feat: Access to selected scan implementation #2295
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Which issue does this PR close?
Closes #.
Rationale for this change
Comet supports multiple scan implementations (e.g., native Comet, DataFusion, Iceberg-compat) and often runs in SCAN_AUTO mode to pick the best option at runtime. Previously, that selection was recomputed each time the rule ran, which adds some overhead. Moreover it is not easy to know in runtime what value has been picked up.
This PR derives the scan implementation once and stores it at the session level, reusing the same choice for subsequent scans. It preserves existing selection semantics, including falling back to native Comet when Parquet encryption is enabled and otherwise delegating to selectScan.
The selected implementation is now exposed via
CometScanRule.getCurrentScanImplementationWhat changes are included in this PR?
How are these changes tested?