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IDS Forum

Re: Indexes on a Fragmented Table

Posted By: ART KAGEL, BLOOMBERG/ 731 LEXIN
Date: Monday, 15 May 2006, at 11:24 a.m.

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

Actually, even if it sorts to merge the data from the two dbspaces, it will only
be sorting the 100 rows that match the filter criteria not the millions of
unfiltered rows in the fragments. Now, in your specific example, you are
ordering on 'price' which is neither the FRAGMENT BY column (order_date) nor the
indexed column in the index you presented (product). In this case the engine
will have no choice but to sort the 100 or so matching data rows regardless of
the optimization strategy. This is one of the examples I had in mind when I
said that the optimizer may not be able to perform a simple merge in all cases.

Art S. Kagel

----- Original Message -----
From: Yunyao (Fra.... <ids@iiug.org>
At: 5/15 10:36:09

HI, Art,

I am curious about your statementt:

".....This will ONLY be true if the index is fragmented on the same expression
as the table and the fragmentation
expression is the same as the ORDER BY clause. If not, then yes sorting will
indeed occur because then the data coming from the fragments in NOT already
sorted. "

Suppose I have a table,

CREATE order ( order_date date, product char(80), price int, ....)
FRAGMENT BY EXPRESSION
order_date >= '2005-01-01 00:00:00' in dbs1,
order_date < '2005-01-01 00:00:00' in dbs2;

CREATE INDEX order_idx1 on order(product);

Then SQL statements,

SELECT FIRST 100 * from order
where product like "TV100%"
order by price

IDS should (will?) search TWO first 100 rows from two fragments ( through
local indexes) separately, then merge it, instead of sorting too many (with
millions) rows.

Thanks,
Frank

ART KAGEL, .... wrote:

><SNIP>
>
>
>>(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
>>
>>
>
>This statement is misleading and incorrect. Alexey we discussed this in Tampa
>so I assume you replied to this before we spoke. It's not that IDS CANNOT use
>the index across fragments and so sorts the data, it's that IDS WILL NOT
>normally use a merge of sorted sinks across fragments and sort instead simply
>because in all of their testing it was FASTER! If you need the initial rows
of
>data back sooner and don't care that the total query time will likely be a
bit
>longer, then you can use the FIRST_ROWS optimization hint for such queries.
>With FIRST ROWS optimization set the optimizer WILL use a merge of the sorted
>data using the indexes on the separate fragments. This will ONLY be true if
>the
>index is fragmented on the same expression as the table and the fragmentation
>expression is the same as the ORDER BY clause. If not, then yes sorting will
>indeed occur because then the data coming from the fragments in NOT already
>sorted.
>
>
>
>>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.
>>
>>
><SNIP>
>
>Art S. Kagel
>
>

>*******************************************************************************

> 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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