Re: New style of hash join proposal

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: pgsql-hackers list <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New style of hash join proposal
Date: 2008-04-06 23:53:51
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Gregory Stark wrote:
> We currently execute a lot of joins as Nested Loops which would be more
> efficient if we could batch together all the outer keys and execute a single
> inner bitmap index scan for all of them together.
> Essentially what I'm saying is that we're missing a trick with Hash Joins
> which currently require that we can execute the inner side once without any
> parameters from the outer side.
> Instead what we could do is build up the hash table, then scan the hash table
> building up an array of keys and pass them as a parameter to the inner side.
> The inner side could do a bitmap index scan to fetch them all at once and
> start returning them just as normal to the hash join.
> There are a couple details:
> 1) Batched hash joins. Actually I think this would be fairly straightforward.
> You want to rescan the inner side once for each batch. That would actually
> be easier than what we currently do with saving tuples to files and all
> that.
> 2) How to pass the keys. This could be a bit tricky especially for
> multi-column keys. My first thought was to build up an actually Array node
> but that only really works for single-column keys I think. Besides it would
> be more efficient to somehow arrange to pass over a reference to the whole
> hash.
> I fear the real complexity would be (as always) in the planner rather than the
> executor. I haven't really looked into what it would take to arrange this or
> how to decide when to do it.

If the scanning of the inner side is a performance problem, why would we
be choosing a nested loop in the first place, vs. another type of join?

Bruce Momjian <bruce(at)momjian(dot)us>

+ If your life is a hard drive, Christ can be your backup. +

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