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 12.10.xC16 and 14.10.xC7. * **************************************************************** * PROBLEM DESCRIPTION: * * See Error Description * **************************************************************** * RECOMMENDATION: * * Upgrade to Informix Server 12.10.xC16Â or 14.10.xC7. * **************************************************************** | |
Local Fix: | |
Solution | |
Workaround | |
**************************************************************** * USERS AFFECTED: * * Users of Informix Server prior to 12.10.xC16 and 14.10.xC7. * **************************************************************** * PROBLEM DESCRIPTION: * * See Error Description * **************************************************************** * RECOMMENDATION: * * Upgrade to Informix Server 12.10.xC16Â or 14.10.xC7. * **************************************************************** | |
Comment | |
Fixed in Informix Server 12.10.xC16 and 14.10.xC7. | |
Timestamps | |
Date - problem reported : Date - problem closed : Date - last modified : | 03.06.2021 02.11.2021 02.05.2023 |
Problem solved at the following versions (IBM BugInfos) | |
Problem solved according to the fixlist(s) of the following version(s) |