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

3 Managing Skew
This section describes these CQDs that are used for managing skew:
• “SKEW_EXPLAIN” (page 15)
• “SKEW_SENSITIVITY_THRESHOLD” (page 15)
SKEW_EXPLAIN
Influencing Query PlansCategory
Turns on the reporting of anti-skew join plan details in EXPLAIN or EXPLAIN options ‘f.Description
Disables the use of SKEW_EXPLAIN.’OFF’
Enables skew information in EXPLAIN.‘ON’
The default value is ‘OFF’.
Values
Not applicableUsage
Not applicableProduction usage
Query plans are not changed by this CQD.Impact
AnyLevel
Allows additional information to be displayed in explain plans. It has no impact on query
plans.
Conflicts/Synergies
Not applicableAddressing the real
problem
Neoview Release 2.3Introduced in release
Not applicableDeprecated in release
SKEW_SENSITIVITY_THRESHOLD
Influencing Query PlansCategory
Used to specify a threshold that determines whether a value in the join column is considered
skewed.
Description
Disables the use of skew buster.’-1’
‘n’ is a floating-point value greater than or equal to 0.‘n’
The default value is ‘-1’.
Values
A value is considered skewed if its occurrence frequency is greater than the threshold value
multiplied by the average number of rows per processing node (CPU). That is, if the average
row count equals the row count divided by the number of processing nodes, then the
frequency of the value is greater than the threshold multiplied by the average row count.
A setting of n, where n ≥ 0, indicates that the value should be considered as skewed if its
occurrence frequency is greater than n times the average number of rows per processing
node.
If some small skew is suspected for hash joins during query execution (detected by observing
spiked CPU busy usage), try to lower this setting. A default setting of 0.1 should eliminate
most skews. Setting the CQD to a very large value, such as 10, is not recommended, as it
effectively turns off skew buster.
Usage
Not applicableProduction usage
Not applicableImpact
AnyLevel
SKEW_EXPLAIN 15