Re: Poor hash Join performance

From: Nuno Souto <nsouto_at_nsw.bigpond.net.au.nospam>
Date: Sat, 27 Jan 2001 11:54:34 GMT
Message-ID: <3a72b3e4.1369829@news-server>

On Fri, 26 Jan 2001 20:38:11 GMT, cshelt_at_acxiom.com wrote:

>while Hash joins are running insanely slow (1.5 million table and 2.5
>million table was going to take around 60 hours for hash and 1 hour for

In general, hash joins are faster if the selected set of rows in one table is much, much smaller than in the other table. When that is not the case, you're better off using merge joins.

Note that this is the subset of rows to be joined, not the number of rows in each table. And it is a rule of thumb, there are always special cases.

In your case, I'm inclined to think you're selecting nearly the same number of rows from each table, in which case the merge join will be much more efficient.

Cheers
Nuno Souto
nsouto_at_bigpond.net.au.nospam
http://www.users.bigpond.net.au/the_Den/index.html Received on Sat Jan 27 2001 - 05:54:34 CST