by your commands, their relative frequency, and
the frequency of data changes.
To answer your other question: If you run aggregate functions (like
SUM(), MIN(), or MAX()) on all records of a table, their results could
be computed by accessing a matching index only. I don't know whether
MySQL does this, I propose you check that yourself using EXPLAIN.
If you run them on subsets of a table only, an index on that column will
not help in general.
In database implementations, there is the concept of a "covering index":
If you have an index on columns A and B of some table, its contents
(without the base data) would suffice to answer
SELECT SUM(B) WHERE A = x
Again, I don't know whether MySQL does this, and I refer you to EXPLAIN.
Joerg Bruehe, MySQL Build Team, joerg.bruehe@stripped
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