Neoview Control Query Default (CQD) Reference Guide (R2.5)

Unit is seconds.Integer
The default value is ‘3600’ (1 hour).
Values
Histogram statistics are “fake” when update statistics is not being run, but instead the
customer is updating the histogram tables directly with statistics to guide the optimizer.
This may be done if the data in the table is very volatile (such as for temporary tables),
update statistics is not possible because of constant flush and fill of the table occurring,
and statistics are manually set to provide some guidance to the optimizer to generate a
good plan.
If these fake statistics are updated constantly to reflect the data churn, this default can be
set to 0. This would ensure that the histograms with fake statistics are not cached, and are
always refreshed. If these fake statistics are set and not touched again, then this interval
could be set very high.
Usage
Not applicableProduction usage
Setting a high interval improves compilation time. However, if statistics are being updated,
the compiler may be working with obsolete histogram statistics, potentially resulting in
poorer plans.
Impact
ServiceLevel
Not applicableConflicts/Synergies
Not applicableAddressing the real
problem
Neoview Release 2.4Introduced in release
Not applicableDeprecated in release
HIST_PREFETCH
HistogramsCategory
Influences the compiler to pre-fetch the histograms and save them in cache.Description
Pre-fetches the histograms.‘ON’
Does not pre-fetch the histograms.‘OFF’
The default value is ‘ON’.
Values
You may want to turn this off if you don’t want to pre-fetch a large number of histograms,
many of which may not be used.
Usage
Not applicableProduction usage
Though it makes compilation time faster, it may result in the histogram cache to be filled
with histograms that may never be used.
Impact
System or ServiceLevel
Use this CQD with CACHE_HISTOGRAMS. If CACHE_HISTOGRAMS is OFF, then this
CQD has no effect.
Conflicts/Synergies
Not applicableAddressing the real
problem
Neoview Release 2.4Introduced in release
Not applicableDeprecated in release
HIST_ROWCOUNT_REQUIRING_STATS
HistogramsCategory
Specifies the minimum row count for which the optimizer needs histograms, in order to
compute better cardinality estimates. The optimizer does not issue any missing statistics
warnings for tables whose size is smaller than the value of this CQD.
Description
34 Managing Histograms