Convert interval to double if it is BigDecimal for further calculation

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/expression/function/udf/SpanFunction.java

@@ -80,8 +80,12 @@ public Expression implement(
       Expression interval = translatedOperands.get(1);
 
       RelDataType fieldType = call.getOperands().get(0).getType();
+      RelDataType intervalType = call.getOperands().get(1).getType();
       RelDataType unitType = call.getOperands().get(2).getType();
 
+      if (SqlTypeUtil.isDecimal(intervalType)) {
+        interval = Expressions.call(interval, "doubleValue");
+      }
       if (SqlTypeUtil.isNull(unitType)) {
         return switch (call.getType().getSqlTypeName()) {
           case BIGINT, INTEGER, SMALLINT, TINYINT -> Expressions.multiply(