From f26622139e99c1b22397f3bcdb25b793f82dcd77 Mon Sep 17 00:00:00 2001
From: discord9 <55937128+discord9@users.noreply.github.com>
Date: Wed, 9 Sep 2026 14:14:39 +0800
Subject: [PATCH] docs: explain instant LastRow filter eligibility

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>
---
 src/query/src/optimizer/scan_hint.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/query/src/optimizer/scan_hint.rs b/src/query/src/optimizer/scan_hint.rs
index 79f41c8f787..777527ba3f0 100644
--- a/src/query/src/optimizer/scan_hint.rs
+++ b/src/query/src/optimizer/scan_hint.rs
@@ -132,6 +132,17 @@ impl ScanHintRule {
         Ok(Transformed::yes(LogicalPlan::TableScan(table_scan)))
     }
 
+    /// Checks whether attached scan predicates permit instant-derived LastRow selection.
+    ///
+    /// Selecting the newest row too early can discard an older matching sample if a
+    /// predicate later rejects that row. Only recognized tag/time predicates are
+    /// allowed: tags select whole series, and supported time predicates constrain
+    /// the scan window before row selection. Field or unrecognized predicates are
+    /// conservatively rejected. Finer-than-millisecond timestamps are also excluded
+    /// because instant evaluation can conflate distinct samples at that precision.
+    ///
+    /// This checks only attached predicates; the rewriter separately rejects residual
+    /// Filter nodes between InstantManipulate and the scan.
     fn filters_preserve_last_row(
         table_scan: &datafusion_expr::logical_plan::TableScan,
         provider: &DummyTableProvider,