From a21deb83fd71bdc0550cb00007d43645fe48b67a Mon Sep 17 00:00:00 2001
From: Alessandro Solimando <asolimando@apache.org>
Date: Fri, 2 Jan 2026 20:36:36 +0100
Subject: [PATCH] feat: improve DECIMAL type handling (#655)

---
 .../isthmus/SubstraitTypeSystem.java          | 19 ++++--
 .../isthmus/SubstraitTypeSystemTest.java      | 63 +++++++++++++++++++
 2 files changed, 78 insertions(+), 4 deletions(-)
 create mode 100644 isthmus/src/test/java/io/substrait/isthmus/SubstraitTypeSystemTest.java

diff --git a/isthmus/src/main/java/io/substrait/isthmus/SubstraitTypeSystem.java b/isthmus/src/main/java/io/substrait/isthmus/SubstraitTypeSystem.java
index 97498e457..31c40905e 100644
--- a/isthmus/src/main/java/io/substrait/isthmus/SubstraitTypeSystem.java
+++ b/isthmus/src/main/java/io/substrait/isthmus/SubstraitTypeSystem.java
@@ -35,18 +35,29 @@ public int getMaxPrecision(final SqlTypeName typeName) {
       case TIMESTAMP:
       case TIMESTAMP_WITH_LOCAL_TIME_ZONE:
         return 6;
+      case DECIMAL:
+        return 38;
     }
     return super.getMaxPrecision(typeName);
   }
 
   @Override
-  public int getMaxNumericScale() {
-    return 38;
+  public int getDefaultPrecision(final SqlTypeName typeName) {
+    switch (typeName) {
+      case DECIMAL:
+        return getMaxPrecision(typeName);
+      default:
+        return super.getDefaultPrecision(typeName);
+    }
   }
 
   @Override
-  public int getMaxNumericPrecision() {
-    return 38;
+  public int getMaxScale(final SqlTypeName typeName) {
+    switch (typeName) {
+      case DECIMAL:
+        return 38;
+    }
+    return super.getMaxScale(typeName);
   }
 
   @Override
diff --git a/isthmus/src/test/java/io/substrait/isthmus/SubstraitTypeSystemTest.java b/isthmus/src/test/java/io/substrait/isthmus/SubstraitTypeSystemTest.java
new file mode 100644
index 000000000..01e7bd0c7
--- /dev/null
+++ b/isthmus/src/test/java/io/substrait/isthmus/SubstraitTypeSystemTest.java
@@ -0,0 +1,63 @@
+package io.substrait.isthmus;
+
+import static io.substrait.isthmus.SubstraitTypeSystem.TYPE_FACTORY;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.apache.calcite.rel.type.RelDataType;
+import org.apache.calcite.rel.type.RelDataTypeSystem;
+import org.apache.calcite.sql.type.SqlTypeName;
+import org.junit.jupiter.api.Test;
+
+class SubstraitTypeSystemTest {
+
+  private final RelDataTypeSystem typeSystem = SubstraitTypeSystem.TYPE_SYSTEM;
+
+  @Test
+  void decimalMaxPrecision() {
+    assertEquals(38, typeSystem.getMaxPrecision(SqlTypeName.DECIMAL));
+  }
+
+  @Test
+  void decimalMaxScale() {
+    assertEquals(38, typeSystem.getMaxScale(SqlTypeName.DECIMAL));
+  }
+
+  @Test
+  void decimalDefaultPrecision() {
+    assertEquals(38, typeSystem.getDefaultPrecision(SqlTypeName.DECIMAL));
+  }
+
+  @Test
+  void decimalDefaultScale() {
+    assertEquals(0, typeSystem.getDefaultScale(SqlTypeName.DECIMAL));
+  }
+
+  @Test
+  void timestampMaxPrecision() {
+    assertEquals(6, typeSystem.getMaxPrecision(SqlTypeName.TIMESTAMP));
+  }
+
+  @Test
+  void timeMaxPrecision() {
+    assertEquals(6, typeSystem.getMaxPrecision(SqlTypeName.TIME));
+  }
+
+  @Test
+  void canCreateDecimalWithMaxPrecision() {
+    RelDataType decimalType = TYPE_FACTORY.createSqlType(SqlTypeName.DECIMAL, 38, 10);
+    assertEquals(38, decimalType.getPrecision());
+    assertEquals(10, decimalType.getScale());
+  }
+
+  @Test
+  void decimalMaxPrecisionAndScaleDifferentFromDefaultTypeSystem() {
+    RelDataTypeSystem defaultTypeSystem = RelDataTypeSystem.DEFAULT;
+    int defaultMaxPrecision = defaultTypeSystem.getMaxPrecision(SqlTypeName.DECIMAL);
+    int defaultMaxScale = defaultTypeSystem.getMaxScale(SqlTypeName.DECIMAL);
+
+    assertEquals(19, defaultMaxPrecision);
+    assertEquals(19, defaultMaxScale);
+    assertEquals(38, typeSystem.getMaxPrecision(SqlTypeName.DECIMAL));
+    assertEquals(38, typeSystem.getMaxScale(SqlTypeName.DECIMAL));
+  }
+}