Plan Pruning Strategies

This task is part of feature #18249

Plan pruning can be adaptable, with several strategies depending on system constraints and goals. Here are a few examples:

- **No pruning until the end:** If enumeration produces a manageable number of partial plans and memory is abundant, we can defer pruning entirely. A cost model is then applied at the final stage to select the best plans.
- **Incremental pruning by level:** To prevent plan space explosion, we retain a subset of plans at each level, using a cost model to discard less promising ones.
- **Memory-aware pruning:** Pruning is triggered only when memory usage exceeds a predefined threshold.
- **Plan count-based pruning:** We prune only when the number of partial plans surpasses a specified limit.
- **Hybrid pruning strategy:** only prune when certain conditions hit and prune to keep certain plans

This flexibility allows pruning strategies to be tailored to system capacity and optimization goals.

Start with feature #18249, which this task belongs to, and review its plan-enumeration requirements before deciding how pruning strategies should be represented. The issue names no files, entry points, tests, limits, or acceptance criteria, so completion cannot be verified from this issue alone.

Written by the indexing model from the issue text.