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Informix - Problem description

Problem IT37120 Status: Closed

CONCURRENT QUERIES USING REVERSE SCAN OF INDEX WITH MANY DELETEDENTRIES
MAY ENCOUNTER POOR PERFORMANCE

product:
INFORMIX SERVER / 5725A3900 / C10 - IDS 12.10
Problem description:
When the execution of a query involves the scan of an index in
the reverse direction some additional precautions are necessary
to ensure that execution does not become deadlocked with other
threads using the index in the forward direction. These
precautions include a back-off and retry mechanism which may
include the need to access many index pages. If there are large
numbers of index pages containing only deleted entries then the
number of pages needing to be accessed will increase which can
increase the chances of triggering the retry mechanism.

Here is an example query plan showing a reverse index scan:

select max(id) as max_id
from     tab1
where    from_time + 10 units second < CURRENT

Estimated Cost: 2
Estimated # of Rows Returned: 1

  1) informix.tab1: INDEX PATH

        Filters: informix.tab1.from_time + interval(10)
second(9) to second < CURRENT year to fraction(3)

    (1) Index Name: informix.itab1_x1
        Index Keys: id  (Reverse)  (Aggregate)  (Serial,
fragments: ALL)

Factors to consider to help avoid the problem include monitoring
of btscanner activity to remove deleted entries from the index
and creation of an index defined using a descending order on the
column(s).
Problem Summary:
****************************************************************
* USERS AFFECTED:                                              *
* Users of Informix Server prior to 14.10.xC7.                 *
****************************************************************
* PROBLEM DESCRIPTION:                                         *
* See Error Description                                        *
****************************************************************
* RECOMMENDATION:                                              *
* Upgrade to Informix Server 14.10.xC7.                        *
****************************************************************
Local Fix:
Solution
Workaround
****************************************************************
* USERS AFFECTED:                                              *
* Users of Informix Server prior to 14.10.xC7.                 *
****************************************************************
* PROBLEM DESCRIPTION:                                         *
* See Error Description                                        *
****************************************************************
* RECOMMENDATION:                                              *
* Upgrade to Informix Server 14.10.xC7.                        *
****************************************************************
Comment
Fixed in Informix Server 14.10.xC7.
Timestamps
Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :
03.06.2021
02.11.2021
02.11.2021
Problem solved at the following versions (IBM BugInfos)
Problem solved according to the fixlist(s) of the following version(s)
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