[DISCUSS] getCumulativeCost vs Planner#getCost(rel, mq)

[Vladimir Sitnikov <si...@gmail.com>]

Hi,

I've found there are two ways to compute the cumulative cost, and they
happen to be quite different.

VolcanoPlanner#getCost seems to be the right one (see [1])
It properly accounts for RelSubset and uses its bestCost

On the other hand, mq.getCumulativeCost(rel) is quite poor (see [2]).
It knows nothing on RelSubsets, so it produces wrong numbers.

Unfortunately, explain with all attributes is using bad API [2] (see [3]).

Any thoughts on unifying the APIs?

I can imagine two approaches:
A) Implement RelSubset#computeSelfCost as "return bestCost". That would
probably make VolcanoPlanner's getCost obsolete.
B) Remove mq.getCumulativeCost and divert all the uses to planner#getCost


[1]:
https://github.com/apache/calcite/blob/763ec429c951acd5fa251c5d4a2b7780fbf648d4/core/src/main/java/org/apache/calcite/plan/volcano/VolcanoPlanner.java#L955-L973
[2]:
https://github.com/apache/calcite/blob/763ec429c951acd5fa251c5d4a2b7780fbf648d4/core/src/main/java/org/apache/calcite/rel/metadata/RelMdPercentageOriginalRows.java#L158-L170
[3]:
https://github.com/apache/calcite/blob/763ec429c951acd5fa251c5d4a2b7780fbf648d4/core/src/main/java/org/apache/calcite/rel/externalize/RelWriterImpl.java#L98-L102

Vladimir