Intro to Postgres Planner

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Workshop Title

Melanie Plageman, PGCon 2019, Intro to Postgres Planner

Date: December 2024

Resources

• Video: https://www.youtube.com/watch?v=j7UPVU5UCV4

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Frédéric: I watch Melnanie's presentation for the next workshop and look at her pointers. I'm not too sure I understand what a path is in planner's jargon. Is it a simplified representation of a potential plan whole tree? Or could it be a subtree?

Robert Haas: Subtree.
You can think if a Path as the precursor to a Plan. We have Plan nodes (like SeqScan and IndexScan) and then we have path nodes (like SeqPath and IndexPath) from which the corresponding Plan nodes are constructed.
A Path takes less work to construct than a Plan, so initially we make Paths, and then the losers are thrown away and the winning Path is turned into a Plan.

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Robert's talk that discussed the Plan data structure: What's in a Plan -- PGCon 2018 & Slides

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Guidelines for new Optimizations

1. Does it always retain semantic correctness?
2. Does it inhibit downstream optimizations?
3. Is the improvement in execution time worth the cost in planning time?
4. Is the complexity cost commensurate the performace benefit?