[SPARK-53868][SQL] Only use signature with Expression[] of visitAggregateFunction in V2ExpressionSQBuilder

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

#50143 This PR introduced `visitAggregateFunction` with `inputs: Array[Expression]`, but it's not the only usage of `visitAggregateFunction` for example here
https://github.com/apache/spark/blob/6eb4d3c9d38f6849b0acfcffdbadce03c8f49ac6/sql/catalyst/src/main/java/org/apache/spark/sql/connector/util/V2ExpressionSQLBuilder.java#L134
the old API is still used but it no longer checked if the function is supported.

This means that if some dialect did not support one of `(MIN, MAX, COUNT, SUM, AVG)` it would not be blocked, but as of now all the dialects have them in the `supportedFunctions` so this is not a behavioral change.

### Why are the changes needed?

To unify the API in case in the future if some dialect does not support an aggregate function of `(MIN, MAX, COUNT, SUM, AVG)` it should be blocked.
### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

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

No

Closes #52573 from alekjarmov/block-aggregates.

Lead-authored-by: alekjarmov <[email protected]>
Co-authored-by: Alek Jarmov <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: 2 people

sql/catalyst/src/main/java/org/apache/spark/sql/connector/util/V2ExpressionSQLBuilder.java

@@ -131,22 +131,17 @@ yield visitBinaryArithmetic(
         default -> visitUnexpectedExpr(expr);
       };
     } else if (expr instanceof Min min) {
-      return visitAggregateFunction("MIN", false,
-        expressionsToStringArray(min.children()));
+      return visitAggregateFunction("MIN", false, min.children());
     } else if (expr instanceof Max max) {
-      return visitAggregateFunction("MAX", false,
-        expressionsToStringArray(max.children()));
+      return visitAggregateFunction("MAX", false, max.children());
     } else if (expr instanceof Count count) {
-      return visitAggregateFunction("COUNT", count.isDistinct(),
-        expressionsToStringArray(count.children()));
+      return visitAggregateFunction("COUNT", count.isDistinct(), count.children());
     } else if (expr instanceof Sum sum) {
-      return visitAggregateFunction("SUM", sum.isDistinct(),
-        expressionsToStringArray(sum.children()));
-    } else if (expr instanceof CountStar) {
-      return visitAggregateFunction("COUNT", false, new String[]{"*"});
+      return visitAggregateFunction("SUM", sum.isDistinct(), sum.children());
+    } else if (expr instanceof CountStar countStar) {
+      return visitAggregateFunction("COUNT", false, countStar.children());
     } else if (expr instanceof Avg avg) {
-      return visitAggregateFunction("AVG", avg.isDistinct(),
-        expressionsToStringArray(avg.children()));
+      return visitAggregateFunction("AVG", avg.isDistinct(), avg.children());
     } else if (expr instanceof GeneralAggregateFunc f) {
       if (f.orderingWithinGroups().length == 0) {
         return visitAggregateFunction(f.name(), f.isDistinct(), f.children());
@@ -282,8 +277,19 @@ protected String visitSQLFunction(String funcName, String[] inputs) {
     return joinArrayToString(inputs, ", ", funcName + "(", ")");
   }
 
+  /**
+   * Builds SQL for an aggregate function.
+   *
+   * In V2ExpressionSQLBuilder, always use this override (with Expression[])
+   * instead of the String[] version, as the String[] version does not validate
+   * whether the function is supported in JDBC dialects.
+   */
   protected String visitAggregateFunction(
       String funcName, boolean isDistinct, Expression[] inputs) {
+    // CountStar has no children but should return with a star
+    if (funcName.equals("COUNT") && inputs == Expression.EMPTY_EXPRESSION) {
+      return visitAggregateFunction(funcName, isDistinct, new String[]{"*"});
+    }
     return visitAggregateFunction(funcName, isDistinct, expressionsToStringArray(inputs));
   }