Re: Range Merge Join v1

From: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
To: Jeff Davis <pgsql(at)j-davis(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Range Merge Join v1
Date: 2017-08-25 17:19:10
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Hi Jeff,

Fast range joins are very nice to have, thank you for working on this.

I've been doing a somewhat related thing recently, trying to teach the
merge join executor to perform full joins on comparison clauses [1]. I
have some comments after reading your patch:

* At the moment, "mergejoinable clause" and "equality clause" mean the
same thing to the planner, and those clauses are used both to create
equivalence classes and to perform merge joins. If we start performing
merge joins for different kinds of clauses, such as comparison or range
intersection, it makes sense to separate these two meanings. I tried
this in my patch and it didn't require many changes. I use
RestrictInfo.equivopfamilies to build equivalence classes, and
RestrictInfo.mergeopfamilies for mergejoinable clauses.

* Semantically, MJCompare() is a function that determines whether you
should emit a join result or advance one of the pointers. This meaning
is not explicit in the code and is not well encapsulated: the function
returns and int and 'compareNoMatch' flag, and the calling code combines
them in various ways to derive the final result. This meaning can be
made explicit by making MJCompare return enum values {Join, NextInner,
NextOuter}, and putting inside it all the logic that decides whether we
join or advance. ExecMergeJoin looks intimidating already, and I think
this change would help readability. Again, you can find an illustration
in my patch.

I hope you find these points helpful.


Alexander Kuzmenkov
Postgres Professional:
The Russian Postgres Company

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