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[CFReview] Red-Black Tree

From: Mark Cave-Ayland <mark(dot)cave-ayland(at)siriusit(dot)co(dot)uk>
To: Robert Haas <robertmhaas(at)gmail(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: [CFReview] Red-Black Tree
Date: 2010-01-29 14:00:23
Message-ID: 4B62E9F7.6000002@siriusit.co.uk (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi Robert,

I've also spent some time reviewing this patch since it is a
pre-requisite to the KNNGiST patch. I did have a much more comprehensive
list of suggestions, but it seems you've managed to resolve most of
these with your latest re-write. Please find some more comments inline:

> Here's an edited version, which I've now reviewed more fully.  Some
> more substantive review comments:

Firstly: the re-worked patch that you have posted seems to contain
remnants of at least 2 other patches. I have extracted the rbtree-only 
sections and re-attached to this email.

The patch was tested against git head 124a3cc... and applied without any 
  fuzz or other issues.

> 1. I'm pretty satisfied that the rbtree code is generally sane,
> although I wonder if we should think about putting it in access/common
> rather than utils/misc.  I'm not sure that I have a sufficiently
> clearly defined notion of what each subdirectory is for to draw a
> definitive conclusion on this point; hopefully someone else will weigh
> in.

I'm happy that the code is a reasonable implementation of an RB-Tree, at
least with respect to the link to the related public domain source that
was posted. In terms of location, I think utils/misc is a reasonable
place for it to live since I see it as analogous to the hash table
implementation, i.e. it's a template RB-Tree implementation designed to
be used as required throughout the codebase. backend/access seems to be
the home of index AMs only.

Other code points:

- The new names for the iterator enum seem much better to me - or at
least it helped understand the meaning of the iterator code.

- You correctly picked up on the namespace issue, although I noticed
that you left RED and BLACK as they were. Maybe RBRED and RBBLACK would
be better, though not that there are any colour related defines around
in a database backend to make a name clash probable.

- I found the name of the "appendator" method misleading - perhaps
"updater" would make more sense?

> 2. I'm a little concerned about the performance implications of this
> patch.  Looking at http://www.sai.msu.su/~megera/wiki/2009-04-03, it's
> clear that the patch is a huge win in some cases.  But I'm also
> surprised by how much performance is lost in some of the other cases
> that you tested.  I suspect, on balance, that it's probably still a
> good idea to put this in, but I wonder if you've profiled this at all
> to see where the extra time is going and/or explored possible ways of
> squashing that overhead down a little more.
>
> 3. In ginInsertEntry(), the "else" branch of the if statement really
> looks like magic when you first read it.  I wonder if it would make
> sense to pull the contents of EAAllocate() directly into this
> function, since there's only one call site anyhow.

God yes. This is not a good example of maintainable code - even with 
your comment I struggled for a while to try and figure it out :(  I 
would suggest that this is refactored appropriately before commit.

> I still have not done any performance testing of my own on this code,
> and it probably needs that.  If anyone else has time to step up and
> help with that, it would be much appreciated.  It would be useful to
> have some plain old benchmarks as well as some profiling data as
> mentioned above.

As part of my testing, I attempted to duplicate some of the benchmarks
at http://www.sai.msu.su/~megera/wiki/2009-04-03. Unfortunately I was 
not really able to reproduce the RND (teodor's) dataset, nor the random 
array test as the SQL used to test the implementation was not present on 
the page above.

For each test, I dropped and recreated the database to ensure that any 
autovacuum impact would be the same.


1) Fixed length random & sequential string arrays

SELECT array_to_string(ARRAY(select '' || a || '.' || b from
generate_series(1,50) b), ' ')::tsvector AS i INTO seq FROM
generate_series(1,100000) a;

SELECT array_to_string(ARRAY(select '' || random() from
generate_series(1,50) b), ' ')::tsvector AS i INTO rnd FROM
generate_series(1,100000) a;


Before patch:

test=# create index seq_idx on seq using gin (i);
CREATE INDEX
Time: 103205.790 ms
test=# create index rnd_idx on rnd using gin (i);
CREATE INDEX
Time: 6770.131 ms


After patch:

test=# create index seq_idx on seq using gin (i);
CREATE INDEX
Time: 87724.953 ms
test=# create index rnd_idx on rnd using gin (i);
CREATE INDEX
Time: 5596.666 ms


2) Identical records, variable length test

select ARRAY(select generate_series(1,len)) as a50  into arr50 from 
generate_series(1,100000) b;


Before patch:

i) len=3

select ARRAY(select generate_series(1,3)) as a3 into arr3 from 
generate_series(1,100000) b;

test=# create index arr3_idx on arr3 using gin (a3);
CREATE INDEX
Time: 324.251 ms


ii) len=30

select ARRAY(select generate_series(1,30)) as a30 into arr30 from 
generate_series(1,100000) b;

test=# create index arr30_idx on arr30 using gin (a30);
CREATE INDEX
Time: 2163.786 ms


iii) len=50

select ARRAY(select generate_series(1,50)) as a50 into arr50 from 
generate_series(1,100000) b;

test=# create index arr50_idx on arr50 using gin (a50);
CREATE INDEX
Time: 3306.074 ms


iv) len=random

select ARRAY(select generate_series(1, (random() * 100)::int)) as arand 
into arrrand from generate_series(1,100000) b;

test=# create index arrrand_idx on arrrand using gin (arand);
CREATE INDEX
Time: 4725.556 ms


After patch:

i) len=3

select ARRAY(select generate_series(1,3)) as a3 into arr3 from 
generate_series(1,100000) b;

test=# create index arr3_idx on arr3 using gin (a3);
CREATE INDEX
Time: 299.090 ms


ii) len=30

select ARRAY(select generate_series(1,30)) as a30 into arr30 from 
generate_series(1,100000) b;

test=# create index arr30_idx on arr30 using gin (a30);
CREATE INDEX
Time: 2828.424 ms


iii) len=50

select ARRAY(select generate_series(1,50)) as a50 into arr50 from 
generate_series(1,100000) b;

test=# create index arr50_idx on arr50 using gin (a50);
CREATE INDEX
Time: 3984.456 ms


iv) len=random

select ARRAY(select generate_series(1, (random() * 100)::int)) as arand 
into arrrand from generate_series(1,100000) b;

test=# create index arrrand_idx on arrrand using gin (arand);
CREATE INDEX
Time: 3514.972 ms


Summary
=======

I believe Robert has done a good deal of the groundwork required to get 
this patch ready for inclusion. With the current version, I was able to 
see a measurable performance improvement in some test cases with no 
significant performance regression. It would have been nice to be able 
to reproduce the whole set of test cases but this was not possible from 
the information specified.

With perhaps some minor tweaks to some of the names and a rework of the 
else clause in ginInsertEntry(), I feel this patch is reasonably close 
to commit.

I shall now continue my review of the associated KNNGiST patch.


ATB,

Mark.

-- 
Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
http://www.siriusit.co.uk
t: +44 870 608 0063

Sirius Labs: http://www.siriusit.co.uk/labs


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