| From: | Atri Sharma <atri(dot)jiit(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Mart Kelder <mart(at)kelder31(dot)nl>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Subject: | Re: Removing INNER JOINs |
| Date: | 2014-12-03 17:54:32 |
| Message-ID: | CAOeZVieWbJvJD5KZu2Pc+7E=6j4dDH-m6Lh6QX8J+sR2bggDig@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Dec 3, 2014 at 11:19 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Atri Sharma <atri(dot)jiit(at)gmail(dot)com> writes:
> > Is it possible to only replan part of the plan in case of this
> > optimization? I think that we might need to only replan parts of the
> > original plan (as you mentioned, join search and above). So we could
> reuse
> > the original plan in part and not do a lot of replanning (an obvious case
> > is scan strategy, which we can assume will not change for the two plans).
>
> I think you assume wrong; or at least, I certainly would not wish to
> hard-wire any such assumption. Skipping some joins could change the
> shape of the join tree *completely*, because the cost estimates will
> change so much. And that could in turn lead to making different choices
> of scan methods, eg, we might or might not care about sort order of
> a scan result if we change join methods.
>
> regards, tom lane
>
Agreed, but in some cases, we could possibly make some assumptions (if
there is no index, if a large fraction of table will be returned in scan,
FunctionScan).
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