[fix](nereids) count in correlated subquery shoud not output null val…

…ue (apache#27064)

consider sql: 

SELECT * FROM t1 WHERE t1.a <= (SELECT COUNT(t2.a) FROM t2 WHERE (t1.b = t2.b));

when unnest correlated subquery, we create a left join node.
Assume outer query is left table and subquery is right one.
If there is no match, the row from right table is filled with nulls.
But COUNT function is always not nullable. 
So wrap COUNT with Nvl to ensure it's result is 0 instead of null to get the correct result

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/expression/rules/FunctionBinder.java

@@ -49,8 +49,11 @@
 import org.apache.doris.nereids.trees.expressions.WhenClause;
 import org.apache.doris.nereids.trees.expressions.functions.BoundFunction;
 import org.apache.doris.nereids.trees.expressions.functions.FunctionBuilder;
+import org.apache.doris.nereids.trees.expressions.functions.agg.Count;
 import org.apache.doris.nereids.trees.expressions.functions.scalar.Lambda;
+import org.apache.doris.nereids.trees.expressions.functions.scalar.Nvl;
 import org.apache.doris.nereids.trees.expressions.functions.udf.AliasUdfBuilder;
+import org.apache.doris.nereids.trees.expressions.literal.BigIntLiteral;
 import org.apache.doris.nereids.trees.expressions.typecoercion.ImplicitCastInputTypes;
 import org.apache.doris.nereids.types.ArrayType;
 import org.apache.doris.nereids.types.BigIntType;
@@ -167,7 +170,21 @@ public Expression visitUnboundFunction(UnboundFunction unboundFunction, Expressi
             // we do type coercion in build function in alias function, so it's ok to return directly.
             return builder.build(functionName, arguments);
         } else {
-            return TypeCoercionUtils.processBoundFunction((BoundFunction) builder.build(functionName, arguments));
+            Expression boundFunction = TypeCoercionUtils
+                    .processBoundFunction((BoundFunction) builder.build(functionName, arguments));
+            if (boundFunction instanceof Count
+                    && context.cascadesContext.getOuterScope().isPresent()
+                    && !context.cascadesContext.getOuterScope().get().getCorrelatedSlots()
+                            .isEmpty()) {
+                // consider sql: SELECT * FROM t1 WHERE t1.a <= (SELECT COUNT(t2.a) FROM t2 WHERE (t1.b = t2.b));
+                // when unnest correlated subquery, we create a left join node.
+                // outer query is left table and subquery is right one
+                // if there is no match, the row from right table is filled with nulls
+                // but COUNT function is always not nullable.
+                // so wrap COUNT with Nvl to ensure it's result is 0 instead of null to get the correct result
+                boundFunction = new Nvl(boundFunction, new BigIntLiteral(0));
+            }
+            return boundFunction;
         }
     }
 

regression-test/suites/nereids_p0/subquery/test_subquery.groovy

@@ -118,6 +118,11 @@ suite("test_subquery") {
         contains("VAGGREGATE")
     }
 
+    explain {
+        sql """SELECT * FROM table_1000_undef_undef t1 WHERE t1.pk <= (SELECT COUNT(t2.pk) FROM table_1000_undef_undef2 t2 WHERE (t1.col_bigint_undef_signed = t2.col_bigint_undef_signed)); """
+        contains("ifnull")
+    }
+
     sql """DROP TABLE IF EXISTS table_1000_undef_undef"""
     sql """DROP TABLE IF EXISTS table_1000_undef_undef2"""