postgresql - 使用 MyBatis 将 PostgreSQL 聚合返回到 HashMap

Question

我有一个 super 简单的表test例如

create table test (
    id serial primary key,
    status varchar (10)
);

insert into test (status) 
     values ('ready'), ('ready'), 
            ('steady'), 
            ('go'), ('go'), ('go'), 
            ('new');

要获得聚合计数，我可以运行:-

1) 使用 group by 的简单多行结果

select status, 
       count(id) as count        
  from test
 group by status

...返回...

-------+-------
status | counts
-------+-------
go     |      3
ready  |      2
new    |      1
steady |      1
-------+-------

2) 使用 jsonb_object_agg 的单行结果

    with stats as (
       select status, 
              count(id) as count        
         from test
     group by status
    )
    select jsonb_object_agg (status, count) as status_counts from stats

...返回...

--------------------------------------------------
status_counts
--------------------------------------------------
{ "go" : 3, "new" : 1, "ready" : 2, "steady" : 1 }
--------------------------------------------------

Mybatis接口(interface)方法。

在我的 Java 代码中(通过 MyBatis)我有一个方法:-

public Map<String, Integer> selectStatusCounts();

我很想知道如何将任一查询映射到 Map<String, Integer>通过 MyBatis 的 Java 对象？

---

更新(一)

关于 a_horse_with_no_name建议和this stackover article我想出了这个:-

3) 使用 hstore 的单行结果

select hstore(array_agg(hs_key), array_agg(hs_value::text))
from (
    select 
        status, 
        count(id) as count        
    from test
    group by status    
) x(hs_key,hs_value)

...返回...

--------------------------------------------------
status_counts
--------------------------------------------------
"go"=>"3", "new"=>"1", "ready"=>"2", "steady"=>"1"
--------------------------------------------------

使用这样的东西可能会起作用:-

https://github.com/gbif/checklistbank/blob/master/checklistbank-mybatis-service/src/main/java/org/gbif/checklistbank/service/mybatis/postgres/HstoreCountTypeHandler.java

马上测试! :-)

---

更新(二)

谢谢 a_horse_with_no_name再次感谢您的贡献 - 我现在非常接近，但仍然对 MyBatis 感到奇怪。这是我创建的类型处理程序(因此我可以在其他地方重用聚合):-

@MappedTypes(LinkedHashMap.class)
@MappedJdbcTypes(JdbcType.OTHER)
public class MyBatisMapHstoreToStringIntegerMap implements TypeHandler<Map<String, Integer>> {

    public MyBatisMapHstoreToStringIntegerMap() {}

    public void setParameter(PreparedStatement ps, int i, Map<String, Integer> map, JdbcType jdbcType) throws SQLException {
        ps.setString(i, HStoreConverter.toString(map));
    }

    public Map<String, Integer> getResult(ResultSet rs, String columnName) throws SQLException {
        return readMap(rs.getString(columnName));
    }

    public Map<String, Integer> getResult(ResultSet rs, int columnIndex) throws SQLException {
        return readMap(rs.getString(columnIndex));
    }

    public Map<String, Integer> getResult(CallableStatement cs, int columnIndex) throws SQLException {
        return readMap(cs.getString(columnIndex));
    }

    private Map<String, Integer> readMap(String hstring) throws SQLException {
        if (hstring != null) {
            Map<String, Integer> map = new LinkedHashMap<String, Integer>();
            Map<String, String> rawMap = HStoreConverter.fromString(hstring);
            for (Map.Entry<String, String> entry : rawMap.entrySet()) {
                map.put(entry.getKey(), Integer.parseInt(entry.getValue())); // convert from <String, String> to <String,Integer>
            }

            return map;
        }
        return null;
    }

}

...这是映射器界面...

public interface TestMapper {

    public Map<String, Integer> selectStatusCounts();

}

... 这是 <select>在 XML 映射器文件中 ...

<select id="selectStatusCounts" resultType="java.util.LinkedHashMap">
    select hstore(array_agg(hs_key), array_agg(hs_value::text)) as status_counts
    from (
        select 
            status, 
            count(id) as count        
        from test
        group by status    
    ) x(hs_key,hs_value)
</select>

但是，它返回一个 Map有一个名为 status_counts 的条目其值是我想要的实际 map ，即 {status_counts={new=1, ready=2, go=3, steady=1}}

以下是我对 PostgreSQL/MyBatis 的 maven 依赖:-

    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis-spring</artifactId>
        <version>1.2.2</version>
    </dependency>
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.3.1</version>
    </dependency>
    <dependency>
        <groupId>org.postgresql</groupId>
        <artifactId>postgresql</artifactId>
        <version>9.4-1201-jdbc41</version>
    </dependency>

Answer 1

最简单的方法是定义一个 hstore_agg()功能:

CREATE AGGREGATE hstore_agg(hstore) 
(
    SFUNC = hs_concat(hstore, hstore),
    STYPE = hstore
);

然后你可以这样做:

select hstore_agg(hstore(status, cnt::text))
from (
  select status, count(*) cnt
  from test
  group by status
) t;

使用当前的 JDBC 驱动程序 Statement.getObject()将返回 Map<String, String> .

作为hstore只存储字符串，它不能返回 Map<String, Integer>