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Jules

From pgsql-hackers-owner+M6154@hub.org Wed Aug 23 14:36:41 2000

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Date: Wed, 23 Aug 2000 18:22:40 +0100 (WEST)

From: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>

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To: Tom Lane <tgl@sss.pgh.pa.us>

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PostgreSQL Hackers list <pgsql-hackers@hub.org>

Subject: Re: [HACKERS] analyze.c

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Status: ORr

On Wed, 23 Aug 2000, Tom Lane wrote:

> > What's the big reason not to do that? I know that

> > there is some code in analyze.c (like comparing) that uses other parts of

> > pg, but that seems to be easily fixed.

>

> Are you proposing not to do any comparisons? It will be interesting to

> see how you can compute a histogram without any idea of equality or

> ordering. But if you want that, then you still need the function-call

> manager as well as the type-specific comparison routines for every

> datatype that you might be asked to operate on (don't forget

> user-defined types here).

I forgot user defined data types :-(, but regarding histograms I think

the code can be made external (at least for testing purposes):

1. I was not suggesting not to do any comparisons, but I think the only

comparison I need is equality, I don't need order as I don't need to

calculate mins or maxs (I just need mins and maxes on frequencies, NOT on

dat itself) to make a histogram.

2. The mapping to text guarantees that I have (PQgetvalue returns

always char* and pg_statistics keeps a "text" anyway) a way of knowing

about equality regardless of type.

But at least anything relating to order has to be in.

> > I'm leaning toward the implementation of end-biased histograms. There is

> > an introductory reference in the IEEE Data Engineering Bulletin, september

> > 1995 (available on microsoft research site).

>

> Sounds interesting. Can you give us an exact URL?

http://www.research.microsoft.com/research/db/debull/default.htm

BTW, you can get access to SIGMOD CDs with lots of goodies for a very low

price (at least in 1999 it was a bargain), check out ACM membership for

sigmod.

I've been reading something about implementation of histograms, and,

AFAIK, in practice histograms is just a cool name for no more than:

1. top ten with frequency for each

2. the same for top ten worse

3. average for the rest

I'm writing code get this info (outside pg for now - for testing

purposes).

Best Regards,

Tiago

PS - again: I'm starting, so, some of my comments can be completly dumb.