alamb commented on code in PR #2200:
URL: https://github.com/apache/arrow-datafusion/pull/2200#discussion_r849772825


##########
datafusion/core/tests/sql/decimal.rs:
##########
@@ -0,0 +1,428 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use super::*;
+
+#[tokio::test]
+async fn decimal_cast() -> Result<()> {
+    let ctx = SessionContext::new();
+    let sql = "select cast(1.23 as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------+",
+        "| CAST(Float64(1.23) AS Decimal(10, 4)) |",
+        "+---------------------------------------+",
+        "| 1.2300                                |",
+        "+---------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(cast(1.23 as decimal(10,3)) as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------------------------------+",
+        "| CAST(CAST(Float64(1.23) AS Decimal(10, 3)) AS Decimal(10, 4)) |",
+        "+---------------------------------------------------------------+",
+        "| 1.2300                                                        |",
+        "+---------------------------------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(1.2345 as decimal(24,2))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(24, 2),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+-----------------------------------------+",
+        "| CAST(Float64(1.2345) AS Decimal(24, 2)) |",
+        "+-----------------------------------------+",
+        "| 1.23                                    |",
+        "+-----------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_sql() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "SELECT c1 from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000010 |",
+        "| 0.000020 |",
+        "| 0.000020 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_filter() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "select c1 from decimal_simple where c1 > 0.000030";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 > c5";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "| 0.000030 | 0.000000000003 | 5  | true  | 0.0000110 |",
+        "| 0.000050 | 0.000000000005 | 8  | false | 0.0000330 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_agg_function() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // min
+    let sql = "select min(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;

Review Comment:
   For anyone else following along, `decimal_group_function` covers grouping by 
decimal columns 👍 
   
   Something like
   ```sql
   select max(c1) from decimal_simple group by c2;
   ```
   
   



##########
datafusion/core/tests/sql/decimal.rs:
##########
@@ -0,0 +1,428 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use super::*;
+
+#[tokio::test]
+async fn decimal_cast() -> Result<()> {
+    let ctx = SessionContext::new();
+    let sql = "select cast(1.23 as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------+",
+        "| CAST(Float64(1.23) AS Decimal(10, 4)) |",
+        "+---------------------------------------+",
+        "| 1.2300                                |",
+        "+---------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(cast(1.23 as decimal(10,3)) as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------------------------------+",
+        "| CAST(CAST(Float64(1.23) AS Decimal(10, 3)) AS Decimal(10, 4)) |",
+        "+---------------------------------------------------------------+",
+        "| 1.2300                                                        |",
+        "+---------------------------------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(1.2345 as decimal(24,2))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(24, 2),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+-----------------------------------------+",
+        "| CAST(Float64(1.2345) AS Decimal(24, 2)) |",
+        "+-----------------------------------------+",
+        "| 1.23                                    |",
+        "+-----------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_sql() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "SELECT c1 from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000010 |",
+        "| 0.000020 |",
+        "| 0.000020 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_filter() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "select c1 from decimal_simple where c1 > 0.000030";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 > c5";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "| 0.000030 | 0.000000000003 | 5  | true  | 0.0000110 |",
+        "| 0.000050 | 0.000000000005 | 8  | false | 0.0000330 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_agg_function() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // min
+    let sql = "select min(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MIN(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000020               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    // max
+    let sql = "select max(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MAX(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000050               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // sum
+    let sql = "select sum(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    // inferred precision is 10+10
+    assert_eq!(
+        &DataType::Decimal(20, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| SUM(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000550               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // avg
+    let sql = "select avg(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(14, 10),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| AVG(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.0000366666           |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_logic_op() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // logic operation: eq
+    let sql = "select * from decimal_simple where c1=0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),

Review Comment:
   is there a reason this test only checks the schema of `c1` and not `c2`? 
   
   It also isn't clear to me why this test is checkout the output schema but 
`decimal_by_filter` isn't (not that this is a problem I am just curious)



