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Date: Tue, 10 May 2005 16:24:01 -0700
From: Mischa Sandberg <mischa.sandberg@telus.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "Jim C. Nasby" <decibel@decibel.org>, Neil Conway <neilc@samurai.com>,
	Christopher Petrilli <petrilli@gmail.com>,
	Ying Lu <ying_lu@cs.concordia.ca>, pgsql-general@postgresql.org,
	pgsql-performance@postgresql.org
Subject: Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL 
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Quoting Tom Lane <tgl@sss.pgh.pa.us>:

> Mischa Sandberg <mischa.sandberg@telus.net> writes:
> > The PG hash join is the simplest possible: build a hash table in
> memory,  and match an input stream against it.
> 
> [ raised eyebrow... ]  Apparently you've not read the code.  It's
> been hybrid hashjoin since we got it from Berkeley.  Probably not the
> best possible implementation of the concept, but we do 
> understand about spill to disk.

Apologies. I stopped reading around line 750 (PG 8.0.1) in
src/backend/executor/nodeHashjoin.c

if (!node->hj_hashdone)
{
    ....
    /*
     * execute the Hash node, to build the hash table
     */
    hashNode->hashtable = hashtable;
    (void) ExecProcNode((PlanState *) hashNode);
    ...

and missed the comment:
    /*
     * Open temp files for outer batches,
     */

Will quietly go and read twice, talk once.