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https://issues.apache.org/jira/browse/TRAFODION-2282?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15589306#comment-15589306
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ASF GitHub Bot commented on TRAFODION-2282:
-------------------------------------------
Github user robertamarton commented on a diff in the pull request:
https://github.com/apache/incubator-trafodion/pull/766#discussion_r84122119
--- Diff: core/sql/optimizer/NATable.cpp ---
@@ -275,75 +282,28 @@ void HistogramCache::getHistograms(NATable& table)
// look up the cache and get a reference to statistics for this table
cachedHistograms = lookUp(table);
- // first thing to check is, if the table to which the histograms are
cached
- // has been updated
+ // (Possibly useless) sanity tests
+
+ // Check to see if the redefinition timestamp has changed. This seems
+ // to be always stubbed to zero today on Trafodion, so this check
+ // seems to never fail.
if (cachedHistograms && (cachedHistograms->getRedefTime() !=
redefTime))
{
deCache(&cachedHistograms);
}
--- End diff --
I actually changed DDL code to use the non-zero redeftime in descriptors as
part of my role invalidation change, but it caused some interesting regression
failures. So I removed the change before delivering the code; hopefully, I can
get back to it sometime.
> Optimizer histogram cache is sometimes never refreshed
> ------------------------------------------------------
>
> Key: TRAFODION-2282
> URL: https://issues.apache.org/jira/browse/TRAFODION-2282
> Project: Apache Trafodion
> Issue Type: Bug
> Components: sql-cmp
> Affects Versions: 2.1-incubating
> Environment: All
> Reporter: David Wayne Birdsall
> Assignee: David Wayne Birdsall
> Fix For: 2.1-incubating
>
>
> Here's the scenario:
> In session 1, create a table T1 and populate it with, say, 200000 or more
> rows.
> In session 2, prepare a query that selects from T1. For example, select
> A,COUNT(*) FROM T1 GROUP BY A. The query should compile fine, but you should
> see missing statistics warning messages (warning codes 6007 and 6008).
> In session 1, do "update statistics for table t1 on every column"
> In session 2, prepare other queries over a period of time. What should happen
> is that after about an hour, the Optimizer decides to refresh the histograms.
> However, due to a bug, it might never do so. So, the missing statistics
> warnings (6007, 6008) never go away. Worse, the prepares never benefit from
> the statistics so plan choice may be poor.
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