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Status Delivered
Workspace Informix
Components Informix Server
Created by Guest
Created on Jun 9, 2021

Add metrics for reverse index scans and number of retries.

As per APAR IT37120: CONCURRENT QUERIES USING REVERSE SCAN OF INDEX WITH MANY DELETEDENTRIES MAY ENCOUNTER POOR PERFORMANCE

reverse index scans can require a retry.

Add metrics for number of reverse scans per partition and number of retries.

Needed by Date Aug 1, 2021
  • Guest
    Reply
    |
    Nov 22, 2021


    This is no use - it does not allow us to track down what is causing the reverse scans.

    The metrics need to be at the partition level and ideally at the session/thread level.


    Having this as the server level may well cause latch contention with thousand of threads trying to update the same metric value.

    This does not scale at the number of sessions increases - where there are thousands of sessions doing selects/inserts this will result in a bottleneck on search/additem, this feature as implement needs to be removed as it will cause a slowdown.


  • Guest
    Reply
    |
    Nov 9, 2021

    I know it's not per partition as requested, but new in 14.10.xc7 there are an onstat -I option giving good information about indexes including global reverse scans


    $ onstat -I
    

    IBM Informix Dynamic Server Version 14.10.FC7AAE -- Read-Only (Sec) -- Up 03:11:40 -- 9141092 Kbytes
    2021-11-09 19:30:49

    Index statistics
    search additem delitem retry rev_scans rs_retries
    10228 0 0 0 0 0

    splits page slot root copyback
    321 321 0 12 0

    compress merges shuffles root
    0 284 0 6