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Re: HASH-JOIN

From: Alan Burlison <alanburlison_at_unn.unisys.com>
Date: 1997/01/26
Message-ID: <32EB9168.1C35@unn.unisys.com>#1/1

> If I remember correctly: the first pass builds a hash table
> for relation A on values of the join attribute, say, A.1. The
> second pass then scans the second relation B and applies the same
> hashing function to join attribute B.1. When B.1 hits A.1 and
> a corresponding check reveals that A.1 equals B.1, the joined
> rows are generated. It's faster than a merge since nothing need
> be sorted.

Yup, thats right. Hash join works best where you are joining a little table to a big table - ideally the hash should be built against the little table, and should fit into memory for best performance. I think has join is really targetted at Decision Support applications

-- 
Alan Burlison
alanburlison_at_unn.unisys.com

My opinions may be incorrect, but they are my own.
Received on Sun Jan 26 1997 - 00:00:00 CST

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