David Wayne Birdsall created TRAFODION-2645:
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Summary: MDAM costing overestimates I/O cost
Key: TRAFODION-2645
URL: https://issues.apache.org/jira/browse/TRAFODION-2645
Project: Apache Trafodion
Issue Type: Bug
Components: sql-cmp
Affects Versions: 2.2-incubating
Environment: All
Reporter: David Wayne Birdsall
Assignee: David Wayne Birdsall
The method MDAMCostWA::compute (optimizer/ScanOptimizer.cpp) has logic to
calculate the total cost of an MDAM plan and compare it to the cost of a single
subset plan. In the case of a Trafodion table, the cost logic for MDAM cost has
an error. It is adding the total number of probes (totalRqsts) to the number of
seeks (totalSeeks) in order to calculate I/O costs.
TotalSeeks models direct access disk seeks. These happen at the block level and
are an I/O cost.
TotalRqsts models the number of probes, which is a CPU cost. A probe is a
direct access within a block that has already been read into memory. This is a
vastly smaller unit of cost than an I/O operation.
Note that TotalRqsts is already modeled as a message cost since it is added to
TotalRows.
The effect of the bug is to vastly penalize MDAM plans having large numbers of
probes. This will show up the most on the largest tables.
The fix is simply not to add totalRqsts to totalSeeks. However, in making this
fix, it is likely that other MDAM-related parameters will need tuning. So this
change needs to be done carefully.
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