Sequence(序列)是数据库经常使用自增列属性，对于单机PostgreSQL实例，数据库维护一个自增变量即可。但是对于Greenplum的MPP架构，如果每个节点都维护自己的Sequence，那么Sequence将会出现重复，那么Greenplum是如何处理的呢？

如何使用Sequence

create table test_sequence(id serial, name text);
 
postgres=> \d
public | test_sequence | table | postgres
public | test_sequence_id_seq | sequence | postgres

postgres=> insert into test_sequence (name) values(1);
INSERT 0 1
postgres=> insert into test_sequence (name) values(2);
INSERT 0 1
postgres=> insert into test_sequence (name) values(3);
INSERT 0 1
postgres=> select * from test_sequence;
 id | name
----+------
  3 | 3
  1 | 1
  2 | 2
(3 rows)

Sequence是谁维护

查看Master的Sequence维护元信息

postgres=> select * from test_sequence_id_seq;
    sequence_name     | last_value | increment_by |      max_value      | min_value | cache_value | log_cnt | is_cycled | is_called
----------------------+------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
 test_sequence_id_seq |          3 |            1 | 9223372036854775807 |         1 |           1 |      30 | f         | t
(1 row)

查看Segment的Sequence维护元信息

postgres=> select * from gp_dist_random('test_sequence_id_seq');
    sequence_name     | last_value | increment_by |      max_value      | min_value | cache_value | log_cnt | is_cycled | is_called
----------------------+------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
 test_sequence_id_seq |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 | f         | f
 test_sequence_id_seq |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 | f         | f
 test_sequence_id_seq |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 | f         | f
 test_sequence_id_seq |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 | f         | f
(4 rows)

通过Master和Segment的元信息可以看出，只有Master一直更新元信息，Segment的Sequence元信息一直不变，所以Sequence是由Master维护的。

Sequence分配过程

如何所示，Master上有一个seqserver进程，专门用来维护全局的Sequence信息。所有的Segment获取最新的Sequence都需要向Master的seqserver请求，然后seqserver更新Sequence云信息，返回给Segment。为了实现Sequence，Master和Segment多了一次交互，这样会影响性能，建议应用层生成自增值。