diff --git a/core/trino-main/src/main/java/io/trino/operator/output/PagePartitioner.java b/core/trino-main/src/main/java/io/trino/operator/output/PagePartitioner.java
index 1e44d90c1081..f4b9f140cb15 100644
--- a/core/trino-main/src/main/java/io/trino/operator/output/PagePartitioner.java
+++ b/core/trino-main/src/main/java/io/trino/operator/output/PagePartitioner.java
@@ -70,7 +70,6 @@ public class PagePartitioner
     private final PositionsAppenderPageBuilder[] positionsAppenders;
     private final boolean replicatesAnyRow;
     private final int nullChannel; // when >= 0, send the position to every partition if this channel is null
-    private final long partitionsInitialRetainedSize;
     private PartitionedOutputInfoSupplier partitionedOutputInfoSupplier;
 
     private boolean hasAnyRowBeenReplicated;
@@ -123,8 +122,7 @@ public PagePartitioner(
             positionsAppenders[i] = PositionsAppenderPageBuilder.withMaxPageSize(pageSize, requireNonNull(sourceTypes, "sourceTypes is null"), positionsAppenderFactory);
         }
         this.memoryContext = aggregatedMemoryContext.newLocalMemoryContext(PagePartitioner.class.getSimpleName());
-        this.partitionsInitialRetainedSize = getRetainedSizeInBytes();
-        this.memoryContext.setBytes(partitionsInitialRetainedSize);
+        updateMemoryUsage();
     }
 
     // sets up this partitioner for the new operator
@@ -154,17 +152,6 @@ public void partitionPage(Page page)
         updateMemoryUsage();
     }
 
-    private void updateMemoryUsage()
-    {
-        // We use getSizeInBytes() here instead of getRetainedSizeInBytes() for an approximation of
-        // the amount of memory used by the pageBuilders, because calculating the retained
-        // size can be expensive especially for complex types.
-        long partitionsSizeInBytes = getSizeInBytes();
-
-        // We also add partitionsInitialRetainedSize as an approximation of the object overhead of the partitions.
-        memoryContext.setBytes(partitionsSizeInBytes + partitionsInitialRetainedSize);
-    }
-
     public void partitionPageByRow(Page page)
     {
         requireNonNull(page, "page is null");
@@ -469,28 +456,14 @@ private List<Slice> splitAndSerializePage(Page page)
         return builder.build();
     }
 
-    private long getSizeInBytes()
-    {
-        // We use a foreach loop instead of streams
-        // as it has much better performance.
-        long sizeInBytes = 0;
-        for (PositionsAppenderPageBuilder pageBuilder : positionsAppenders) {
-            sizeInBytes += pageBuilder.getSizeInBytes();
-        }
-        return sizeInBytes;
-    }
-
-    /**
-     * This method can be expensive for complex types.
-     */
-    private long getRetainedSizeInBytes()
+    private void updateMemoryUsage()
     {
-        long sizeInBytes = 0;
+        long retainedSizeInBytes = 0;
         for (PositionsAppenderPageBuilder pageBuilder : positionsAppenders) {
-            sizeInBytes += pageBuilder.getRetainedSizeInBytes();
+            retainedSizeInBytes += pageBuilder.getRetainedSizeInBytes();
         }
-        sizeInBytes += serializer.getRetainedSizeInBytes();
-        return sizeInBytes;
+        retainedSizeInBytes += serializer.getRetainedSizeInBytes();
+        memoryContext.setBytes(retainedSizeInBytes);
     }
 
     /**
diff --git a/core/trino-main/src/main/java/io/trino/operator/output/RowPositionsAppender.java b/core/trino-main/src/main/java/io/trino/operator/output/RowPositionsAppender.java
index 84711ad278c3..9dd477808be6 100644
--- a/core/trino-main/src/main/java/io/trino/operator/output/RowPositionsAppender.java
+++ b/core/trino-main/src/main/java/io/trino/operator/output/RowPositionsAppender.java
@@ -43,8 +43,8 @@ public class RowPositionsAppender
     private boolean hasNullRow;
     private boolean hasNonNullRow;
     private boolean[] rowIsNull = new boolean[0];
-    private long retainedSizeInBytes;
-    private long sizeInBytes;
+    private long retainedSizeInBytes = -1;
+    private long sizeInBytes = -1;
 
     public static RowPositionsAppender createRowAppender(
             PositionsAppenderFactory positionsAppenderFactory,
@@ -63,7 +63,7 @@ private RowPositionsAppender(PositionsAppender[] fieldAppenders, int expectedPos
     {
         this.fieldAppenders = requireNonNull(fieldAppenders, "fields is null");
         this.initialEntryCount = expectedPositions;
-        updateRetainedSize();
+        resetSize();
     }
 
     @Override
@@ -103,7 +103,7 @@ else if (allPositionsNull(positions, block)) {
         }
 
         positionCount += positions.size();
-        updateSize();
+        resetSize();
     }
 
     @Override
@@ -135,7 +135,7 @@ else if (value.isNull(0)) {
             throw new IllegalArgumentException("unsupported block type: " + value);
         }
         positionCount += rlePositionCount;
-        updateSize();
+        resetSize();
     }
 
     @Override
@@ -165,7 +165,7 @@ else if (value.isNull(position)) {
             throw new IllegalArgumentException("unsupported block type: " + value);
         }
         positionCount++;
-        updateSize();
+        resetSize();
     }
 
     @Override
@@ -191,16 +191,32 @@ public Block build()
     @Override
     public long getRetainedSizeInBytes()
     {
-        long size = retainedSizeInBytes;
+        if (retainedSizeInBytes != -1) {
+            return retainedSizeInBytes;
+        }
+
+        long size = INSTANCE_SIZE + sizeOf(rowIsNull);
         for (PositionsAppender field : fieldAppenders) {
             size += field.getRetainedSizeInBytes();
         }
+
+        retainedSizeInBytes = size;
         return size;
     }
 
     @Override
     public long getSizeInBytes()
     {
+        if (sizeInBytes != -1) {
+            return sizeInBytes;
+        }
+
+        long size = (Integer.BYTES + Byte.BYTES) * (long) positionCount;
+        for (PositionsAppender field : fieldAppenders) {
+            size += field.getSizeInBytes();
+        }
+
+        sizeInBytes = size;
         return sizeInBytes;
     }
 
@@ -210,10 +226,9 @@ private void reset()
         initialized = false;
         rowIsNull = new boolean[0];
         positionCount = 0;
-        sizeInBytes = 0;
         hasNonNullRow = false;
         hasNullRow = false;
-        updateRetainedSize();
+        resetSize();
     }
 
     private boolean allPositionsNull(IntArrayList positions, Block block)
@@ -265,21 +280,13 @@ private void ensureCapacity(int additionalCapacity)
 
             int newCapacity = Math.max(newSize, positionCount + additionalCapacity);
             rowIsNull = Arrays.copyOf(rowIsNull, newCapacity);
-            updateRetainedSize();
+            resetSize();
         }
     }
 
-    private void updateSize()
-    {
-        long size = (Integer.BYTES + Byte.BYTES) * (long) positionCount;
-        for (PositionsAppender field : fieldAppenders) {
-            size += field.getSizeInBytes();
-        }
-        sizeInBytes = size;
-    }
-
-    private void updateRetainedSize()
+    private void resetSize()
     {
-        retainedSizeInBytes = INSTANCE_SIZE + sizeOf(rowIsNull);
+        sizeInBytes = -1;
+        retainedSizeInBytes = -1;
     }
 }