Question

I currently have a summary table to keep track of my users’ post counts, and I run SELECTs on that table to sort them by counts, like WHERE count > 10, for example. Now I know having an index on columns used in WHERE clauses speeds things up, but since these fields will also be updated quite often, would indexing provide better or worse performance?

Answer 1

If you have a query like

SELECT count(*) as rowcount
FROM table1
GROUP BY name

Then you cannot put an index on count, you need to put an index on the group by field instead.

If you have a field named count

Then putting an index in this query may speed up the query, it may also make no difference at all:

SELECT id, `count`
FROM table1
WHERE `count` > 10

Whether an index on count will speed up the query really depends on what percentage of the rows satisfy the where clause. If it’s more than 30%, MySQL (or any SQL for that matter) will refuse to use an index.
It will just stubbornly insist on doing a full table scan. (i.e. read all rows)
This is because using an index requires reading 2 files (1 index file and then the real table file with the actual data).
If you select a large percentage of rows, reading the extra index file is not worth it and just reading all the rows in order will be faster.

If only a few rows pass the sets, using an index will speed up this query a lot

Know your data

Using explain select will tell you what indexes MySQL has available and which one it picked and (kind of/sort of in a complicated kind of way) why.

See: http://dev.mysql.com/doc/refman/5.0/en/explain.html