[GLUTEN-12356][VL] Preserve precision for long decimal to double cast

backends-velox/src/main/scala/org/apache/gluten/backendsapi/velox/VeloxSparkPlanExecApi.scala

@@ -1305,7 +1305,19 @@ class VeloxSparkPlanExecApi extends SparkPlanExecApi with Logging {
     // ISOControl characters, refer java.lang.Character.isISOControl(int)
     val isoControlStr = (('\u0000' to '\u001F') ++ ('\u007F' to '\u009F')).toList.mkString
     // scalastyle:on nonascii
-    if (VeloxConfig.get.castFromVarcharAddTrimNode && c.child.dataType == StringType) {
+    val isLongDecimalToDouble = c.dataType == DoubleType && (c.child.dataType match {
+      case decimalType: DecimalType => decimalType.precision > 18
+      case _ => false
+    })
+    if (isLongDecimalToDouble) {
+      // Velox converts long-decimal unscaled int128 to double before applying the scale factor,
+      // so precision can be lost. Precision above 18 uses Velox's long-decimal representation;
+      // parsing its exact string value preserves Spark's correctly rounded conversion.
+      // Use the 3-arg Cast (no evalMode) for cross-version compatibility: Cast.evalMode does
+      // not exist before Spark 3.4, and decimal-to-string is lossless regardless of eval mode,
+      // so the outer cast (kept via withNewChildren) still carries the original ANSI semantics.
+      c.withNewChildren(Seq(Cast(c.child, StringType, c.timeZoneId))).asInstanceOf[Cast]
+    } else if (VeloxConfig.get.castFromVarcharAddTrimNode && c.child.dataType == StringType) {
       val trimStr = c.dataType match {
         case BinaryType | _: ArrayType | _: MapType | _: StructType | _: UserDefinedType[_] =>
           None

backends-velox/src/test/scala/org/apache/gluten/functions/MathFunctionsValidateSuite.scala

@@ -16,12 +16,15 @@
  */
 package org.apache.gluten.functions
 
+import org.apache.gluten.backendsapi.BackendsApiManager
 import org.apache.gluten.config.GlutenConfig
 import org.apache.gluten.execution.{BatchScanExecTransformer, ProjectExecTransformer}
 
 import org.apache.spark.SparkConf
 import org.apache.spark.sql.Row
+import org.apache.spark.sql.catalyst.expressions.{AttributeReference, Cast}
 import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.types.{DecimalType, DoubleType, StringType}
 
 class MathFunctionsValidateSuiteAnsiOn extends FunctionsValidateSuite {
 
@@ -449,6 +452,91 @@ class MathFunctionsValidateSuite extends FunctionsValidateSuite {
     }
   }
 
+  test("decimal to double preserves precision after decimal division") {
+    withSQLConf(
+      "spark.sql.optimizer.excludedRules" ->
+        "org.apache.spark.sql.catalyst.optimizer.ConstantFolding",
+      "spark.sql.decimalOperations.allowPrecisionLoss" -> "false"
+    ) {
+      runQueryAndCompare(
+        "SELECT CAST(2.8 / CAST(0.0130597014925373134 AS DECIMAL(38,19)) AS DOUBLE)") {
+        checkGlutenPlan[ProjectExecTransformer]
+      }
+    }
+  }
+
+  test("GLUTEN-12356: long decimal to double rewrite applies only to precision > 18") {
+    val api = BackendsApiManager.getSparkPlanExecApiInstance
+    def castOf(precision: Int, scale: Int): Cast =
+      Cast(AttributeReference("c", DecimalType(precision, scale))(), DoubleType)
+    // Precision <= 18 keeps Velox's direct conversion.
+    assert(api.genCastWithNewChild(castOf(18, 6)).child.dataType == DecimalType(18, 6))
+    // Precision > 18 goes through the exact string representation.
+    assert(api.genCastWithNewChild(castOf(19, 0)).child.dataType == StringType)
+    assert(api.genCastWithNewChild(castOf(38, 19)).child.dataType == StringType)
+  }
+
+  test("GLUTEN-12356: direct cast of decimal to double matches vanilla Spark") {
+    withTempView("decimal_double_cast") {
+      withTempPath {
+        path =>
+          // Cover short decimal (no rewrite), long decimal at both precision bounds,
+          // zero/negative/NULL values, values around 2^53, and near-max 38-digit values.
+          spark
+            .sql("""
+                   |SELECT
+                   |  CAST(v18 AS DECIMAL(18,6)) AS d18_6,
+                   |  CAST(v19 AS DECIMAL(19,0)) AS d19_0,
+                   |  CAST(v38_0 AS DECIMAL(38,0)) AS d38_0,
+                   |  CAST(v38_19 AS DECIMAL(38,19)) AS d38_19,
+                   |  CAST(v38_38 AS DECIMAL(38,38)) AS d38_38
+                   |FROM VALUES
+                   |  ('123456789012.345678', '9007199254740991',
+                   |   '12345678901234567890123456789012345678',
+                   |   '214.4000000000000006143270301075587414',
+                   |   '0.99999999999999999999999999999999999999'),
+                   |  ('999999999999.999999', '9007199254740992',
+                   |   '9007199254740993',
+                   |   '9007199254740993.0000000000000000001',
+                   |   '0.00000000000000000000000000000000000001'),
+                   |  ('-123456789012.345678', '-9007199254740993',
+                   |   '-99999999999999999999999999999999999999',
+                   |   '-0.0130597014925373134',
+                   |   '-0.5'),
+                   |  ('0', '0', '0', '0', '0'),
+                   |  (NULL, NULL, NULL, NULL, NULL),
+                   |  ('999999999999.999999', '9999999999999999999',
+                   |   '99999999999999999999999999999999999999',
+                   |   '9999999999999999999.9999999999999999999',
+                   |   '0.12345678901234567890123456789012345678')
+                   |AS t(v18, v19, v38_0, v38_19, v38_38)
+                   |""".stripMargin)
+            .write
+            .parquet(path.getCanonicalPath)
+          spark.read.parquet(path.getCanonicalPath).createOrReplaceTempView("decimal_double_cast")
+
+          Seq("true", "false").foreach {
+            ansi =>
+              withSQLConf(
+                SQLConf.ANSI_ENABLED.key -> ansi,
+                GlutenConfig.GLUTEN_ANSI_FALLBACK_ENABLED.key -> "false") {
+                runQueryAndCompare("""
+                                     |SELECT
+                                     |  CAST(d18_6 AS DOUBLE),
+                                     |  CAST(d19_0 AS DOUBLE),
+                                     |  CAST(d38_0 AS DOUBLE),
+                                     |  CAST(d38_19 AS DOUBLE),
+                                     |  CAST(d38_38 AS DOUBLE)
+                                     |FROM decimal_double_cast
+                                     |""".stripMargin) {
+                  checkGlutenPlan[ProjectExecTransformer]
+                }
+              }
+          }
+      }
+    }
+  }
+
   testWithMinSparkVersion(
     "decimal arithmetic respects allowPrecisionLoss captured at view analysis time",
     "4.1") {