Re: [GENERAL] B-tree index on a VARCHAR(4000) column

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От John Turner
Тема Re: [GENERAL] B-tree index on a VARCHAR(4000) column
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Msg-id CAMAP1Q=J9wo2xNxk0Z+54YD8Yv7QTAYuHOcVk38=HaZULu+LvA@mail.gmail.com
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Ответ на Re: [GENERAL] B-tree index on a VARCHAR(4000) column  (Merlin Moncure <mmoncure@gmail.com>)
Ответы Re: [GENERAL] B-tree index on a VARCHAR(4000) column  (Tom Lane <tgl@sss.pgh.pa.us>)
Список pgsql-general

On Sun, Sep 10, 2017 at 10:42 AM Merlin Moncure <mmoncure@gmail.com> wrote:
On Friday, September 8, 2017, John Turner <fenwayriffs@gmail.com> wrote:


On Fri, Sep 8, 2017 at 6:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Ron Johnson <ron.l.johnson@cox.net> writes:
> Based on LENGTH(offending_column), none of the values are more than 144
> bytes in this 44.2M row table.  Even though VARCHAR is, by definition,
> variable length, are there any internal design issues which would make
> things more efficient if it were dropped to, for example, VARCHAR(256)?

No.

So the declarative column length has no bearing on memory grants during plan generation/execution?

Nope.  Memory usage is proportional to the size of the string, not the maximum length for varchar.  Maximum length is a constraint.

Ok, thanks for verifying.  I was curious since other platforms seem to handle this aspect of memory allocation differently (more crudely, perhaps) based on estimation of how fully populated the column _might_ be given a size constraint:

John

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