原作者：彭冲
openGauss/MogDB数据库里bytea二进制类型受segment size编译参数限制，默认不能超过1GB，如果字段存储数据超过1GB可以使用lo（Large Object）扩展类型。
lo类型需要先创建lo extension
解释$ gsql -p5432 -Uomm postgres -r
gsql ((MogDB 2.0.1 build f892ccb7) compiled at 2021-07-09 16:15:21 commit 0 last mr )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.
postgres=# create extension lo;
CREATE EXTENSION
创建完lo扩展，我们新建test_lo表，info字段使用lo类型。
postgres=# create table test_lo(id int,info lo);
CREATE TABLE
创建test_lo表管理触发器，对update和delete操作使用lo_manage函数管理，不然会产生孤立大对象。
解释postgres=# create trigger test_lo before UPDATE OR DELETE ON test_lo FOR EACH ROW EXECUTE procedure lo_manage(info);
WARNING: Trigger function with non-plpgsql type is not recommended.
DETAIL: Non-plpgsql trigger function are not shippable by default.
HINT: Unshippable trigger may lead to bad performance.
CREATE TRIGGER
使用dd生成2GB文件
解释postgres=# \! dd if=/dev/zero of=test_lo bs=1M count=2048 && sync
记录了2048+0 的读入
记录了2048+0 的写出
2147483648字节（2.1 GB，2.0 GiB）已复制，0.805435 s，2.7 GB/s
测试lo_import函数导入数据到数据表
postgres=# insert into test_lo values(1,lo_import('/home/omm/test_lo'));
INSERT 0 1
可以看到数据可以正常导入，如果不使用lo类型，使用bytea类型会提示下面的报错。
ERROR: requested length too large
测试lo_export函数导出数据表数据到文件
解释postgres=# select lo_export(test_lo.info,'/home/omm/test_ext_lo') from test_lo where id=1;
lo_export
-----------
1
(1 row)
可以看到数据正常导出。
查看导入导出的数据文件，也可以使用diff命令进行比对。
postgres=# \! ls -lh test_*
-rw-r--r-- 1 omm dbgrp 2.0G 12月 17 13:00 test_ext_lo
-rw------- 1 omm dbgrp 2.0G 12月 17 12:58 test_lo
查看数据表大对象字段大小
分两步进行，首先查大对象字段的oid（lo类型字段在用户表里面只存储一个oid引用指针，并不实际存数据）
解释postgres=# select * from test_lo;
id | info
----+-------
1 | 16392
(1 row)
实际数据使用多条bytea记录存储在pg_largeobject表，可以根据oid查询统计字段的大小
解释postgres=# select loid,pg_size_pretty(sum(octet_length(data)))
from pg_largeobject
where loid =16392
group by loid;
loid | pg_size_pretty
-------+----------------
16392 | 2048 MB
(1 row)
也可以使用如下函数来查询
解释create or replace function get_lo_size(oid)
returns bigint
volatile strict
as $function$
declare
fd integer;
sz bigint;
begin
fd := lo_open($1, x'40000'::int);
perform lo_lseek64(fd, 0, 2);
sz := lo_tell64(fd);
perform lo_close(fd);
return sz;
end;
$function$ language plpgsql;
查询结果如下
解释postgres=# select pg_size_pretty(get_lo_size(16392));
pg_size_pretty
----------------
2048 MB
(1 row)
再来测试JDBC应用层的使用
JDBC-Java文件入库
解释 public static void main(String[] args) throws Exception{
Class.forName("org.postgresql.Driver");
Connection conn = DriverManager.getConnection("jdbc:postgresql://ip:port/dbname","username","password");
conn.setAutoCommit(false);
LargeObjectManager lobj = conn.unwrap(org.postgresql.PGConnection.class).getLargeObjectAPI();
long oid = lobj.createLO(LargeObjectManager.READ | LargeObjectManager.WRITE);
LargeObject obj = lobj.open(oid, LargeObjectManager.WRITE);
File file = new File("c:/work/test_lo");
FileInputStream fis = new FileInputStream(file);
byte buf[] = new byte[10*1024*1024];
int s, tl = 0;
while ((s = fis.read(buf, 0, 2048)) > 0)
{
obj.write(buf, 0, s);
tl += s;
}
obj.close();
PreparedStatement ps = conn.prepareStatement("INSERT INTO test_lo VALUES (?, ?)");
ps.setInt(1, 100);
ps.setLong(2, oid);
ps.executeUpdate();
ps.close();
fis.close();
conn.commit();
conn.close();
}
JDBC-Java读数据输出到文件
解释 public static void main(String[] args) throws Exception{
Class.forName("org.postgresql.Driver");
Connection conn = DriverManager.getConnection("jdbc:postgresql://ip:port/dbname","username","password");
conn.setAutoCommit(false);
LargeObjectManager lobj = conn.unwrap(org.postgresql.PGConnection.class).getLargeObjectAPI();
PreparedStatement ps = conn.prepareStatement("SELECT info FROM test_lo WHERE id = ?");
ps.setInt(1, 100);
ResultSet rs = ps.executeQuery();
File file = new File("c:/work/test_out_lo");
FileOutputStream fos = new FileOutputStream(file);
while (rs.next())
{
long oid = rs.getLong(1);
LargeObject obj = lobj.open(oid, LargeObjectManager.READ);
byte buf[] = new byte[10*1024*1024];
int s, tl = 0;
while ((s = obj.read(buf, 0, 2048)) > 0)
{
fos.write(buf, 0, s);
tl += s;
}
obj.close();
}
rs.close();
ps.close();
fos.close();
conn.commit();
conn.close();
}
Jdbc-Java Large Object示例参考
