David Wayne Birdsall created TRAFODION-3158:
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Summary: Bad histogram values are sometimes not reported
Key: TRAFODION-3158
URL: https://issues.apache.org/jira/browse/TRAFODION-3158
Project: Apache Trafodion
Issue Type: Bug
Components: sql-cmp
Affects Versions: 2.3
Reporter: David Wayne Birdsall
Assignee: David Wayne Birdsall
Function EncodedValue::constructorFunction has logic to raise a 6003 warning if
it encounters invalid values in histogram intervals. When this happens, the
compiler generates a default histogram for that column (that is, it behaves as
if it has no statistics for that column.) This can lead to poor query plans. In
some circumstances, however, this warning does not get reported, so the user is
not aware of the possible compromise in plan quality.
An example of this can be created as follows:
# Create a Hive table Test1 with columns (a int, b int). Put some data into
it, and in Trafodion, use UPDATE STATISTICS FOR TABLE HIVE.HIVE.TEST1 to
generate statistics for it.
# Outside of Trafodion (in the Hive shell, say), drop and recreate the Hive
table, but this time with columns (a string, b string). Populate that with a
few rows.
# In a fresh sqlci session, do "prepare s1 from select a,b from
hive.hive.test1 where a < 'abcd';". This will appear to be successful, however
in fact 6003 warnings were generated internally but not reported.
Another way to create an example is to create a Trafodion table, test1, with
columns (a char(4), b char(4). Put some rows into it and do UPDATE STATISTICS
on it. Then manually alter the boundary value in one of the
SB_HISTOGRAM_INTERVALS tables, changing a value from a character string literal
to a numeric literal. For example, change "('abcd')" to "(3)".
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