count(*) v. count(pk)

From: <brendan_o'brien_at_wrightexpress.com>
Date: Mon, 27 Apr 1998 11:45:19 -0600
Message-ID: <6i2cn9$u1h$1@nnrp1.dejanews.com>

Hi folks.

Saw a post recently where someone recommended doing a count(pk) instead of a count(*) as count(*) always does a full table scan.

Questions:

Is this in fact true (that a count(*) always does a full table scan)?
Isn't counting on the primary key an impossibility if the key is comprised of multiple columns?
Can anyone suggest any other method for quickly retieving total rows from large, multi-column-PK tables?

I tested this theory on a table with 250,000 rows and a one-column PK using both queries, and got no difference in response time. I can think of no way to test this on a table with a 2+ column PK as count() accepts only one argument.

This isn't of prime importance, more to settle an internal debate here.

Thanks,
-Brendan

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