##########
datafusion/core/tests/sql/decimal.rs:
##########
@@ -0,0 +1,428 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use super::*;
+
+#[tokio::test]
+async fn decimal_cast() -> Result<()> {
+    let ctx = SessionContext::new();
+    let sql = "select cast(1.23 as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------+",
+        "| CAST(Float64(1.23) AS Decimal(10, 4)) |",
+        "+---------------------------------------+",
+        "| 1.2300                                |",
+        "+---------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(cast(1.23 as decimal(10,3)) as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------------------------------+",
+        "| CAST(CAST(Float64(1.23) AS Decimal(10, 3)) AS Decimal(10, 4)) |",
+        "+---------------------------------------------------------------+",
+        "| 1.2300                                                        |",
+        "+---------------------------------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(1.2345 as decimal(24,2))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(24, 2),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+-----------------------------------------+",
+        "| CAST(Float64(1.2345) AS Decimal(24, 2)) |",
+        "+-----------------------------------------+",
+        "| 1.23                                    |",
+        "+-----------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_sql() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "SELECT c1 from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000010 |",
+        "| 0.000020 |",
+        "| 0.000020 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_filter() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "select c1 from decimal_simple where c1 > 0.000030";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 > c5";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "| 0.000030 | 0.000000000003 | 5  | true  | 0.0000110 |",
+        "| 0.000050 | 0.000000000005 | 8  | false | 0.0000330 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_agg_function() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // min
+    let sql = "select min(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MIN(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000020               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    // max
+    let sql = "select max(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MAX(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000050               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // sum
+    let sql = "select sum(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    // inferred precision is 10+10
+    assert_eq!(
+        &DataType::Decimal(20, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| SUM(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000550               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // avg
+    let sql = "select avg(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(14, 10),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| AVG(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.0000366666           |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_logic_op() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // logic operation: eq
+    let sql = "select * from decimal_simple where c1=0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 2  | true  | 0.0000250 |",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: not eq
+    let sql = "select c2,c3 from decimal_simple where c1!=0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------------+-----+",
+        "| c2             | c3  |",
+        "+----------------+-----+",
+        "| 0.000000000001 | 1   |",
+        "| 0.000000000003 | 4   |",
+        "| 0.000000000003 | 5   |",
+        "| 0.000000000003 | 5   |",
+        "| 0.000000000004 | 5   |",
+        "| 0.000000000004 | 12  |",
+        "| 0.000000000004 | 14  |",
+        "| 0.000000000004 | 8   |",
+        "| 0.000000000005 | 9   |",
+        "| 0.000000000005 | 4   |",
+        "| 0.000000000005 | 8   |",
+        "| 0.000000000005 | 100 |",
+        "| 0.000000000005 | 1   |",
+        "+----------------+-----+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    // logic operation: lt
+    let sql = "select * from decimal_simple where c1 < 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+------+-----------+",
+        "| c1       | c2             | c3 | c4   | c5        |",
+        "+----------+----------------+----+------+-----------+",
+        "| 0.000010 | 0.000000000001 | 1  | true | 0.0000140 |",
+        "+----------+----------------+----+------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: lteq
+    let sql = "select * from decimal_simple where c1 <= 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000010 | 0.000000000001 | 1  | true  | 0.0000140 |",
+        "| 0.000020 | 0.000000000002 | 2  | true  | 0.0000250 |",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: gt
+    let sql = "select * from decimal_simple where c1 > 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+-----+-------+-----------+",
+        "| c1       | c2             | c3  | c4    | c5        |",
+        "+----------+----------------+-----+-------+-----------+",
+        "| 0.000030 | 0.000000000003 | 4   | true  | 0.0000320 |",
+        "| 0.000030 | 0.000000000003 | 5   | false | 0.0000350 |",
+        "| 0.000030 | 0.000000000003 | 5   | true  | 0.0000110 |",
+        "| 0.000040 | 0.000000000004 | 5   | true  | 0.0000440 |",
+        "| 0.000040 | 0.000000000004 | 12  | false | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 14  | true  | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 8   | false | 0.0000440 |",
+        "| 0.000050 | 0.000000000005 | 9   | true  | 0.0000520 |",
+        "| 0.000050 | 0.000000000005 | 4   | true  | 0.0000780 |",
+        "| 0.000050 | 0.000000000005 | 8   | false | 0.0000330 |",
+        "| 0.000050 | 0.000000000005 | 100 | true  | 0.0000680 |",
+        "| 0.000050 | 0.000000000005 | 1   | false | 0.0001000 |",
+        "+----------+----------------+-----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: gteq

Review Comment:
   Is it worth checking somewhere what happens with different decimal scales?
   
