Index: oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdate.java
===================================================================
--- oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdate.java	(revision 1601598)
+++ oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdate.java	(working copy)
@@ -19,6 +19,7 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.collect.Lists.newArrayList;
 import static com.google.common.collect.Lists.newArrayListWithCapacity;
+import static org.apache.jackrabbit.oak.api.Type.BOOLEAN;
 import static org.apache.jackrabbit.oak.commons.PathUtils.concat;
 import static org.apache.jackrabbit.oak.plugins.index.IndexConstants.ASYNC_PROPERTY_NAME;
 import static org.apache.jackrabbit.oak.plugins.index.IndexConstants.ASYNC_REINDEX_VALUE;
@@ -108,7 +109,7 @@
     @Override
     public void enter(NodeState before, NodeState after)
             throws CommitFailedException {
-        collectIndexEditors(builder.getChildNode(INDEX_DEFINITIONS_NAME));
+        collectIndexEditors(builder.getChildNode(INDEX_DEFINITIONS_NAME), before);
 
         // no-op when reindex is empty
         CommitFailedException exception = EditorDiff.process(
@@ -122,17 +123,30 @@
         }
     }
 
-    private void collectIndexEditors(NodeBuilder definitions)
-            throws CommitFailedException {
+    private boolean shouldReindex(NodeBuilder definition, NodeState before,
+            String name) {
+        PropertyState ps = definition.getProperty(REINDEX_PROPERTY_NAME);
+        if (ps != null) {
+            return ps.getValue(BOOLEAN);
+        }
+        // reindex only in the case this is a new node, even though there is
+        // no other way in which the property would be missing
+        return !before.getChildNode(INDEX_DEFINITIONS_NAME).hasChildNode(name);
+    }
+
+    private void collectIndexEditors(NodeBuilder definitions,
+            NodeState before) throws CommitFailedException {
         for (String name : definitions.getChildNodeNames()) {
             NodeBuilder definition = definitions.getChildNode(name);
             if (Objects.equal(async, definition.getString(ASYNC_PROPERTY_NAME))) {
                 String type = definition.getString(TYPE_PROPERTY_NAME);
+                boolean shouldReindex = shouldReindex(definition,
+                        before, name);
                 Editor editor = provider.getIndexEditor(type, definition, root, updateCallback);
                 if (editor == null) {
                     // trigger reindexing when an indexer becomes available
                     definition.setProperty(REINDEX_PROPERTY_NAME, true);
-                } else if (definition.getBoolean(REINDEX_PROPERTY_NAME)) {
+                } else if (shouldReindex) {
                     if (definition.getBoolean(REINDEX_ASYNC_PROPERTY_NAME)
                             && definition.getString(ASYNC_PROPERTY_NAME) == null) {
                         // switch index to an async update mode
Index: oak-core/src/test/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdateTest.java
===================================================================
--- oak-core/src/test/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdateTest.java	(revision 1601598)
+++ oak-core/src/test/java/org/apache/jackrabbit/oak/plugins/index/IndexUpdateTest.java	(working copy)
@@ -172,6 +172,41 @@
         assertEquals(ImmutableSet.of("testRoot"), find(lookup, "foo", "abc"));
     }
 
+    /**
+     * Auto Reindex Test
+     * <ul>
+     * <li>Add some content</li>
+     * <li>Add an index definition without a reindex flag (see OAK-1874)</li>
+     * <li>Search & verify</li>
+     * </ul>
+     */
+    @Test
+    public void testReindexAuto() throws Exception {
+        builder.child("testRoot").setProperty("foo", "abc");
+        NodeState before = builder.getNodeState();
+
+        NodeBuilder index = createIndexDefinition(
+                builder.child(INDEX_DEFINITIONS_NAME), "rootIndex", true,
+                false, ImmutableSet.of("foo"), null);
+        index.removeProperty(REINDEX_PROPERTY_NAME);
+
+        NodeState after = builder.getNodeState();
+
+        NodeState indexed = HOOK.processCommit(before, after, CommitInfo.EMPTY);
+
+        // first check that the index content nodes exist
+        NodeState ns = checkPathExists(indexed, INDEX_DEFINITIONS_NAME,
+                "rootIndex");
+        checkPathExists(ns, INDEX_CONTENT_NODE_NAME);
+        PropertyState ps = ns.getProperty(REINDEX_PROPERTY_NAME);
+        assertNotNull(ps);
+        assertFalse(ps.getValue(Type.BOOLEAN));
+
+        // next, lookup
+        PropertyIndexLookup lookup = new PropertyIndexLookup(indexed);
+        assertEquals(ImmutableSet.of("testRoot"), find(lookup, "foo", "abc"));
+    }
+
     @Test
     public void testIndexDefinitions() throws Exception {
         createIndexDefinition(builder.child(INDEX_DEFINITIONS_NAME),