refactor: make DynamicFilterPhysicalExpr.fetch a shared reference to SortExec.fetch

[zhuqi-lucas]

Background

In #21580, DynamicFilterPhysicalExpr was given a fetch: Option<usize> field that stores the TopK K value. This value is copied from SortExec.fetch at filter creation time in SortExec::create_filter().

Problem

If a future optimizer calls SortExec::with_fetch() independently (without recreating the filter), the DynamicFilterPhysicalExpr.fetch would go stale. Currently this doesn't happen because create_filter and fetch are set in the same method, but the coupling is fragile.

Raised by @xudong963 in #21580 (comment)

Proposed fix

Replace fetch: Option<usize> with a shared reference (e.g., Arc<AtomicUsize>) that reads directly from SortExec.fetch. This way any update to fetch is automatically visible to the parquet reader's stats init and cumulative pruning logic.

Related

• feat: statistics-driven TopK optimization for parquet (file reorder + RG reorder + threshold init + cumulative prune) #21580 — PR that introduced fetch on DynamicFilterPhysicalExpr