Fast pushdown projection

[robinskil]

Enable the fast pushdown of selected columns when using the JSON API. This can provide big performance improvements. Its is feature blocked by the flag BEACON_ENABLE_PUSHDOWN_PROJECTION

[Copilot]

Pull Request Overview

This PR implements fast pushdown projection to improve query performance by filtering columns early in the query processing pipeline. The feature is controlled by the BEACON_ENABLE_PUSHDOWN_PROJECTION configuration flag.

• Adds column collection logic to gather selected columns from query SELECT clauses
• Implements pushdown projection by filtering schemas to only include requested columns
• Adds configuration flag to enable/disable the feature

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
beacon-config/src/lib.rs	Adds configuration flag for enabling pushdown projection
beacon-query/src/lib.rs	Implements column collection method for Select enum
beacon-query/src/parser.rs	Adds conditional logic to collect columns and pass them for projection
beacon-query/src/from.rs	Updates table initialization to support projection parameter
beacon-data-lake/src/files/collection.rs	Implements schema filtering for pushdown projection

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The HashSet maybe_set_projection is created but then immediately checked for emptiness, while the original projection Vec is used for filtering. This creates unnecessary overhead - you can check projection.is_empty() directly and use the Vec for filtering without creating the HashSet.

Suggested change

	let maybe_set_projection: HashSet<String> = HashSet::from_iter(projection.iter().cloned());
	if !maybe_set_projection.is_empty() {
	// Only keep fields that are in the projection
	// Only keep fields that are in the projection if projection is not empty
	if !projection.is_empty() {