Re: Data warehousing requirements

----- Original Message -----
From: "Gabriele Bartolini" <angusgb(at)tin(dot)it>
To: "Aaron Werman" <awerman2(at)hotmail(dot)com>;
<pgsql-performance(at)postgresql(dot)org>
Sent: Thursday, October 07, 2004 1:07 PM
Subject: Re: [PERFORM] Data warehousing requirements

> At 13.30 07/10/2004, Aaron Werman wrote:
> >Consider how the fact table is going to be used, and review hacking it up
> >based on usage. Fact tables should be fairly narrow, so if there are
extra
> >columns beyond keys and dimension keys consider breaking it into parallel
> >tables (vertical partitioning).
>
> Hmm ... I have only an extra column. Sorry if I ask you to confirm this,
> but practically vertical partitioning allows me to divide a table into 2
> tables (like if I cut them vertically, right?) having the same key. If I
> had 2 extra columns, that could be the case, couldn't it?

Yes - it's splitting a table's columns and copying the PK. If you have only
one column and it's narrow - partitioning becomes harder to justify.

>
> >Horizontal partitioning is your friend; especially if it is large -
consider
> >slicing the data into chunks. If the fact table is date driven it might
be
> >worthwhile to break it into separate tables based on date key. This wins
in
> >reducing the working set of queries and in buffering. If there is a real
> >hotspot, such as current month's activity, you might want to keep a
separate
> >table with just the (most) active data.Static tables of unchanged data
can
> >simplify backups, etc., as well.
>
> In this case, you mean I can chunk data into: "facts_04_08" for the august
> 2004 facts. Is this the case?

Exactly. The problem is when you need to query across the chunks. There was
a discussion here of creating views ala

create view facts as
select * from facts_04_07 where datekey between '01/07/2004' and
'31/07/2004'
union all
select * from facts_04_08 where datekey between '01/08/2004' and
'31/08/2004'
union all
select * from facts_04_09 where datekey between '01/09/2004' and
'30/09/2004'
...

hoping the restrictions would help the planner prune chunks out. Has anyone
tried this?

>
> Otherwise, is it right my point of view that I can get good results by
> using a different approach, based on mixing vertical partitioning and the
> CLUSTER facility of PostgreSQL? Can I vertically partition also dimension
> keys from the fact table or not?

If you can do that, you probably should beyond a star schema. The standard
definition of a star schema is a single very large fact table with very
small dimension tables. The point of a star is that it can be used to
efficiantly restrict results out by merging the dimensional restrictions and
only extracting matches from the fact table. E.g.,

select
count(*)
from
people_fact, /* 270M */
states_dim, /* only 50 something */
gender_dim, /* 2 */
age_dim /* say 115 */
where
age_dim.age > 65
and
gender_dim.gender = 'F'
and
states_dim.state_code in ('PR', 'ME')
and
age_dim.age_key = people_fact.age_key
and
gender_dim.gender_key = people_fact.gender_key
and
states_dim.state_key = people_fact.state_key

(I had to write out this trivial query because most DBAs don't realize going
in how ugly star queries are.) If you split the fact table so ages were in a
vertical partition you would optimize queries which didn't use the age data,
but if you needed the age data, you would have to join two large tables -
which is not a star query.

What you're thinking about on the cluster front is fun. You can split groups
of dimension keys off to seperate vertical partitions, but you can only
cluster each on a single key. So you need to split each one off, which
results in your inventing the index! (-:

>
> However, this subject is awesome and interesting. Far out ... data
> warehousing seems to be really continous modeling, doesn't it! :-)
>
> >Consider summary tables if you know what type of queries you'll hit.
>
> At this stage, I can't predict it yet. But of course I need some sort of
> summary. I will keep it in mind.
>
> >Especially here, MVCC is not your friend because it has extra work to do
for
> >aggregate functions.
>
> Why does it have extra work? Do you mind being more precise, Aaron? It is
> really interesting. (thanks)

The standard reasons - that a lot of queries that seem intuitively to be
resolvable statically or through indices have to walk the data to find
current versions. Keeping aggregates (especially if you can allow them to be
slightly stale) can reduce lots of reads. A big goal of horizontal
partitioning is to give the planner some way of reducing the query scope.

>
> >Cluster helps if you bulk load.
>
> Is it maybe because I can update or build them once the load operation has
> finished?

If you have streaming loads, clustering can be a pain to implement well.

>
> >In most warehouses, the data is downstream data from existing operational
> >systems.
>
> That's my case too.
>
> >Because of that you're not able to use database features to
> >preserve integrity. In most cases, the data goes through an
> >extract/transform/load process - and the output is considered acceptable.
> >So, no RI is correct for star or snowflake design. Pretty much no
anything
> >else that adds intelligence - no triggers, no objects, no constraints of
any
> >sort. Many designers try hard to avoid nulls.
>
> That's another interesting argument. Again, I had in mind the space
> efficiency principle and I decided to use null IDs for dimension tables if
> I don't have the information. I noticed though that in those cases I can't
> use any index and performances result very poor.
>
> I have a dimension table 'categories' referenced through the 'id_category'
> field in the facts table. I decided to set it to NULL in case I don't have
> any category to associate to it. I believe it is better to set a '0' value
> if I don't have any category, allowing me not to use a "SELECT * from
facts
> where id_category IS NULL" which does not use the INDEX I had previously
> created on that field.

(Sorry for being a pain in the neck, but BTW - that is not a star query; it
should be

SELECT
facts.*
from
facts,
id_dim
where
facts.id_key = id_dim.id_key
and
id_dim.id_category IS NULL

[and it really gets to the whole problem of indexing low cardinality
fields])

>
> >On the hardware side - RAID5 might work here because of the low volume if
> >you can pay the write performance penalty. To size hardware you need to
> >estimate load in terms of transaction type (I usually make bucket
categories
> >of small, medium, and large effort needs) and transaction rate. Then try
to
> >estimate how much CPU and I/O they'll use.
>
> Thank you so much again Aaron. Your contribution has been really important
> to me.
>
> Ciao,
> -Gabriele
>
> >"Let us not speak of them; but look, and pass on."
>
> P.S.: Dante rules ... :-)

:-)

that quote was not a reference to anyone in this group!

Good luck,
/Aaron

>
> --
> Gabriele Bartolini: Web Programmer, ht://Dig & IWA/HWG Member, ht://Check
> maintainer
> Current Location: Prato, Toscana, Italia
> angusgb(at)tin(dot)it | http://www.prato.linux.it/~gbartolini | ICQ#129221447
> > "Leave every hope, ye who enter!", Dante Alighieri, Divine Comedy, The
> Inferno
>