| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ALTER TABLE ... ALTER COLUMN ... SET DISTINCT |
| Date: | 2009-04-06 02:00:36 |
| Message-ID: | 7785.1238983236@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Sun, Apr 5, 2009 at 7:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> [ shrug... ] Precision is not important for this value: we are not
>> anywhere near needing more than six significant digits for our
>> statistical estimates. Range, on the other hand, could be important
>> when dealing with really large tables.
> I thought about that, and if you think that's better, I can implement
> it that way. Personally, I'm unconvinced. The use case for
> specifying a number of distinct values in excess of 2 billion as an
> absolute number rather than as a percentage of the table size seems
> pretty weak to me.
I was more concerned about the other end of it. Your patch sets a
not-too-generous lower bound on the percentage that can be represented ...
regards, tom lane
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