openGauss/MogDB数据库里bytea二进制类型受segment size编译参数限度，默认不能超过1GB，如果字段存储数据超过1GB能够应用lo（Large Object）扩大类型。
01
lo类型须要先创立lo extension

$ gsql -p5432 -Uomm postgres -rgsql ((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

创立完lo扩大，咱们新建test_lo表，info字段应用lo类型。

postgres=# create table test_lo(id int,info lo);CREATE TABLE

创立test_lo表治理触发器，对update和delete操作应用lo_manage函数治理，不然会产生孤立大对象。

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

应用dd生成2GB文件：

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

02
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

能够看到数据能够失常导入，如果不应用lo类型，应用bytea类型会提醒上面的报错。

ERROR: requested length too large

03
测试lo_export函数导出数据表数据到文件

postgres=# select lo_export(test_lo.info,'/home/omm/test_ext_lo') from test_lo where id=1; lo_export -----------         1(1 row)

能够看到数据失常导出。

查看导入导出的数据文件，也能够应用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

04
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类型字段在用户表外面只存储一个oid援用指针，并不理论存数据）

postgres=# select * from test_lo; id | info  ----+-------  1 | 16392(1 row)

理论数据应用多条bytea记录存储在pg_largeobject表，能够依据oid查问统计字段的大小。

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)

也能够应用如下函数来查问：

create or replace function get_lo_size(oid)returns bigintvolatile strictas $function$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;

查问后果如下：

postgres=# select pg_size_pretty(get_lo_size(16392)); pg_size_pretty ---------------- 2048 MB(1 row)

再来测试JDBC应用层的应用：

05
JDBC-Java文件入库

    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();            }

06
JDBC-Java读数据输入到文件

    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：（复制链接至浏览器中浏览）

https://jdbc.postgresql.org/d...

墨天轮原文链接：https://www.modb.pro/db/21475...（复制链接至浏览器或点击文末浏览原文查看)

对于作者
彭冲，云和恩墨PG技术顾问，网名“多米爸比”，PG社区认证专家，中国首期PostgreSQL ACE Partner，多年从事基于PostgreSQL数据库的软件研发，擅长于PL/PGSQL业务迁徙及优化，Oracle到PostgreSQL的迁徙降级，异构数据库整合；作为墨天轮PostgreSQL实际专栏作者，热衷于PostgreSQL实际技术分享，在本人的岗位踊跃推广PostgreSQL，致力为PG社区多做贡献。