(N + 1) solution does not yield same results as lazy loading solution
---------------------------------------------------------------------
Key: IBATIS-69
URL: http://issues.apache.org/jira/browse/IBATIS-69
Project: iBatis for Java
Type: Bug
Components: SQL Maps
Versions: 2.0.9
Environment: XP Pro, WSAD 5.1.2
Reporter: Mark Nabours
Attachments: Category.java, Item.java, Item.xml
Hello,
We have a situation where it would be advantageous for performance reasons to
avoid lazy loading and utilize the recently implemented (N + 1) solution.
Unfortunately, when utilizing a left join the results are not the same as the
lazy loading solution.
To illustrate the problem I've put together a very simple example.
Please consider the following simple tables of data, categories and items:
+---------------+---------------+
| category_code | category_name |
+---------------+---------------+
| cat1 | Category 1 |
| cat2 | Category 2 |
| cat3 | Category 3 |
+---------------+---------------+
+---------+-----------+---------------+
| item_id | item_name | category_code |
+---------+-----------+---------------+
| item1 | Item 1 | cat1 |
| item2 | Item 2 | cat1 |
| item3 | Item 3 | cat1 |
| item4 | Item 4 | cat1 |
| item5 | Item 5 | cat2 |
| item6 | Item 6 | cat2 |
+---------+-----------+---------------+
The challenge is to load a List of items for each category.
Here are the two extremely simple data objects used to illustrate the issue:
//==============================================================
// Category.java
//==============================================================
package com.alliancesys.ibatisdemo.domain;
import java.util.Iterator;
import java.util.List;
public class Category {
private String code;
private String name;
private List items;
public Category() {
super();
}
public String getCode() {
return code;
}
public List getItems() {
return items;
}
public String getName() {
return name;
}
public void setCode(String string) {
code = string;
}
public void setItems(List list) {
items = list;
}
public void setName(String string) {
name = string;
}
/**
* @see java.lang.Object#toString()
*/
public String toString() {
StringBuffer buffer = new StringBuffer();
buffer.append(this.getClass().getName());
buffer.append("-");
buffer.append("code:");
buffer.append(code);
buffer.append(",name:");
buffer.append(name);
buffer.append(",items:[");
List items = getItems();
if (items != null) {
for (Iterator iter = items.iterator(); iter.hasNext();)
{
buffer.append(iter.next());
buffer.append(";");
}
}
buffer.append("]");
return buffer.toString();
}
}
//==============================================================
// Item.java
//==============================================================
package com.alliancesys.ibatisdemo.domain;
public class Item {
private String id;
private String name;
private String categoryCode;
public Item() {
super();
}
public String getCategoryCode() {
return categoryCode;
}
public String getId() {
return id;
}
public String getName() {
return name;
}
public void setCategoryCode(String categoryCode) {
this.categoryCode = categoryCode;
}
public void setId(String id) {
this.id = id;
}
public void setName(String name) {
this.name = name;
}
/**
* @see java.lang.Object#toString()
*/
public String toString() {
StringBuffer buffer = new StringBuffer();
buffer.append(this.getClass().getName());
buffer.append("-");
buffer.append("id:");
buffer.append(id);
buffer.append(",name:");
buffer.append(name);
buffer.append(",categoryCode:");
buffer.append(categoryCode);
return buffer.toString();
}
}
Here is the sqlMap file that contains both the lazy loading solution
(getAllCategoriesWithLazyLoad) and the N + 1 solution
(getAllCategoriesWithNPlusOne):
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE sqlMap PUBLIC "-//iBATIS.com//DTD SQL Map 2.0//EN"
"http://www.ibatis.com/dtd/sql-map-2.dtd";>
<sqlMap namespace="Item">
<typeAlias
alias="item"
type="com.alliancesys.ibatisdemo.domain.Item" />
<typeAlias
alias="category"
type="com.alliancesys.ibatisdemo.domain.Category" />
<resultMap
id="itemMap"
class="item">
<result
property="id"
column="item_id" />
<result
property="name"
column="item_name" />
<result
property="categoryCode"
column="category_code" />
</resultMap>
<resultMap
id="categoryWithLazyLoad"
class="category">
<result
property="code"
column="category_code"/>
<result
property="name"
column="category_name"/>
<result
property="items"
column="category_code"
select="getItemsByCategoryCode" />
</resultMap>
<resultMap
id="categoryWithNPlusOne"
class="category"
groupBy="code">
<result
property="code"
column="category_code"/>
<result
property="name"
column="category_name" />
<result
property="items"
resultMap="Item.itemMap" />
</resultMap>
<select
id="getItemsByCategoryCode"
parameterClass="java.lang.String"
resultMap="itemMap">
select
item_id,
item_name,
category_code
from
items
where
category_code = #value#;
</select>
<select
id="getAllCategoriesWithLazyLoad"
resultMap="categoryWithLazyLoad">
select
category_code,
category_name
from
categories;
</select>
<select
id="getAllCategoriesWithNPlusOne"
resultMap="categoryWithNPlusOne">
select
c.category_code,
c.category_name,
i.item_id,
i.item_name
from
categories c
left join items i on c.category_code = i.category_code
order by
c.category_code;
</select>
</sqlMap>
The following test case exercises the two approaches:
package com.alliancesys.ibatis.testing;
import java.io.Reader;
import java.sql.SQLException;
import java.util.Iterator;
import java.util.List;
import com.alliancesys.ibatisdemo.domain.Category;
import com.ibatis.common.resources.Resources;
import com.ibatis.sqlmap.client.SqlMapClient;
import com.ibatis.sqlmap.client.SqlMapClientBuilder;
import junit.framework.TestCase;
public class IBATISAggregateObjectTest extends TestCase {
/**
* Constructor for IBATISAggregateObjectTest.
