MVStore: convert an empty root node to a leaf

h2/src/main/org/h2/mvstore/MVMap.java

@@ -554,6 +554,10 @@ public class MVMap<K, V> extends AbstractMap<K, V>
             Page p = copyOnWrite(root, v);
             @SuppressWarnings("unchecked")
             V result = (V) remove(p, v, key);
+            if (!p.isLeaf() && p.getTotalCount() == 0) {
+                p.removePage();
+                p = Page.createEmpty(this,  p.getVersion());
+            }
             newRoot(p);
             return result;
         } finally {
@@ -682,7 +686,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
             if (p.getKeyCount() == 0) {
                 p.setChild(index, c);
                 p.setCounts(index, c);
-                p.removePage();
+               c.removePage();
             } else {
                 p.remove(index);
             }
@@ -1006,8 +1010,9 @@ public class MVMap<K, V> extends AbstractMap<K, V>
 
     @Override
     public boolean isEmpty() {
+        // could also use (sizeAsLong() == 0)
         checkOpen();
-        return 0 == (root.isLeaf() ? root.getKeyCount() : root.getChildPageCount());
+        return root.isLeaf() && root.getKeyCount() == 0;
     }
 
     public long getCreateVersion() {

h2/src/main/org/h2/mvstore/MVMapConcurrent.java

@@ -77,6 +77,10 @@ public class MVMapConcurrent<K, V> extends MVMap<K, V> {
             synchronized (this) {
                 Page p = copyOnWrite(root, v);
                 result = (V) remove(p, v, key);
+                if (!p.isLeaf() && p.getTotalCount() == 0) {
+                    p.removePage();
+                    p = Page.createEmpty(this,  p.getVersion());
+                }
                 newRoot(p);
             }
             return result;

h2/src/main/org/h2/mvstore/rtree/MVRTreeMap.java

@@ -155,9 +155,6 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
                 if (keyType.equals(p.getKey(i), key)) {
                     result = p.getValue(i);
                     p.remove(i);
-                    if (p.getKeyCount() == 0) {
-                        p.removePage();
-                    }
                     break;
                 }
             }
@@ -181,7 +178,7 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
                     // this child was deleted
                     p.remove(i);
                     if (p.getKeyCount() == 0) {
-                        p.removePage();
+                        c.removePage();
                     }
                     break;
                 }