Remove redundant size in serialized variable width block

Avoids encoding the slice length separately from the offsets, which can
be used to infer the value directly.

Author: pettyjamesm

core/trino-main/src/test/java/io/trino/execution/buffer/TestPagesSerde.java

@@ -211,14 +211,14 @@ public void testVarcharSerializedSize()
         // empty page
         Page page = new Page(builder.build());
         int pageSize = serializedSize(ImmutableList.of(VARCHAR), page);
-        assertThat(pageSize).isEqualTo(48);
+        assertThat(pageSize).isEqualTo(44);
 
         // page with one value
         VARCHAR.writeString(builder, "alice");
         pageSize = 44; // Now we have moved to the normal block implementation so the page size overhead is 44
         page = new Page(builder.build());
         int firstValueSize = serializedSize(ImmutableList.of(VARCHAR), page) - pageSize;
-        assertThat(firstValueSize).isEqualTo(8 + 5); // ending offsets + nonNullsCount + "alice"
+        assertThat(firstValueSize).isEqualTo(4 + 5); // ending offset + nonNullsCount + "alice"
 
         // page with two values
         VARCHAR.writeString(builder, "bob");

core/trino-spi/src/main/java/io/trino/spi/block/VariableWidthBlockEncoding.java

@@ -60,9 +60,7 @@ public void writeBlock(BlockEncodingSerde blockEncodingSerde, SliceOutput sliceO
         int startingOffset = rawOffsets[arrayBaseOffset];
         int totalLength = rawOffsets[positionCount + arrayBaseOffset] - startingOffset;
 
-        sliceOutput
-                .appendInt(totalLength)
-                .writeBytes(variableWidthBlock.getRawSlice(), startingOffset, totalLength);
+        sliceOutput.writeBytes(variableWidthBlock.getRawSlice(), startingOffset, totalLength);
     }
 
     @Override
@@ -73,8 +71,8 @@ public Block readBlock(BlockEncodingSerde blockEncodingSerde, SliceInput sliceIn
 
         int[] offsets = readOffsetsWithNullsCompacted(sliceInput, valueIsNull, positionCount);
 
-        int blockSize = sliceInput.readInt();
-        Slice slice = Slices.allocate(blockSize);
+        int sliceSize = offsets[offsets.length - 1];
+        Slice slice = Slices.allocate(sliceSize);
         sliceInput.readBytes(slice);
 
         return new VariableWidthBlock(0, positionCount, slice, offsets, valueIsNull);