* @param arg0
*/
public IBATISAggregateObjectTest(String arg0) {
super(arg0);
}
public static void main(String[] args) {
junit.textui.TestRunner.run(IBATISAggregateObjectTest.class);
}
public void testGetCategories() throws Exception {
Reader reader =
Resources.getResourceAsReader(
getClass().getClassLoader(),
"sql-map-config.xml");
SqlMapClient sqlMapClient =
SqlMapClientBuilder.buildSqlMapClient(reader);
try {
sqlMapClient.startTransaction();
//Fetch a list of categories with lazy load of items.
List categoriesWithLazyLoad =
sqlMapClient.queryForList("getAllCategoriesWithLazyLoad", null);
for (Iterator iter = categoriesWithLazyLoad.iterator();
iter.hasNext();
) {
//force lazy load
((Category)iter.next()).getItems().iterator();
}
//print each category
for (Iterator iter = categoriesWithLazyLoad.iterator();
iter.hasNext();
) {
System.out.println(iter.next());
}
// Fetch a list of categories with N+1 solution
List categoriesWithNPlusOne =
sqlMapClient.queryForList("getAllCategoriesWithNPlusOne", null);
// print categories expect same output as before
for (Iterator iter = categoriesWithNPlusOne.iterator();
iter.hasNext();
) {
System.out.println(iter.next());
}
} catch (SQLException e) {
throw e;
} finally {
sqlMapClient.endTransaction();
}
}
}
We expect both approaches to yield the same output. But for the cat3 category
under the lazy loading approach the following is outputted:
com.alliancesys.ibatisdemo.domain.Category-code:cat3,name:Category 3,items:[]
Indicating that the items property was loaded with an empty List. This is
exactly what I expect would happen since there are no items for that category.
All is well except that performance stinks for our real world needs because of
the N + 1 issue.
Looking at cat3 for the N + 1 solution approach, the following is outputted:
com.alliancesys.ibatisdemo.domain.Category-code:cat3,name:Category
3,items:[com.alliancesys.ibatisdemo.domain.Item-id:null,name:null,categoryCode:cat3;]
Indicating that a nonexistent item was loaded into the list for the items
property.
I believe the issue is that the (N + 1) solution approach does not handle left
joins. (It's not acceptable for me to switch the join to an inner join or I
would basically lose cat3 from the collection.) The results from the left join
are as follows:
+---------------+---------------+---------+-----------+
| category_code | category_name | item_id | item_name |
+---------------+---------------+---------+-----------+
| cat1 | Category 1 | item3 | Item 3 |
| cat1 | Category 1 | item4 | Item 4 |
| cat1 | Category 1 | item1 | Item 1 |
| cat1 | Category 1 | item2 | Item 2 |
| cat2 | Category 2 | item5 | Item 5 |
| cat2 | Category 2 | item6 | Item 6 |
| cat3 | Category 3 | NULL | NULL |
+---------------+---------------+---------+-----------+
The cat3 row seems to be problematic unless there are additional configuration
settings in the SQLMap file that address this.
So in summary, the lazy loading approach yields the correct results, but we
need to utilize the (N + 1) solution for performance but it yields bogus
results. Is there already a solution for this? Or should this be logged in
JIRA?
We would appreciate a quick turnaround on this because the (N + 1) support is
such a great feature that we want to be able to utilize to simplify our code.
Thanks,
Mark
--
This message is automatically generated by JIRA.
-
If you think it was sent incorrectly contact one of the administrators:
http://issues.apache.org/jira/secure/Administrators.jspa
-
If you want more information on JIRA, or have a bug to report see:
http://www.atlassian.com/software/jira
