diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
index 73be7902b998e..4c41042c53853 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
@@ -1215,6 +1215,15 @@ object InferFiltersFromConstraints extends Rule[LogicalPlan]
     }
   }
 
+  // Whether the result of this expression may be null. For example: CAST(strCol AS double)
+  // We will infer an IsNotNull expression for this expression to avoid skew join.
+  private def resultMayBeNull(exp: Expression): Boolean = exp match {
+    case e if !e.nullable => false
+    case Cast(child: Attribute, dataType, _, _) => !Cast.canUpCast(child.dataType, dataType)
+    case c: Coalesce if c.children.forall(_.isInstanceOf[Attribute]) => true
+    case _ => false
+  }
+
   private def inferFilters(plan: LogicalPlan): LogicalPlan = plan.transformWithPruning(
     _.containsAnyPattern(FILTER, JOIN)) {
     case filter @ Filter(condition, child) =>
@@ -1227,25 +1236,42 @@ object InferFiltersFromConstraints extends Rule[LogicalPlan]
       }
 
     case join @ Join(left, right, joinType, conditionOpt, _) =>
+      val leftKeys = new mutable.HashSet[Expression]
+      val rightKeys = new mutable.HashSet[Expression]
+      conditionOpt.foreach { condition =>
+        splitConjunctivePredicates(condition).foreach {
+          case EqualTo(l, r) =>
+            if (resultMayBeNull(l)) {
+              if (canEvaluate(l, left)) leftKeys.add(l)
+              if (canEvaluate(l, right)) rightKeys.add(l)
+            }
+            if (resultMayBeNull(r)) {
+              if (canEvaluate(r, left)) leftKeys.add(r)
+              if (canEvaluate(r, right)) rightKeys.add(r)
+            }
+          case _ =>
+        }
+      }
+
       joinType match {
         // For inner join, we can infer additional filters for both sides. LeftSemi is kind of an
         // inner join, it just drops the right side in the final output.
         case _: InnerLike | LeftSemi =>
           val allConstraints = getAllConstraints(left, right, conditionOpt)
-          val newLeft = inferNewFilter(left, allConstraints)
-          val newRight = inferNewFilter(right, allConstraints)
+          val newLeft = inferNewFilter(left, allConstraints, leftKeys)
+          val newRight = inferNewFilter(right, allConstraints, rightKeys)
           join.copy(left = newLeft, right = newRight)
 
         // For right outer join, we can only infer additional filters for left side.
         case RightOuter =>
           val allConstraints = getAllConstraints(left, right, conditionOpt)
-          val newLeft = inferNewFilter(left, allConstraints)
+          val newLeft = inferNewFilter(left, allConstraints, leftKeys)
           join.copy(left = newLeft)
 
         // For left join, we can only infer additional filters for right side.
         case LeftOuter | LeftAnti =>
           val allConstraints = getAllConstraints(left, right, conditionOpt)
-          val newRight = inferNewFilter(right, allConstraints)
+          val newRight = inferNewFilter(right, allConstraints, rightKeys)
           join.copy(right = newRight)
 
         case _ => join
@@ -1261,9 +1287,13 @@ object InferFiltersFromConstraints extends Rule[LogicalPlan]
     baseConstraints.union(inferAdditionalConstraints(baseConstraints))
   }
 
-  private def inferNewFilter(plan: LogicalPlan, constraints: ExpressionSet): LogicalPlan = {
+  private def inferNewFilter(
+      plan: LogicalPlan,
+      constraints: ExpressionSet,
+      joinKeys: mutable.HashSet[Expression]): LogicalPlan = {
     val newPredicates = constraints
       .union(constructIsNotNullConstraints(constraints, plan.output))
+      .union(ExpressionSet(joinKeys.map(IsNotNull)))
       .filter { c =>
         c.references.nonEmpty && c.references.subsetOf(plan.outputSet) && c.deterministic
       } -- plan.constraints
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala
index 72ad6ca24c1f1..3d5d2ff37ecf7 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala
@@ -271,12 +271,15 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
     val originalLeft = testRelation1.subquery('left)
     val originalRight = testRelation2.where('b === 1L).subquery('right)
 
