MVStore: support concurrent transactions (MVCC style, should match the behavior of PostgreSQL)

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

@@ -831,7 +831,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         return store;
     }
 
-    int getId() {
+    public int getId() {
         return id;
     }
 
@@ -980,7 +980,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         return root.getTotalCount();
     }
 
-    long getCreateVersion() {
+    public long getCreateVersion() {
         return createVersion;
     }
 
@@ -1010,7 +1010,10 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         Page newest = null;
         // need to copy because it can change
         Page r = root;
-        if (r.getVersion() <= version && r.getVersion() >= 0) {
+        if (version >= r.getVersion() && 
+                (r.getVersion() >= 0 || 
+                version <= createVersion || 
+                store.getFile() == null)) {
             newest = r;
         } else {
             // find the newest page that has a getVersion() <= version

h2/src/test/org/h2/test/store/TestMVStore.java

@@ -42,6 +42,7 @@ public class TestMVStore extends TestBase {
         FileUtils.deleteRecursive(getBaseDir(), true);
         FileUtils.createDirectories(getBaseDir());
 
+        testOldVersion();
         testAtomicOperations();
         testWriteBuffer();
         testWriteDelay();
@@ -732,6 +733,31 @@ public class TestMVStore extends TestBase {
         assertEquals("[10, 11, 12, 13, 14, 50, 100, 90, 91, 92]", list.toString());
         s.close();
     }
+    
+    private void testOldVersion() {
+        MVStore s;
+        for (int op = 0; op <= 1; op++) {
+            for (int i = 0; i < 5; i++) {
+                s = openStore(null);
+                s.setRetainVersion(0);
+                MVMap<String, String> m;
+                m = s.openMap("data");
+                for (int j = 0; j < 5; j++) {
+                    if (op == 1) {
+                        m.put("1", "" + s.getCurrentVersion());
+                    }
+                    s.incrementVersion();
+                }
+                for (int j = 0; j < s.getCurrentVersion(); j++) {
+                    MVMap<String, String> old = m.openVersion(j);
+                    if (op == 1) {
+                        assertEquals("" + j, old.get("1"));
+                    }
+                }
+                s.close();
+            }
+        }
+    }
 
     private void testVersion() {
         String fileName = getBaseDir() + "/testVersion.h3";