优化
如果您使用Polars的惰性API，Polars将对您的查询进行多项优化。其中一些优化会提前执行，而其他优化则会在具体化数据到达时即时确定。
以下是polars所做优化的不完全概述，它们的作用以及它们运行的频率。
| Optimization | Explanation | runs |
|---|---|---|
| Predicate pushdown | Applies filters as early as possible/ at scan level. | 1 time |
| Projection pushdown | Select only the columns that are needed at the scan level. | 1 time |
| Slice pushdown | Only load the required slice from the scan level. Don't materialize sliced outputs (e.g. join.head(10)). | 1 time |
| Common subplan elimination | Cache subtrees/file scans that are used by multiple subtrees in the query plan. | 1 time |
| Simplify expressions | Various optimizations, such as constant folding and replacing expensive operations with faster alternatives. | until fixed point |
| Join ordering | Estimates the branches of joins that should be executed first in order to reduce memory pressure. | 1 time |
| Type coercion | Coerce types such that operations succeed and run on minimal required memory. | until fixed point |
| Cardinality estimation | Estimates cardinality in order to determine optimal group by strategy. | 0/n times; dependent on query |