tiny docs and fixes

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

src/metric-engine/src/engine/create.rs

@@ -313,6 +313,8 @@ impl MetricEngineInner {
                         .unwrap())
                     .clone();
                     if metadata.semantic_type == SemanticType::Field {
+                        // Physical split-value fields are nullable because each row uses only one
+                        // half. Preserve the user-facing field schema in logical metadata.
                         column_metadata.column_schema = metadata.column_schema.clone();
                     }
                     column_metadata
@@ -619,6 +621,8 @@ fn configure_metric_value_int_columns(
             || (metadata.semantic_type == SemanticType::Field
                 && metadata.column_schema.data_type == ConcreteDataType::float64_datatype())
         {
+            // Every split row leaves either the float or integer half null. Logical metadata
+            // retains the original nullability constraint.
             metadata.column_schema.set_nullable();
         }
         value_split_enabled || !is_int_column

src/mito2/src/metric_value.rs

@@ -29,7 +29,9 @@ use crate::error::{InvalidMetadataSnafu, Result};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) struct MetricValueColumn {
+    /// Index of the logical Float64 value in `RegionMetadata::column_metadatas`.
     pub(crate) value_index: usize,
+    /// Index of its Int64 companion in `RegionMetadata::column_metadatas`.
     pub(crate) int_index: usize,
 }
 

src/mito2/src/sst/parquet/metric_value_split.rs

@@ -24,11 +24,16 @@ use snafu::{OptionExt, ResultExt};
 use store_api::metadata::RegionMetadata;
 
 use crate::error::{InvalidRecordBatchSnafu, NewRecordBatchSnafu, Result};
-use crate::metric_value::metric_value_columns;
+use crate::metric_value::{MetricValueColumn, metric_value_columns};
 
+/// Maps a split metric value pair in region metadata to an Arrow batch schema.
 #[derive(Debug, Clone)]
 pub(crate) struct MetricValueSplitColumn {
+    /// The value pair's indices in region metadata.
+    pub(crate) region_column: MetricValueColumn,
+    /// Index of the Float64 value in the Arrow batch schema.
     pub(crate) float_index: usize,
+    /// Index of the Int64 companion in the Arrow batch schema.
     pub(crate) int_index: usize,
 }
 
@@ -46,6 +51,7 @@ pub(crate) fn metric_value_split_columns(
                 .ok()?;
             let int_index = arrow_schema.index_of(&int_column.column_schema.name).ok()?;
             Some(MetricValueSplitColumn {
+                region_column: column,
                 float_index,
                 int_index,
             })
@@ -293,6 +299,10 @@ mod tests {
         let batch = split_metric_value_columns(
             &batch,
             &[MetricValueSplitColumn {
+                region_column: MetricValueColumn {
+                    value_index: 0,
+                    int_index: 1,
+                },
                 float_index: 0,
                 int_index: 1,
             }],

src/mito2/src/sst/parquet/writer.rs

@@ -408,10 +408,18 @@ where
         metric_value_split_columns(region_metadata, schema)
             .into_iter()
             .fold(builder, |builder, split_column| {
-                let float_col =
-                    ColumnPath::new(vec![schema.field(split_column.float_index).name().clone()]);
-                let int_col =
-                    ColumnPath::new(vec![schema.field(split_column.int_index).name().clone()]);
+                let float_col = ColumnPath::new(vec![
+                    region_metadata.column_metadatas[split_column.region_column.value_index]
+                        .column_schema
+                        .name
+                        .clone(),
+                ]);
+                let int_col = ColumnPath::new(vec![
+                    region_metadata.column_metadatas[split_column.region_column.int_index]
+                        .column_schema
+                        .name
+                        .clone(),
+                ]);
                 builder
                     .set_column_encoding(float_col.clone(), Encoding::PLAIN)
                     .set_column_dictionary_enabled(float_col, false)