IDS Forum

Re: AW: Unwanted index fragmentation

Posted By: John Miller iii
Date: Tuesday, 22 January 2008, at 9:33 a.m.

In Response To: AW: Unwanted index fragmentation (Andreas.KUTSCHE@spar.at)

Actually index on varchar columns do not allocate the maximum
space in the index, but only allocate the space required by
the data. Here is a little test which print out the data in the index
pages

I create a table with a varchar column sized of 200 bytes, create an
index insert two rows filling just a small portion of the row. I then
print out the index to see the size of each row. As you
can see each index slot is only 10 and 12 bytes long, not the
200 bytes of the fully sized varchar column.

John

create table vchar (c1 varchar(200));
create index vchar_ix1 on vchar(c1);
insert into vchar values("JOHN");
insert into vchar values("MILLER");

oncheck -pp 2097217 1
addr stamp chksum nslots flag type frptr frcnt n=
ext
prev
2:2430 6418868 f8a9 2 b0 BTREE 46 10182 0=

0

slot ptr len flg

1 24 10 0

2 34 12 0
slot 1:

0: 4 4a 4f 48 4e 0 0 1 1 0 .JOHN.........=
..
slot 2:

0: 6 4d 49 4c 4c 45 52 0 0 1 2 0 .MILLER.......=
..

=

"Andreas.KUTSCHE@ =

spar.at" =

<andreas.kutsche@ =
To

spar.at> ids@iiug.org =

Sent by: =
cc

ids-bounces@iiug. =

org Subj=
ect

AW: Unwanted index fragmentation=

[11049] =

01/22/2008 06:06 =

AM =

=

=

Please respond to =

ids@iiug.org =

=

=

Hi,

Indices on varchar-columns always allocate the maximum possible column
length
to store the index keys.

Regards,
Andreas

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-----Urspr=FCngliche Nachricht-----

> Von: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] Im Auftrag vo=
n
> ANDY KENT
> Gesendet: Dienstag, 22. J=E4nner 2008 14:49
> An: ids@iiug.org
> Betreff: Unwanted index fragmentation [11046]
>
> Re-sizing a database for migration and taking care to get the extent
sizes
> just how we want them, I was shocked to see indexes immediately grabb=
ing
> 20+
> extents. It seems worse on tables with lots of varchars, where I've
really
> pulled the extent sizes back to suit the actual rather than maximum r=
ow
> lengths.
>
> Any workaround that doesn't involve oversizing the base extents?
>
>
>
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Forum Note: Use "Reply" to post a response in the discussion forum.

See you at the IIUG Informix 2008 Conference
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April 27 - 30, 2008 Marriott Overland Park (Kansas City), Kansas
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Messages In This Thread

• Unwanted index fragmentation
  ANDY KENT -- Tuesday, 22 January 2008, at 8:49 a.m.
  • AW: Unwanted index fragmentation
    Andreas.KUTSCHE@spar.at -- Tuesday, 22 January 2008, at 9:06 a.m.
    • Re: AW: Unwanted index fragmentation
      John Miller iii -- Tuesday, 22 January 2008, at 9:33 a.m.
      • Re: AW: Unwanted index fragmentation
        Art S. Kagel (Oninit LLC) -- Tuesday, 22 January 2008, at 9:45 a.m.
  • Re: Unwanted index fragmentation
    Art S. Kagel (Oninit LLC) -- Tuesday, 22 January 2008, at 9:38 a.m.

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