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IDS Forum

RE: Indexes on a Fragmented Table

Posted By: Alexey Sonkin
Date: Friday, 12 May 2006, at 6:15 p.m.

In Response To: Re: Indexes on a Fragmented Table (Yunyao (Frank) Qu)

Actually, the default behavior of indexes on fragmented tables
in Informix is completely equivalent to the behavior of local indexes
on fragmented tables in Oracle: each fragment of index,
created without 'IN dbspaceXXX', is created in the same dbspace,
as table fragment.
'Detach fragment' in Informix executes momentarily when indexes are
LOCAL
(fragmented same way, as tables).

The are still some disadvantages of using local indexes
(or any type of fragmented index) over global non-fragmented index.
Consider the following:

CREATE TABLE transactions (tx_id INT, tx_date DATETIME YEAR TO SECOND,
...)
FRAGMENT BY EXPRESSION
tx_date >= '2005-01-01 00:00:00' and tx_date < '2005-02-01 00:00:00' in
dbs1,
tx_date >= '2005-02-01 00:00:00' and tx_date < '2005-03-01 00:00:00' in
dbs2;
CREATE INDEX tx_idx1 on transactions(tx_date);

This DDL creates local fragmented index, co-allocated with table data
fragments.

The biggest problem with that index is about running queries like:

SELECT first 100 *
FROM transactions
WHERE tx_date >= '2005-01-29 00:00:00' and tx_date < '2005-02-02
00:00:00'
ORDER BY tx_date
(Note, that date range spans across two table fragments)

Informix (including Informix 10) is unable to use index on TX_DATE
to do the sort!!!!!!!!! It creates a temporary table (may be, few
million rows), sorts it, and then returns first 100 rows
of the sorted result set.
Optimizer directives (like --+FIRST_ROWS) do not help.

If date range for SELECT goes to a single fragment, then Informix
is able to use index fragment to do the sort the data for ORDER BY,
and doesn't create the TEMP table.

That is, the major benefit of global (non-fragmented) index
is that it works much better for SELECT with ORDER BY

The benefits of fragmented index are faster inserts, and dramatically
faster 'ALTER FRAGMENT ON tab_name DETACH frag_name'

-Alexey

> -----Original Message-----
> From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of
Yunyao
> (Fra....
> Sent: Friday, May 12, 2006 11:45 AM
> To: ids@iiug.org
> Subject: Re: Indexes on a Fragmented Table [6708]
>
>
> You really raised an excellent question!
>
> Actually we desperately hope a index could be partitioned
AUTOMATICALLY
> by Informix if we would declare the index being LOCAL(, and also we do
> NOT want the index columns must be a subset of fragmenting columns).
>
> I saw the local index from Oracle. Basically, a local index is
> associated with a partition(fragment) and is maintained by DBMS.
> whenever you add or drop a fragment, the associated local index is
> added or dropped FAST, automatically and independently (NO performance
> suffering and NO impact to other fragments or indexes if there is no
> global index associated with the table)! How NICE the feature would
be!
>
> I did not see this feature in Informix.
>
> Thanks,
> Frank
>
> Demeis, Tony wrote:
>
> >Hi,
> >
> >If a table is fragmented by expression, should all of its indexes be
> >fragmented (regardless of whether they're unique or non-unique)?
> >
> >At one time, I was told to keep any unique indexes non-fragmented in
a
> >separate dbspace.
> >
> >Platform: IDS 9.40.FC3 / AIX 5.1
> >
> >Thank you.
> >
> >
>
>
>***********************************************************************
******
> **
> > Forum Note: Use "Reply" to post a response in the discussion forum.
> >
> >
> >
> >
>
> --
> Yunyao "Frank" Qu
> Computer Sciences Corporation(CSC)
> NOAA/CLASS, (301)817-4696
>
>
>
************************************************************************
******
> *
> Forum Note: Use "Reply" to post a response in the discussion forum.
>

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