A primary key automatically creates an index and not-null and unique
constraints, too. So you don't need to explicitly create an index on
a primary key column.
If your queries are going to have WHERE clauses (as they most likely
are) I'm not sure how the small-index suggestion would make the query
any faster - the WHERE clause would preclude the use of that index in
computing the rows - but I'm probably missing something here.
When you say that you need to know the number of rows returned before
executing the query, do you mean before you start getting rows back
or before you actually execute the query? I don't think it's
possible to know how many rows the query will return without actually
executing it, but you might well want to know how many rows you get
before you start processing rows.
Have you looked at the SQL_CALC_FOUND_ROWS option on SELECT, and the
accompanying FOUND_ROWS() function? http://dev.mysql.com/doc/refman/
It will tell you the total number of rows which would have been
found if you hadn't used a LIMIT clause. I think it is a connection-
specific function; if you created a second statement handle and did a
SELECT FOUND_ROWS() on the same connection, perhaps that would give
what you want.
On Sep 6, 2006, at 11:29 PM, <prasad.ramisetti@stripped>
> Hi Dan,
> Thanks for yur response. Does it makes sense to create an index on a
> primary key ..as that is my smallest field ?
> -----Original Message-----
> From: Dan Nelson [mailto:dnelson@stripped]
> Sent: Monday, September 04, 2006 9:53 AM
> To: Prasad Ramisetti (WT01 - Broadband Networks)
> Cc: dmagick@stripped; mysql@stripped
> Subject: Re: problem with InnoDB
> In the last episode (Sep 04), prasad.ramisetti@stripped said:
>> Actually there is some requirement, where I need to know the
>> number of
>> rows that I will get for my queries before actually executing the
>> query. Could you please suggest some way for this.
> Your best bet is to create an index on the smallest column you can
> (maybe even create a char(1) and leave it empty), and "SELECT COUNT(*)
> FROM innotable USE INDEX (smallcolumn)". That way mysql only has to
> scan a small secondary index instead of the main table index.
> Dan Nelson
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