From 9355abe256d3e389d343fad52a802ca94d8ee6cd Mon Sep 17 00:00:00 2001
From: starocean999 <12095047@qq.com>
Date: Wed, 15 Nov 2023 20:27:36 +0800
Subject: [PATCH] [fix](nereids)count in correlated subquery shoud not output
 null value

---
 .../expression/rules/FunctionBinder.java      | 19 ++++++++++++++++++-
 .../nereids_p0/subquery/test_subquery.groovy  |  5 +++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/fe/fe-core/src/main/java/org/apache/doris/nereids/rules/expression/rules/FunctionBinder.java b/fe/fe-core/src/main/java/org/apache/doris/nereids/rules/expression/rules/FunctionBinder.java
index ac6a83f9cf4680..30e36c273fe445 100644
--- a/fe/fe-core/src/main/java/org/apache/doris/nereids/rules/expression/rules/FunctionBinder.java
+++ b/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;
         }
     }
 
diff --git a/regression-test/suites/nereids_p0/subquery/test_subquery.groovy b/regression-test/suites/nereids_p0/subquery/test_subquery.groovy
index 01e347031f82ef..7170fd20d57019 100644
--- a/regression-test/suites/nereids_p0/subquery/test_subquery.groovy
+++ b/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"""