   Something like
   ```sql 
   select * from decimal_simple where c1 >= CAST(0.00002 as Decimal(10,8))
   ```
   
   I am not sure if the coercion to an appropriate decimal type is covered 
elsewhere



##########
datafusion/core/tests/sql/decimal.rs:
##########
@@ -0,0 +1,428 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use super::*;
+
+#[tokio::test]
+async fn decimal_cast() -> Result<()> {
+    let ctx = SessionContext::new();
+    let sql = "select cast(1.23 as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------+",
+        "| CAST(Float64(1.23) AS Decimal(10, 4)) |",
+        "+---------------------------------------+",
+        "| 1.2300                                |",
+        "+---------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(cast(1.23 as decimal(10,3)) as decimal(10,4))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 4),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+---------------------------------------------------------------+",
+        "| CAST(CAST(Float64(1.23) AS Decimal(10, 3)) AS Decimal(10, 4)) |",
+        "+---------------------------------------------------------------+",
+        "| 1.2300                                                        |",
+        "+---------------------------------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select cast(1.2345 as decimal(24,2))";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(24, 2),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+-----------------------------------------+",
+        "| CAST(Float64(1.2345) AS Decimal(24, 2)) |",
+        "+-----------------------------------------+",
+        "| 1.23                                    |",
+        "+-----------------------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_sql() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "SELECT c1 from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000010 |",
+        "| 0.000020 |",
+        "| 0.000020 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000030 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_by_filter() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "select c1 from decimal_simple where c1 > 0.000030";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+",
+        "| c1       |",
+        "+----------+",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000040 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "| 0.000050 |",
+        "+----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 > c5";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "| 0.000030 | 0.000000000003 | 5  | true  | 0.0000110 |",
+        "| 0.000050 | 0.000000000005 | 8  | false | 0.0000330 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_agg_function() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // min
+    let sql = "select min(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MIN(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000020               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    // max
+    let sql = "select max(c1) from decimal_simple where c4=false";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| MAX(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000050               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // sum
+    let sql = "select sum(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    // inferred precision is 10+10
+    assert_eq!(
+        &DataType::Decimal(20, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| SUM(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.000550               |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // avg
+    let sql = "select avg(c1) from decimal_simple";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(14, 10),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+------------------------+",
+        "| AVG(decimal_simple.c1) |",
+        "+------------------------+",
+        "| 0.0000366666           |",
+        "+------------------------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_logic_op() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    // logic operation: eq
+    let sql = "select * from decimal_simple where c1=0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    assert_eq!(
+        &DataType::Decimal(10, 6),
+        actual[0].schema().field(0).data_type()
+    );
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 2  | true  | 0.0000250 |",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: not eq
+    let sql = "select c2,c3 from decimal_simple where c1!=0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------------+-----+",
+        "| c2             | c3  |",
+        "+----------------+-----+",
+        "| 0.000000000001 | 1   |",
+        "| 0.000000000003 | 4   |",
+        "| 0.000000000003 | 5   |",
+        "| 0.000000000003 | 5   |",
+        "| 0.000000000004 | 5   |",
+        "| 0.000000000004 | 12  |",
+        "| 0.000000000004 | 14  |",
+        "| 0.000000000004 | 8   |",
+        "| 0.000000000005 | 9   |",
+        "| 0.000000000005 | 4   |",
+        "| 0.000000000005 | 8   |",
+        "| 0.000000000005 | 100 |",
+        "| 0.000000000005 | 1   |",
+        "+----------------+-----+",
+    ];
+    assert_batches_eq!(expected, &actual);
+    // logic operation: lt
+    let sql = "select * from decimal_simple where c1 < 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+------+-----------+",
+        "| c1       | c2             | c3 | c4   | c5        |",
+        "+----------+----------------+----+------+-----------+",
+        "| 0.000010 | 0.000000000001 | 1  | true | 0.0000140 |",
