| From: | "Zeugswetter Andreas ADI SD" <ZeugswetterA(at)spardat(dot)at> |
|---|---|
| To: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>, "Dann Corbit" <DCorbit(at)connx(dot)com> |
| Cc: | "Hannu Krosing" <hannu(at)skype(dot)net>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Selecting a constant question |
| Date: | 2007-06-12 10:47:55 |
| Message-ID: | E1539E0ED7043848906A8FF995BDA57902240FD1@m0143.s-mxs.net |
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| Lists: | pgsql-hackers |
> Thats exactly the point. Consider
> select mytext from mytable ;
>
> How can PostgreSQL possibly know the maximum length of the
> returned values *before* it has scanned the whole table?
I think this focuses too much on those cases where it is not possible.
When it is not feasible like with a text column, clients deal with it
already (obviously some better than others).
It is for those cases where it would be feasible, like constants (or
concateneted columns), where the max length if properly returned could
be used to improve performance.
Andreas
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