-    val left = testRelation1.where(IsNotNull('a) && 'a.cast(LongType) === 1L).subquery('left)
-    val right = testRelation2.where(IsNotNull('b) && 'b === 1L).subquery('right)
-
     Seq(Some("left.a".attr.cast(LongType) === "right.b".attr),
       Some("right.b".attr === "left.a".attr.cast(LongType))).foreach { condition =>
-      testConstraintsAfterJoin(originalLeft, originalRight, left, right, Inner, condition)
+      testConstraintsAfterJoin(
+        originalLeft,
+        originalRight,
+        testRelation1.where(IsNotNull('a) && 'a.cast(LongType) === 1L).subquery('left),
+        testRelation2.where(IsNotNull('b) && 'b === 1L).subquery('right),
+        Inner,
+        condition)
     }
 
     Seq(Some("left.a".attr === "right.b".attr.cast(IntegerType)),
@@ -285,7 +288,8 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
         originalLeft,
         originalRight,
         testRelation1.where(IsNotNull('a)).subquery('left),
-        right,
+        testRelation2.where(IsNotNull('b) && IsNotNull('b.cast(IntegerType)) &&
+          'b === 1L).subquery('right),
         Inner,
         condition)
     }
@@ -302,7 +306,13 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
 
     Seq(Some("left.a".attr.cast(LongType) === "right.b".attr),
       Some("right.b".attr === "left.a".attr.cast(LongType))).foreach { condition =>
-      testConstraintsAfterJoin(originalLeft, originalRight, left, right, Inner, condition)
+      testConstraintsAfterJoin(
+        originalLeft,
+        originalRight,
+        testRelation1.where(IsNotNull('a) && 'a === 1).subquery('left),
+        testRelation2.where(IsNotNull('b)).subquery('right),
+        Inner,
+        condition)
     }
 
     Seq(Some("left.a".attr === "right.b".attr.cast(IntegerType)),
@@ -310,8 +320,9 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
       testConstraintsAfterJoin(
         originalLeft,
         originalRight,
-        left,
-        testRelation2.where(IsNotNull('b) && 'b.attr.cast(IntegerType) === 1).subquery('right),
+        testRelation1.where(IsNotNull('a) && 'a === 1).subquery('left),
+        testRelation2.where(IsNotNull('b) && IsNotNull('b.cast(IntegerType)) &&
+          'b.attr.cast(IntegerType) === 1).subquery('right),
         Inner,
         condition)
     }
@@ -361,4 +372,32 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
     val optimized = Optimize.execute(originalQuery)
     comparePlans(optimized, correctAnswer)
   }
+
+  test("SPARK-31809: Infer IsNotNull for join condition") {
+    val testRelation2 = LocalRelation('a.string, 'b.int)
+
+    testConstraintsAfterJoin(
+      testRelation.subquery('left),
+      testRelation2.subquery('right),
+      testRelation.where(IsNotNull('a)).subquery('left),
+      testRelation2.where(IsNotNull('a.cast(IntegerType)) && IsNotNull('a)).subquery('right),
+      Inner,
+      Some("left.a".attr === "right.a".attr))
+
+    testConstraintsAfterJoin(
+      testRelation.subquery('left),
+      testRelation2.subquery('right),
+      testRelation.where(IsNotNull('a)).subquery('left),
+      testRelation2.subquery('right),
+      RightOuter,
+      Some("left.a".attr === "right.a".attr))
+
+    testConstraintsAfterJoin(
+      testRelation.subquery('left),
+      testRelation.subquery('right),
+      testRelation.where(IsNotNull(Coalesce(Seq('a, 'b)))).subquery('left),
+      testRelation.where(IsNotNull('c)).subquery('right),
+      Inner,
+      Some(Coalesce(Seq("left.a".attr, "left.b".attr)) === "right.c".attr))
+  }
 }