USTAT_USE_IS_WHEN_NO_STATS could use a different default setting

Bug #1392555 reported by Suresh Subbiah on 2014-11-14
6
This bug affects 1 person
Affects Status Importance Assigned to Milestone
Trafodion
Fix Committed
Medium
Barry Fritchman

Bug Description

When doing update stats on a table that has been newly loaded (i.e. has no previous stats), if we use the 'on every column' option, then the command takes a long time, particularly if the table has several columns and a large number of rows (even if sampling is used). It was found that when USTAT_USE_IS_WHEN_NO_STATS is set to ON, the time takes is improved several fold (Thank you Barry). This LP is a request to consider changing the default setting.

Changed in trafodion:
status: New → In Progress

Reviewed: https://review.trafodion.org/717
Committed: https://github.com/trafodion/core/commit/582b09c7bc8325af01b35b8b2b52ae6d7a9d0d77
Submitter: Trafodion Jenkins
Branch: master

commit 582b09c7bc8325af01b35b8b2b52ae6d7a9d0d77
Author: Barry Fritchman <email address hidden>
Date: Thu Nov 20 22:42:14 2014 +0000

    Change default for USTAT_USE_IS_WHEN_NO_STATS

    This cqd was originally set to OFF by default, causing
    internal sort not to be used in cases where there are
    no existing histograms. Internal sort is known to
    underperform when there are a small number of unique
    values for a column, and without prior histograms, the
    unique entry count is unknown.

    However, experience with POCs has shown that using internal
    sort in these situations (unique count unknown) is in
    general significantly faster than not using it, so the
    default setting has been changed to ON.

    Running developer regressions with this setting exposed a
    bug that occurs when internal sort is used on an empty table
    that previously did contain rows. This change includes a known
    diffs file for the failing test until the bug (LP 1393930)
    can be fixed.

    Change-Id: I9ff6672a168016fb83fadb26b40a30c7fa3eee6c
    Closes-Bug: #1392555

Changed in trafodion:
status: In Progress → Fix Committed
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