GitHub user DaveBirdsall opened a pull request:
https://github.com/apache/trafodion/pull/1403
[TRAFODION-2913] Tweak some MDAM-related heuristics
There are two changes in this pull request.
1. The default value for CQD MDAM_SELECTION_DEFAULT has been changed from
0.5 to 8.0. This is a recalibration of this default; it had not been changed
since predecessor product days. The old value resulted in MDAM not being
considered at times when it is the better plan. Details of how this
recalibration was done will be documented in the JIRA.
2. The heuristic implemented by IndexDesc::pruneMdam is now limited to
index elimination logic and no longer used by the scan optimizer. As a
practical matter this was already true in most circumstances:
ScanOptimizer::getMdamStatus ignored the mdamFlag when CQD
RANGESPEC_TRANSFORMATION is set to 'ON'. (And 'ON' is the default value for
that CQD.) The one case when this was not true was if CQD FSO_IN_USE was set to
'0'. So, ironically, when the user is trying to encourage MDAM using CQD
FSO_IN_USE, we would pay attention to this heuristic that prevents MDAM.
It should be noted that the heuristic in IndexDesc::pruneMdam depends on
counting the number of leading columns without a key predicate. It does so
without taking into account parallelization, which means it does not see
potential equality predicates on a leading "_SALT_" column, which can cause it
to reject MDAM when MDAM would be useful. Even so, the heuristic there seems
reasonable for narrowing the set of indexes that we might consider so it is
still good for index elimination purposes though not perfect. But once an index
is being considered, it is better to let the costing code, which does know
about parallelization, to have the final word.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/DaveBirdsall/trafodion Trafodion2913
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/trafodion/pull/1403.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #1403
----
commit ae17030878f48aab2be36db7a57d7b11c2413016
Author: Dave Birdsall <dbirdsall@...>
Date: 2018-01-17T17:22:24Z
[TRAFODION-2913] Tweak some MDAM-related heuristics
----
---
