SEQUENCE 算子用于计算伪列 SEQUENCE 的值。

伪列 SEQUENCE 是由 CREATE SEQUENCE 创建的序列，下层算子每输出一行执行一次计算。

示例：计算伪列 SEQUENCE 的当前值与下一个值

obclient>CREATE TABLE t1(c1 INT, c2 INT);
Query OK, 0 rows affected (0.09 sec)
obclient>CREATE TABLE t2(c1 INT, c2 INT);
Query OK, 0 rows affected (0.09 sec)
obclient>CREATE SEQUENCE seq INCREMENT BY 1 START WITH 1;
Query OK, 0 rows affected (0.09 sec)
obclient>EXPLAIN SELECT seq.NEXTVAL, seq.CURRVAL FROM t1\G;
*************************** 1. row ***************************
Query Plan: 
|=====================================
|ID|OPERATOR   |NAME|EST. ROWS|COST |
-------------------------------------
|0 |SEQUENCE   |    |100000   |77868|
|1 | TABLE SCAN|T1  |100000   |64066|
=====================================
Outputs & filters: 
-------------------------------------
  0 - output([SEQ.NEXTVAL], [SEQ.CURRVAL]), filter(nil)
  1 - output([T1.__pk_increment]), filter(nil), 
      access([T1.__pk_increment]), partitions(p0)

上述示例中，执行计划展示中 0 号算子 SEQUENCE 用来计算序列的值，output([SEQ.NEXTVAL],[SEQ.CURRVAL] 指定了 SEQUENCE 需要计算序列的当前值与下一个值。t1 表每输出一行数据，SEQUENCE 就会计算一次序列的值。执行计划展示中 outputs & filters 详细列出了 SEQUENCE 算子的输出信息如下：