在测试 PG/PPAS 的时候，一个很大的查询，总是会很快就报OOM。把heap dump出来以后，发现一个一个ResultSet占用了大概600M；而同样的代码在oracle上面就没有问题。然后google了一下，发现是（默认）PG会一次把 query 执行完，并把结果返回。
可以用如下的方式设置 fetchsize
http://jdbc.postgresql.org/documentation/head/query.html#fetchsize-example
Changing code to cursor mode is as simple as setting the fetch size of the Statement to the appropriate size. Setting the fetch size back to 0 will cause all rows to be cached (the default behaviour).
// make sure autocommit is off
conn.setAutoCommit(false);
Statement st = conn.createStatement();
// Turn use of the cursor on.
st.setFetchSize(50);
ResultSet rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next())
{
System.out.print("a row was returned.");
}
rs.close();
// Turn the cursor off.
st.setFetchSize(0);
rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next())
{
System.out.print("many rows were returned.");
}
rs.close();
// Close the statement.
st.close();
同时，查询文档，发现 oracle jdbc（如果没有设置），默认是每次返回10条数据
