| From: | Hannu Krosing <hannu(at)trust(dot)ee> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Thomas Lockhart <lockhart(at)alumni(dot)caltech(dot)edu>, Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>, Zeugswetter Andreas IZ5 <Andreas(dot)Zeugswetter(at)telecom(dot)at>, pgsql-hackers(at)postgreSQL(dot)org |
| Subject: | Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple se lect) |
| Date: | 1999-07-29 05:42:45 |
| Message-ID: | 379FE9D5.430BB918@trust.ee |
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| Lists: | pgsql-hackers |
Tom Lane wrote:
>
> (2) The statistics that we have available are stats about a column.
> Not stats about a column given the predicate of some index. So there's
> no gain in our statistical knowledge either.
If we added just count of NULLs we would cover for the NOT NULL case
> Partial indexes might be a component of a solution, but they are
> very far from being a solution all by themselves.
>
> regards, tom lane
>
> PS: a quick glance at gram.y shows that we don't actually accept
> partial-index predicates in CREATE INDEX, so Andreas was right that
> the feature got ripped out at some point. I have no idea how much
> work might be required to re-enable it... but I'll bet it's not
> trivial.
That's why I suggested getting just the simplest case (NOT NULL)
working first.
The more general approach would of course be to gather stats by index.
--------------
Hannu
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