+        "+----------+----------------+----+------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: lteq
+    let sql = "select * from decimal_simple where c1 <= 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+----+-------+-----------+",
+        "| c1       | c2             | c3 | c4    | c5        |",
+        "+----------+----------------+----+-------+-----------+",
+        "| 0.000010 | 0.000000000001 | 1  | true  | 0.0000140 |",
+        "| 0.000020 | 0.000000000002 | 2  | true  | 0.0000250 |",
+        "| 0.000020 | 0.000000000002 | 3  | false | 0.0000190 |",
+        "+----------+----------------+----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: gt
+    let sql = "select * from decimal_simple where c1 > 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+-----+-------+-----------+",
+        "| c1       | c2             | c3  | c4    | c5        |",
+        "+----------+----------------+-----+-------+-----------+",
+        "| 0.000030 | 0.000000000003 | 4   | true  | 0.0000320 |",
+        "| 0.000030 | 0.000000000003 | 5   | false | 0.0000350 |",
+        "| 0.000030 | 0.000000000003 | 5   | true  | 0.0000110 |",
+        "| 0.000040 | 0.000000000004 | 5   | true  | 0.0000440 |",
+        "| 0.000040 | 0.000000000004 | 12  | false | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 14  | true  | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 8   | false | 0.0000440 |",
+        "| 0.000050 | 0.000000000005 | 9   | true  | 0.0000520 |",
+        "| 0.000050 | 0.000000000005 | 4   | true  | 0.0000780 |",
+        "| 0.000050 | 0.000000000005 | 8   | false | 0.0000330 |",
+        "| 0.000050 | 0.000000000005 | 100 | true  | 0.0000680 |",
+        "| 0.000050 | 0.000000000005 | 1   | false | 0.0001000 |",
+        "+----------+----------------+-----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    // logic operation: gteq
+    let sql = "select * from decimal_simple where c1 >= 0.00002";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+-----+-------+-----------+",
+        "| c1       | c2             | c3  | c4    | c5        |",
+        "+----------+----------------+-----+-------+-----------+",
+        "| 0.000020 | 0.000000000002 | 2   | true  | 0.0000250 |",
+        "| 0.000020 | 0.000000000002 | 3   | false | 0.0000190 |",
+        "| 0.000030 | 0.000000000003 | 4   | true  | 0.0000320 |",
+        "| 0.000030 | 0.000000000003 | 5   | false | 0.0000350 |",
+        "| 0.000030 | 0.000000000003 | 5   | true  | 0.0000110 |",
+        "| 0.000040 | 0.000000000004 | 5   | true  | 0.0000440 |",
+        "| 0.000040 | 0.000000000004 | 12  | false | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 14  | true  | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 8   | false | 0.0000440 |",
+        "| 0.000050 | 0.000000000005 | 9   | true  | 0.0000520 |",
+        "| 0.000050 | 0.000000000005 | 4   | true  | 0.0000780 |",
+        "| 0.000050 | 0.000000000005 | 8   | false | 0.0000330 |",
+        "| 0.000050 | 0.000000000005 | 100 | true  | 0.0000680 |",
+        "| 0.000050 | 0.000000000005 | 1   | false | 0.0001000 |",
+        "+----------+----------------+-----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn decimal_sort() -> Result<()> {
+    let ctx = SessionContext::new();
+    register_decimal_csv_table_by_sql(&ctx).await;
+    let sql = "select * from decimal_simple where c1 >= 0.00004 order by c1";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+-----+-------+-----------+",
+        "| c1       | c2             | c3  | c4    | c5        |",
+        "+----------+----------------+-----+-------+-----------+",
+        "| 0.000040 | 0.000000000004 | 5   | true  | 0.0000440 |",
+        "| 0.000040 | 0.000000000004 | 12  | false | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 14  | true  | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 8   | false | 0.0000440 |",
+        "| 0.000050 | 0.000000000005 | 9   | true  | 0.0000520 |",
+        "| 0.000050 | 0.000000000005 | 4   | true  | 0.0000780 |",
+        "| 0.000050 | 0.000000000005 | 8   | false | 0.0000330 |",
+        "| 0.000050 | 0.000000000005 | 100 | true  | 0.0000680 |",
+        "| 0.000050 | 0.000000000005 | 1   | false | 0.0001000 |",
+        "+----------+----------------+-----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 >= 0.00004 order by c1 
desc";
+    let actual = execute_to_batches(&ctx, sql).await;
+    let expected = vec![
+        "+----------+----------------+-----+-------+-----------+",
+        "| c1       | c2             | c3  | c4    | c5        |",
+        "+----------+----------------+-----+-------+-----------+",
+        "| 0.000050 | 0.000000000005 | 9   | true  | 0.0000520 |",
+        "| 0.000050 | 0.000000000005 | 4   | true  | 0.0000780 |",
+        "| 0.000050 | 0.000000000005 | 8   | false | 0.0000330 |",
+        "| 0.000050 | 0.000000000005 | 100 | true  | 0.0000680 |",
+        "| 0.000050 | 0.000000000005 | 1   | false | 0.0001000 |",
+        "| 0.000040 | 0.000000000004 | 5   | true  | 0.0000440 |",
+        "| 0.000040 | 0.000000000004 | 12  | false | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 14  | true  | 0.0000400 |",
+        "| 0.000040 | 0.000000000004 | 8   | false | 0.0000440 |",
+        "+----------+----------------+-----+-------+-----------+",
+    ];
+    assert_batches_eq!(expected, &actual);
+
+    let sql = "select * from decimal_simple where c1 < 0.00003 order by c1 
desc,c4";

Review Comment:
   👍 



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