From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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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 03:33:14 |
Message-ID: | 9224.1238988794@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> (It's also worth pointing out that the calculations we do with
> ndistinct are pretty approximations anyway. If the difference between
> stadistinct = -1 x 10^-6 and stadistinct = -1.4^10-6 is the thing
> that's determining whether the planner is picking the correct plan on
> your 4-billion-row table,
No, it's the loss of ability to set stadistinct to -1e-9 or -1e-12 or
-1e-15 or so that is bothering me. In a table with billions of rows
that could become important.
Or maybe not; but the real bottom line here is that it is 100% silly to
use a different representation in this column than is used in the
underlying stadistinct column. All you accomplish by that is to impose
on the user the intersection of the accuracy/range limits of the two
different representations.
regards, tom lane
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