diff --git a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
index 805e6af67..1aec29224 100644
--- a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
+++ b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2013 the original author or authors.
+ * Copyright 2006-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,7 +51,8 @@ import org.springframework.util.ClassUtils;
  * needed as {@link #read()} method is called, returning an object corresponding
  * to current position. On restart it uses the last sort key value to locate the
  * first page to read (so it doesn't matter if the successfully processed items
- * have been removed or modified).
+ * have been removed or modified).  It is important to have a unique key constraint on the
+ * sort key to guarantee that no data is lost between executions.
  * </p>
  * 
  * <p>
diff --git a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
index d71d6e030..5dfd788ce 100644
--- a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
+++ b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
@@ -16,6 +16,12 @@
 
 package org.springframework.batch.item.database.support;
 
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import javax.sql.DataSource;
+
 import org.springframework.batch.item.database.JdbcParameterUtils;
 import org.springframework.batch.item.database.Order;
 import org.springframework.batch.item.database.PagingQueryProvider;
@@ -23,12 +29,6 @@ import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
-import javax.sql.DataSource;
-import java.util.ArrayList;
-import java.util.LinkedHashMap;
-import java.util.List;
-import java.util.Map;
-
 /**
  * Abstract SQL Paging Query Provider to serve as a base class for all provided
  * SQL paging query providers.
@@ -42,7 +42,9 @@ import java.util.Map;
  * Provides properties and preparation for the mandatory "selectClause" and
  * "fromClause" as well as for the optional "whereClause". Also provides
  * property for the mandatory "sortKeys".  <b>Note:</b> The columns that make up 
- * the sort key must be a true key and not just a column to order by.
+ * the sort key must be a true key and not just a column to order by.  It is important
+ * to have a unique key constraint on the sort key to guarantee that no data is lost
+ * between executions.
  * 
  * @author Thomas Risberg
  * @author Dave Syer
diff --git a/src/site/docbook/reference/readersAndWriters.xml b/src/site/docbook/reference/readersAndWriters.xml
index 1df2bd29c..436485040 100644
--- a/src/site/docbook/reference/readersAndWriters.xml
+++ b/src/site/docbook/reference/readersAndWriters.xml
@@ -2192,6 +2192,9 @@ itemReader.close(executionContext);</programlisting>
         also provide an optional where clause. These clauses will be used to
         build an SQL statement combined with the required sortKey.</para>
 
+        <note>It is important to have a unique key constraint on the `sortKey` to
+          guarantee that no data is lost between executions.</note>
+
         <para>After the reader has been opened, it will pass back one item per
         call to <methodname>read</methodname> in the same basic fashion as any
         other <classname>ItemReader</classname>. The paging happens behind the