Index: lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java
===================================================================
--- lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java	(revision 1527612)
+++ lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java	(working copy)
@@ -30,6 +30,7 @@
 import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.LuceneTestCase;
 import org.apache.lucene.util.LuceneTestCase.SuppressCodecs;
+import org.apache.lucene.util.RamUsageEstimator;
 import org.apache.lucene.util._TestUtil;
 import org.junit.Test;
 
@@ -1211,5 +1212,74 @@
     writer.close();
     dir.close();
   }
+
+  @Test
+  public void testTonsOfUpdates() throws Exception {
+    // LUCENE-5248: make sure that when there are many updates, we don't use too much RAM
+    Directory dir = newFSDirectory(_TestUtil.getTempDir("tonsOfUpdates"));
+    final Random random = random();
+    IndexWriterConfig conf = newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random));
+    conf.setRAMBufferSizeMB(IndexWriterConfig.DEFAULT_RAM_BUFFER_SIZE_MB);
+    conf.setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH); // don't flush by doc
+    IndexWriter writer = new IndexWriter(dir, conf);
+    
+    // test data: lots of documents (few 10Ks) and lots of update terms (few hundreds)
+    final int numDocs = atLeast(40000);
+    final int numNumericFields = atLeast(5);
+    final int numTerms = _TestUtil.nextInt(random, 10, 100); // terms should affect many docs
+    Set<String> updateTerms = new HashSet<String>();
+    while (updateTerms.size() < numTerms) {
+      updateTerms.add(_TestUtil.randomSimpleString(random));
+    }
+
+    System.out.println("numDocs=" + numDocs + " numNumericFields=" + numNumericFields + " numTerms=" + numTerms); // nocommit __COMMENT
+    
+    // build a large index with many NDV fields and update terms
+    for (int i = 0; i < numDocs; i++) {
+      Document doc = new Document();
+      int numUpdateTerms = _TestUtil.nextInt(random, 1, numTerms / 10);
+      for (int j = 0; j < numUpdateTerms; j++) {
+        doc.add(new StringField("upd", RandomPicks.randomFrom(random, updateTerms), Store.NO));
+      }
+      for (int j = 0; j < numNumericFields; j++) {
+        long val = random.nextInt();
+        doc.add(new NumericDocValuesField("f" + j, val));
+        doc.add(new NumericDocValuesField("cf" + j, val * 2));
+      }
+      writer.addDocument(doc);
+    }
+
+    // set to flush every 512 bytes, so we get many flushes during numeric updates
+    writer.getConfig().setRAMBufferSizeMB(0.05);
+    final int numUpdates = atLeast(3000);
+    System.out.println("numUpdates=" + numUpdates); // nocommit __COMMENT
+    for (int i = 0; i < numUpdates; i++) {
+      int field = random.nextInt(numNumericFields);
+      Term updateTerm = new Term("upd", RandomPicks.randomFrom(random, updateTerms));
+      long value = random.nextInt();
+      writer.updateNumericDocValue(updateTerm, "f" + field, value);
+      writer.updateNumericDocValue(updateTerm, "cf" + field, value * 2);
+      
+      if (i % 100 == 0) { // nocommit use it to assert?
+        System.out.println(RamUsageEstimator.sizeOf(writer));
+      }
+    }
+
+    writer.close();
+    
+    DirectoryReader reader = DirectoryReader.open(dir);
+    for (AtomicReaderContext context : reader.leaves()) {
+      for (int i = 0; i < numNumericFields; i++) {
+        NumericDocValues f = context.reader().getNumericDocValues("f" + i);
+        NumericDocValues cf = context.reader().getNumericDocValues("cf" + i);
+        for (int j = 0; j < context.reader().maxDoc(); j++) {
+          assertEquals(cf.get(j), f.get(j) * 2);
+        }
+      }
+    }
+    reader.close();
+    
+    dir.close();
+  }
   
 }