GitHub user DaveBirdsall opened a pull request:
https://github.com/apache/incubator-trafodion/pull/186
[TRAFODION-1641] Fix MDAM costing bugs
This set of changes is a partial fix to [TRAFODION-1641]. It does resolve
the issue described in the JIRA (that is, the query now gets a better choice of
MDAM plans), however as in many Optimizer changes, there are side effects that
still remain to be fully evaluated. Follow-on work will address these if judged
to be significant.
JIRA 1641 describes two bugs. This set of changes fixes bug #1: MDAM
costing now takes into account the cumulative cost of each key column
materialized. This set of changes partially fixes bug #2: It removes a bit of
code that set the stop column for all MDAM plans to the last column (which
caused MDAM plans to always traverse all keys, which can be grossly
inefficient).
For queries not containing RangeSpec predicates with OR subtrees, these
changes will allow MDAM to traverse a leading proper subset of columns, which
often will be far more efficient. (For RangeSpec predicates containing OR
subtrees, there is a heuristic in the code that presently causes all key
columns to be traversed. This could be improved, and is part of the follow-on
work.)
A side effect of these changes is that the cost computed for MDAM plans of
2 or more key columns will be somewhat higher (and much higher when traversing
key columns with high UEC). This may cause MDAM to be chosen less often than
previously.
One example of this occurs in the regression test suites (seabase/TEST010):
A single query there now chooses a full table scan instead of an index scan
using MDAM. Not using MDAM is a good decision for this particular plan. I have
not investigated, however, why a full table scan is chosen over a full index
scan.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/DaveBirdsall/incubator-trafodion Traf1641
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/incubator-trafodion/pull/186.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 #186
----
commit 3415443dee6e4e1074c22342222e683c21ff31eb
Author: Dave Birdsall <[email protected]>
Date: 2015-11-24T19:55:38Z
[TRAFODION-1641] Fix MDAM costing bugs
----
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
