Re: PoC: full merge join on comparison clause

From: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
To: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Jeff Davis <pgsql(at)j-davis(dot)com>
Subject: Re: PoC: full merge join on comparison clause
Date: 2017-09-28 15:27:09
Message-ID: 22906fd3-6f56-2473-9897-21a9532e6da3@postgrespro.ru
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Hi Ashutosh,

Thanks for the review.

*Jeff*, I'm copying you because this is relevant to our discussion about
what to do with mergeopfamilies when adding new merge join types.

> You have renamed RestrictInfo member mergeopfamilies as
> equivopfamilies. I don't think that's a good name; it doesn't convey
> that these are opfamilies containing merge operators. The changes in
> check_mergejoinable() suggest that an operator may act as equality
> operator in few operator families and comparison operator in others.
> That looks odd. Actually an operator family contains operators other
> than equality operators, so you may want to retain this member and add
> a new member to specify whether the clause is an equality clause or
> not.

For mergeopfamilies, I'm not sure what is the best thing to do. I'll try
to explain my understanding of the situation, please correct me if I'm
wrong.

Before the patch, mergeopfamilies was used for two things: creating
equivalence classes and performing merge joins.

For equivalence classes: we look at the restriction clauses, and if they
have mergeopfamilies set, it means that these clause are based on an
equality operator, and the left and right variables must be equal. To
record this fact, we create an equivalence class. The variables might be
equal for one equality operator and not equal for another, so we record
the particular operator families to which our equality operator belongs.

For merge join: we look at the join clauses, and if they have
mergeopfamilies set, it means that these clauses are based on an
equality operator, and we can try performing this particular join as
merge join. These opfamilies are also used beforehand to create the
equivalence classes for left and right variables. The equivalence
classes are used to match the join clauses to pathkeys describing the
ordering of join inputs.

So, if we want to start doing merge joins for operators other than
equality, we still need to record their opfamilies, but only use them
for the second case and not the first. I chose to put these opfamilies
to different variables, and
name the one used for equivalence classes 'equivopfamilies' and the one
used for merge joins 'mergeopfamilies'. The equality operators are used
for both cases, so we put their opfamilies into both of these variables.

I agree this might look confusing. Indeed, we could keep a single
variable for opfamilies, and add separate flags that show how they can
be used, be that for equivalence classes, merge joins, range joins or
some combination of them. This is similar to what Jeff did in his range
merge join patch [1]. I will think more about this and try to produce an
updated patch.

> In mergejoinscansel() you have just removed Assert(op_strategy ==
> BTEqualStrategyNumber); Probably this function is written considering
> on equality operators. But now that we are using this for all other
> operators, we will need more changes to this function. That may be the
> reason why INNER join in your earlier example doesn't choose right
> costing.

I changed mergejoinscansel() slightly to reflect the fact that the inner
relation is scanned from the beginning if we have an inequality merge
clause.

> The comment change in final_cost_mergejoin() needs more work. n1, n2,
> n3 are number of rows on inner side with values 1, 2, 3 resp. So n1 +
> n2 + n3 + ... = size of inner relation is correct. In that context I
> am not able to understand your change
> + * If the merge clauses contain inequality, (n1 + n2 + ...) ~=
> + * (size of inner relation)^2.

I extended the comment in final_cost_mergejoin(). Not sure if that
approximation makes any sense, but this is the best I could think of.

Style problems are fixed.

Attached please find the new version of the patch that addresses all the
review comments except mergeopfamilies.

The current commitfest is ending, but I'd like to continue working on
this patch, so I am moving it to the next one.

[1]
https://www.postgresql.org/message-id/flat/CAMp0ubfwAFFW3O_NgKqpRPmm56M4weTEXjprb2gP_NrDaEC4Eg%40mail(dot)gmail(dot)com#CAMp0ubfwAFFW3O_NgKqpRPmm56M4weTEXjprb2gP_NrDaEC4Eg(at)mail(dot)gmail(dot)com

--
Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

Attachment Content-Type Size
full-merge-join-v3.patch text/x-patch 50.8 KB

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