From 7465a164e1112b01c0c6642fdfb25602669a0b52 Mon Sep 17 00:00:00 2001
From: Gabriel <45515538+gabotechs@users.noreply.github.com>
Date: Wed, 28 Jan 2026 12:45:05 +0100
Subject: [PATCH] Fix constant value from stats (#20042)

## Which issue does this PR close?

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- Closes https://github.com/apache/datafusion/issues/20041

## Rationale for this change

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Why are you proposing this change? If this is already explained clearly
in the issue then this section is not needed.
Explaining clearly why changes are proposed helps reviewers understand
your changes and offer better suggestions for fixes.
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Fix a bug that appeared when reading constant values from stats

## What changes are included in this PR?

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There is no need to duplicate the description in the issue here but it
is sometimes worth providing a summary of the individual changes in this
PR.
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- A 2-line bug fix
- A test that reproduces the issue on `main` and succeeds in this PR

## Are these changes tested?

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Yes, by a new unit test.

Additionally, I tested this changes with
https://github.com/datafusion-contrib/datafusion-distributed and the
problem is solved also there.

## Are there any user-facing changes?

Nothing besides the bug fix

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---
 .../src/datasource/physical_plan/parquet.rs   | 48 +++++++++++++++++--
 datafusion/datasource-parquet/src/opener.rs   |  4 ++
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/datafusion/core/src/datasource/physical_plan/parquet.rs b/datafusion/core/src/datasource/physical_plan/parquet.rs
index 4703b55ecc0de..dde40cc0603b4 100644
--- a/datafusion/core/src/datasource/physical_plan/parquet.rs
+++ b/datafusion/core/src/datasource/physical_plan/parquet.rs
@@ -38,10 +38,10 @@ mod tests {
     use crate::prelude::{ParquetReadOptions, SessionConfig, SessionContext};
     use crate::test::object_store::local_unpartitioned_file;
     use arrow::array::{
-        ArrayRef, AsArray, Date64Array, Int8Array, Int32Array, Int64Array, StringArray,
-        StringViewArray, StructArray, TimestampNanosecondArray,
+        ArrayRef, AsArray, Date64Array, DictionaryArray, Int8Array, Int32Array,
+        Int64Array, StringArray, StringViewArray, StructArray, TimestampNanosecondArray,
     };
-    use arrow::datatypes::{DataType, Field, Fields, Schema, SchemaBuilder};
+    use arrow::datatypes::{DataType, Field, Fields, Schema, SchemaBuilder, UInt16Type};
     use arrow::record_batch::RecordBatch;
     use arrow::util::pretty::pretty_format_batches;
     use arrow_schema::{SchemaRef, TimeUnit};
@@ -2249,6 +2249,48 @@ mod tests {
         Ok(())
     }
 
+    /// Tests that constant dictionary columns (where min == max in statistics)
+    /// are correctly handled. This reproduced a bug where the constant value
+    /// from statistics had type Utf8 but the schema expected Dictionary.
+    #[tokio::test]
+    async fn test_constant_dictionary_column_parquet() -> Result<()> {
+        let tmp_dir = TempDir::new()?;
+        let path = tmp_dir.path().to_str().unwrap().to_string() + "/test.parquet";
+
+        // Write parquet with dictionary column where all values are the same
+        let schema = Arc::new(Schema::new(vec![Field::new(
+            "status",
+            DataType::Dictionary(Box::new(DataType::UInt16), Box::new(DataType::Utf8)),
+            false,
+        )]));
+        let status: DictionaryArray<UInt16Type> =
+            vec!["active", "active"].into_iter().collect();
+        let batch = RecordBatch::try_new(schema.clone(), vec![Arc::new(status)])?;
+        let file = File::create(&path)?;
+        let props = WriterProperties::builder()
+            .set_statistics_enabled(parquet::file::properties::EnabledStatistics::Page)
+            .build();
+        let mut writer = ArrowWriter::try_new(file, schema, Some(props))?;
+        writer.write(&batch)?;
+        writer.close()?;
+
+        // Query the constant dictionary column
+        let ctx = SessionContext::new();
+        ctx.register_parquet("t", &path, ParquetReadOptions::default())
+            .await?;
+        let result = ctx.sql("SELECT status FROM t").await?.collect().await?;
+
+        insta::assert_snapshot!(batches_to_string(&result),@r"
+        +--------+
+        | status |
+        +--------+
+        | active |
+        | active |
+        +--------+
+        ");
+        Ok(())
+    }
+
     fn write_file(file: &String) {
         let struct_fields = Fields::from(vec![
             Field::new("id", DataType::Int64, false),
diff --git a/datafusion/datasource-parquet/src/opener.rs b/datafusion/datasource-parquet/src/opener.rs
index 83bdf79c8fcc0..719a3afc764f8 100644
--- a/datafusion/datasource-parquet/src/opener.rs
+++ b/datafusion/datasource-parquet/src/opener.rs
@@ -696,6 +696,10 @@ fn constant_value_from_stats(
         && !min.is_null()
         && matches!(column_stats.null_count, Precision::Exact(0))
     {
+        // Cast to the expected data type if needed (e.g., Utf8 -> Dictionary)
+        if min.data_type() != *data_type {
+            return min.cast_to(data_type).ok();
+        }
         return Some(min.clone());
     }