Returning PostgreSQL aggregations to a HashMap using MyBatis - postgresql

Question

I have a super simple table test e.g.
create table test (
id serial primary key,
status varchar (10)
);
insert into test (status)
values ('ready'), ('ready'),
('steady'),
('go'), ('go'), ('go'),
('new');
To get the aggregated counts I can run: -
1) Simple multi-row result using group by
select status,
count(id) as count
from test
group by status
... which returns ...
-------+-------
status | counts
-------+-------
go | 3
ready | 2
new | 1
steady | 1
-------+-------
2) Single-Row Result using jsonb_object_agg
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
... which returns ...
--------------------------------------------------
status_counts
--------------------------------------------------
{ "go" : 3, "new" : 1, "ready" : 2, "steady" : 1 }
--------------------------------------------------
Mybatis Interface method.
In my Java code (via MyBatis) I have a method: -
public Map<String, Integer> selectStatusCounts();
What I'm keen to figure out is how to map either query to the Map<String, Integer> Java object via MyBatis?
Update (1)
On a_horse_with_no_name advice and this stackover article I've come up with this: -
3) Single-Row Result using hstore
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)
... which returns ...
--------------------------------------------------
status_counts
--------------------------------------------------
"go"=>"3", "new"=>"1", "ready"=>"2", "steady"=>"1"
--------------------------------------------------
Using something like this could maybe work: -
https://github.com/gbif/checklistbank/blob/master/checklistbank-mybatis-service/src/main/java/org/gbif/checklistbank/service/mybatis/postgres/HstoreCountTypeHandler.java
Will test now! :-)
Update (2)
Thanks a_horse_with_no_name again for your contributions - I'm very close now but still weirdness with MyBatis. Here is a type handler I created (so I can reuse the aggregations elsewhere): -
#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;
}
}
... and here's the mapper interface ...
public interface TestMapper {
public Map<String, Integer> selectStatusCounts();
}
... and here is the <select> inside the XML mapper file ...
<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>
However, it returns a Map with one entry called status_counts the value of which is the actual map I want i.e. {status_counts={new=1, ready=2, go=3, steady=1}}
The following is my maven dependencies with regards PostgreSQL / MyBatis: -
<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

The easiest way is to define a hstore_agg() function:
CREATE AGGREGATE hstore_agg(hstore)
(
SFUNC = hs_concat(hstore, hstore),
STYPE = hstore
);
Then you can do this:
select hstore_agg(hstore(status, cnt::text))
from (
select status, count(*) cnt
from test
group by status
) t;
With the current JDBC driver Statement.getObject() will return a Map<String, String>.
As hstore only stores strings, it can't return a Map<String, Integer>

Answer 2

Posting an answer (which isn't perfect) but keen to see if anyone else has a better solution.
My answer is based on this stackoverflow answer: -
Return HashMap in mybatis and use it as ModelAttribute in spring MVC
I've created a POJO class called KeyValue: -
public class KeyValue<K, V> {
private K key;
private V value;
public KeyValue() {
}
public KeyValue(K key, V value) {
this.key = key;
this.value = value;
}
public K getKey() {
return key;
}
public void setKey(K key) {
this.key = key;
}
public V getValue() {
return value;
}
public void setValue(V value) {
this.value = value;
}
}
... and changed the test mapper method to ...
#MapKey("key")
public Map<String, KeyValue<String, Integer>> selectStatusCounts();
Note use of #MapKey parameter
I'm using the "1) Simple multi-row result using group by" SQL from original question and changed the result columns to key + value (so it maps to the new KeyValue object) as follows: -
<select id="selectStatusCounts" resultType="test.KeyValue">
select status as key,
count(id) as value
from bulk_upload
group by status
</select>
Accessing this in Java is achieved as follows: -
Map<String, KeyValue<String, Integer>> statusCounts = mapper.selectStatusCounts();
And to retrieve e.g. the value from map of new items we simply do: -
int numberOfstatusCounts = statusCounts.get("new").getValue();
I'm fairly happy with this solution but I would still prefer a Map<String, Integer> as opposed to a Map<String, KeyValue<String, Integer>> so not going to accept my solution - it's here purely just to show how I've got something working (for now).