[SPARK-54750][SQL] Fix ROUND returning NULL for Decimal values with l…

### What changes were proposed in this pull request?

Fixed a bug in `RoundBase` where `ROUND` function incorrectly returns `NULL` for certain Decimal values. The issue was caused by using the input decimal's runtime precision instead of the target type's precision when calling `toPrecision()`.

For example, `ROUND(PERCENTILE_APPROX(2150 / 1000.0, 0.95), 3)` incorrectly returned `NULL` instead of `2.15`.

### Why are the changes needed?

The fix changes `decimal.toPrecision(decimal.precision, s, mode)` to `decimal.toPrecision(p, s, mode)` in both `nullSafeEval` and `doGenCode` methods, where `p` is the target DecimalType's precision.

### Does this PR introduce _any_ user-facing change?

Yes, this fixes a bug where ROUND could return NULL for valid decimal inputs.

### How was this patch tested?

Added regression test in `ApproximatePercentileQuerySuite`.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #53529 from qindongliang/SPARK-54750-fix-round-precision.

Lead-authored-by: qindongliang <[email protected]>
Co-authored-by: Kent Yao <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

Author: qindongliang

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala

@@ -1572,7 +1572,7 @@ abstract class RoundBase(child: Expression, scale: Expression,
         val decimal = input1.asInstanceOf[Decimal]
         if (_scale >= 0) {
           // Overflow cannot happen, so no need to control nullOnOverflow
-          decimal.toPrecision(decimal.precision, s, mode)
+          decimal.toPrecision(p, s, mode)
         } else {
           Decimal(decimal.toBigDecimal.setScale(_scale, mode), p, s)
         }
@@ -1644,10 +1644,9 @@ abstract class RoundBase(child: Expression, scale: Expression,
       case DecimalType.Fixed(p, s) =>
         if (_scale >= 0) {
           s"""
-            ${ev.value} = ${ce.value}.toPrecision(${ce.value}.precision(), $s,
-            Decimal.$modeStr(), true, null);
+            ${ev.value} = ${ce.value}.toPrecision($p, $s, Decimal.$modeStr(), true, null);
             ${ev.isNull} = ${ev.value} == null;"""
-       } else {
+        } else {
           s"""
             ${ev.value} = new Decimal().set(${ce.value}.toBigDecimal()
             .setScale(${_scale}, Decimal.$modeStr()), $p, $s);

sql/core/src/test/scala/org/apache/spark/sql/ApproximatePercentileQuerySuite.scala

@@ -370,4 +370,16 @@ class ApproximatePercentileQuerySuite extends QueryTest with SharedSparkSession
       context = ExpectedContext(
         "", "", 8, 40, "percentile_approx(col, NULL, 100)"))
   }
+
+  test("SPARK-54750: percentile_approx returns NULL for certain decimal values") {
+    // Regression test: ROUND(PERCENTILE_APPROX(2150/1000.0, 0.95), 3) should return 2.15
+    checkAnswer(
+      spark.sql("SELECT ROUND(PERCENTILE_APPROX(2150 / 1000.0, 0.95), 3) as p95"),
+      Row(2.15)
+    )
+    checkAnswer(
+      spark.sql("SELECT ROUND(PERCENTILE_APPROX(2151 / 1000.0, 0.95), 3) as p95"),
+      Row(2.151)
+    )
+  }
 }