[GitHub] [spark] tanelk commented on pull request #30965: [SPARK-33935][SQL] Fix CBO cost function

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tanelk commented on pull request #30965:
URL: https://github.com/apache/spark/pull/30965#issuecomment-810810870


   @wzhfy and @cloud-fan 
   
   I'm not a fan of adding up the relative costs.
   
   A simple example, where the weight is 0.5:
   If this plans size (bytes) is 2x larger, then no matter how many times more rows does the other plan have, the other plan will allways be considered to be better - `0.5*2 + 0.5*0.00000000000001  > 1`.
   This basically the same situation, where one cost overwhelms the other.
   
   Perhaps this would be a best of both worlds:
   `(this.card / other.card) ^ cardWeight * (this.size / other.size) ^ (1 - cardWeight) < 1`.
   In short - multiply the relative costs instead of adding them.
   
   
   


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