Clear LazyBlockLoader reference after block load

LazyBlockLoaders for ORC retains large memory allocations, which
are not needed once the block is fully loaded.

presto-spi/src/main/java/com/facebook/presto/spi/block/LazyFixedWidthBlock.java

@@ -24,7 +24,7 @@ public class LazyFixedWidthBlock
  extends AbstractFixedWidthBlock
 {
  private final int positionCount;
- private final LazyBlockLoader<LazyFixedWidthBlock> loader;
+ private LazyBlockLoader<LazyFixedWidthBlock> loader;
  private Slice slice;
  private boolean[] valueIsNull;
 
@@ -102,6 +102,9 @@ public void assureLoaded()
  if (slice == null) {
  throw new IllegalArgumentException("Lazy block loader did not load this block");
  }
+
+ // clear reference to loader to free resources, since load was successful
+ loader = null;
  }
 
  public void setRawSlice(Slice slice)

presto-spi/src/main/java/com/facebook/presto/spi/block/LazySliceArrayBlock.java

@@ -25,7 +25,7 @@ public class LazySliceArrayBlock
  extends AbstractVariableWidthBlock
 {
  private final int positionCount;
- private final LazyBlockLoader<LazySliceArrayBlock> loader;
+ private LazyBlockLoader<LazySliceArrayBlock> loader;
  private Slice[] values;
  private final AtomicInteger sizeInBytes = new AtomicInteger(-1);
 
@@ -124,6 +124,9 @@ public void assureLoaded()
  if (values == null) {
  throw new IllegalArgumentException("Lazy block loader did not load this block");
  }
+
+ // clear reference to loader to free resources, since load was successful
+ loader = null;
  }
 
  @Override