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Re: Fwd: Indexes (Disk space)

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: Aldor <an(at)mediaroot(dot)de>
Cc: jose fuenmayor <jafn82(at)gmail(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: Re: Fwd: Indexes (Disk space)
Date: 2005-08-24 18:17:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
I think the best you could do would be to estimate it based on index key
size and knowing the overhead for the index structure. Note that it
probably wouldn't be too hard to codify this; it's just that no one has
done it. would be a
good start at figuring out how much space the index will take.

On Wed, Aug 24, 2005 at 02:51:30PM +0100, Aldor wrote:
> I don't think this is possible...
> you could try a explain on the create
> index command but... this is like playinng lotto, didn't try it and 
> don't have the time to try it;-)
> i think getting the size is only possible after creating the index, 
> because when the index is created it will gather the information about 
> the data indexed.
> if this answer is not enough: why do you need to know the size of the 
> index BEFORE creating it?
> jose fuenmayor wrote:
> >Ok I understand but I need to know the size of the index before i create it
> >I need to know  how much space it will ocupy (before actually create
> >the index, having the information of the table on wich I will build
> >the index), is this possible?
> >thanks in advance to everyone that can help me.
> >
> >On 8/23/05, Aldor <an(at)mediaroot(dot)de> wrote:
> >
> >>select * from pg_class;
> >>
> >>or
> >>
> >>select relname, relpages from pg_class where relname = '[index-name]';
> >>
> >>the pages give you the information about the space the index uses, a
> >>page has 8kb.
> >>
> >>[...]Every table and index is stored as an array of pages of a fixed
> >>size (usually 8Kb, although a different page size can be selected when
> >>compiling the server). In a table, all the pages are logically
> >>equivalent, so a particular item (row) can be stored in any page. In
> >>indexes, the first page is generally reserved as a metapage holding
> >>control information, and there may be different types of pages within
> >>the index, depending on the index access method. [...]
> >>
> >>Source-URL:
> >>
> >>
> >>jose fuenmayor wrote:
> >>
> >>>---------- Forwarded message ----------
> >>>From: jose fuenmayor <jafn82(at)gmail(dot)com>
> >>>Date: Aug 22, 2005 1:37 PM
> >>>Subject: Indexes (Disk space)
> >>>To: pgsql-admin(at)postgresql(dot)org
> >>>
> >>>
> >>>Hi all,
> >>>I have the following question.
> >>>Is there anyway for me to know how much space on disk will ocupy an
> >>>index, created in a determined row or rows of a table?, anything like
> >>>a rule, formula, calculation? that allow me to know in advance how
> >>>much space will the index use before actually created it.
> >>>
> >>>I aprecciate all the help you can give me
> >>>
> >>>Thanks in advance.
> >>>
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> >>
> >
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> TIP 5: don't forget to increase your free space map settings

Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software        512-569-9461

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