摘要：通过一些自定义的视图或者查询语句，批量识别集群的sequence相关问题

本文分享自华为云社区《GaussDB(DWS)运维 -- sequence常见运维操作》，作者： 譡里个檔。

GaussDB(DWS)是一个share nothing架构的产品，seqeunce编号需要通过GTM统一分配。集群规模越大、节点数越多、sequence调用越频繁，sequnce对GTM的压力就越大。对于seqeunce的使用一般有如下建议：

1. 建议业务中不要大量使用sequence，建议不要超过50个
2. 如果业务允许，建议使用uuid替换sequence
3. 使用sequence的时候，需要定义cache值不小于100

建议定义如下视图进行查询

CREATE OR REPLACE FUNCTION public.dfm_get_seqence_info
(
OUT namespace text, -- 序列的schema
OUT sequencename text, -- 序列名称
OUT min_value bigint, -- 序列最小值
OUT max_value bigint, -- 序列最大值
OUT start_value bigint, -- 起始值
OUT increment_by bigint, -- 增加步长
OUT cache_value bigint, -- cache值
OUT is_cycled boolean -- 是否可以循环使用
)
RETURNS SETOF record
LANGUAGE plpgsql
NOT FENCED NOT SHIPPABLE
AS $function$
DECLARE
rowObject record;
rowDetail record;
queryObjectStr text;
queryDetailStr text;
BEGIN
--Get all the node names
queryObjectStr := 'SELECT c.oid, relname, n.nspname FROM pg_class c INNER JOIN pg_namespace n on c.relnamespace = n.oid WHERE c.relkind = ''S''';
FOR rowObject IN EXECUTE(queryObjectStr) LOOP
queryDetailStr := 'SELECT start_value, increment_by, max_value, min_value, cache_value, is_cycled FROM ' || quote_ident(rowObject.nspname) || '.' || quote_ident(rowObject.relname);
FOR rowDetail IN EXECUTE(queryDetailStr) LOOP
namespace = rowObject.nspname;
sequencename = rowObject.relname;
start_value = rowDetail.start_value;
increment_by = rowDetail.increment_by;
max_value = rowDetail.max_value;
min_value = rowDetail.min_value;
cache_value = rowDetail.cache_value;
is_cycled = rowDetail.is_cycled;
RETURN next;
END LOOP;
END LOOP;
RETURN;
END; $function$
;

CREATE VIEW public.dfm_get_seqence_info AS SELECT * FROM public.dfm_get_seqence_info();

示例

postgres=# CREATE SEQUENCE public.test_seq;
CREATE SEQUENCE
Time: 13.917 ms
postgres=# CREATE TABLE public.test_serial(a bigserial, b int) DISTRIBUTE BY ROUNDROBIN;
NOTICE: CREATE TABLE will create implicit sequence "test_serial_a_seq" for serial column "test_serial.a"
SQLSTATE: 00000
LOCATION: CreateSeqOwnedByTable, parse_utilcmd.cpp:144
CREATE TABLE
Time: 16.572 ms
postgres=# CREATE SEQUENCE public.test_seq_1;
CREATE SEQUENCE
Time: 13.412 ms
postgres=# CREATE TABLE public.test_serial_default(a bigint default nextval('public.test_seq_1'::regclass), b int) DISTRIBUTE BY ROUNDROBIN;
CREATE TABLE
Time: 12.006 ms
postgres=# SELECT * FROM public.dfm_get_seqence_info;
namespace | sequencename | min_value | max_value | start_value | increment_by | cache_value | is_cycled
-----------+-------------------+-----------+---------------------+-------------+--------------+-------------+-----------
public | test_seq | 1 | 9223372036854775807 | 1 | 1 | 1 | f
public | test_serial_a_seq | 1 | 9223372036854775807 | 1 | 1 | 1 | f
public | test_seq_1 | 1 | 9223372036854775807 | 1 | 1 | 1 | f
(3 rows)

查询sequence和表的bind关系，主要场景有两个

1. 表的字段类型为bigserial或者serial类型，内置生成一个序列
2. 表的字段的默认值为序列的序列号

这两个场景在上述里面都有case

postgres=# SELECT
postgres-# pg_get_userbyid(c1.relowner) AS seqowner, -- sequence的用户
postgres-# n1.nspname AS seqschema, -- sequence的schema
postgres-# c1.relname AS seqname, -- -- sequence的名称
postgres-# (pg_sequence_parameters(c1.oid)).minimum_value AS minvalue, -- sequence的最小值
postgres-# (pg_sequence_parameters(c1.oid)).maximum_value AS maxvalue, -- sequence的最大值
postgres-# (pg_sequence_parameters(c1.oid)).increment AS incrementby, -- sequence的步长
postgres-# pg_get_userbyid(c.relowner) AS tableowner, -- 关联的表的owner
postgres-# n.nspname AS tableschema, -- 关联的表的schema
postgres-# c.relname AS tablename,-- 关联的表的名称
postgres-# a.attname, -- -- 关联的表的列的名称
postgres-# d.adsrc as defaultexpression -- 关联的sequence的表达式
postgres-# FROM pg_catalog.pg_class c
postgres-# INNER JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
postgres-# INNER JOIN pg_catalog.pg_attribute a ON a.attrelid = c.oid
postgres-# INNER JOIN pg_catalog.pg_attrdef d ON d.adrelid = c.oid AND d.adnum = a.attnum
postgres-# INNER JOIN pg_catalog.pg_depend p ON p.objid = d.oid
postgres-# INNER JOIN pg_catalog.pg_class c1 ON c1.oid = p.refobjid
postgres-# INNER JOIN pg_catalog.pg_namespace n1 ON n1.oid = c1.relnamespace
postgres-# WHERE c.relkind = 'r'::"char" AND c.oid > 16384
postgres-# AND n.nspname not in ('pg_toast', 'cstore', 'pg_catalog')
postgres-# AND a.attnum > 0
postgres-# AND p.classid='pg_catalog.pg_attrdef'::regclass
postgres-# AND refclassid = 'pg_catalog.pg_class'::regclass
postgres-# AND p.refobjsubid = 0
postgres-# AND c1.relkind = 'S'::"char"
postgres-# ;
seqowner | seqschema | seqname | minvalue | maxvalue | incrementby | tableowner | tableschema | tablename | attname | defaultexpression
-----------+-----------+-------------------+----------+---------------------+-------------+------------+-------------+---------------------+---------+----------------------------------------
j00565968 | public | test_serial_a_seq | 1 | 9223372036854775807 | 1 | j00565968 | public | test_serial | a | nextval('test_serial_a_seq'::regclass)
j00565968 | public | test_seq_1 | 1 | 9223372036854775807 | 1 | j00565968 | public | test_serial_default | a | nextval('test_seq_1'::regclass)
(2 rows)

实际业务中使用sequence一般有两个场景

• 保证表数据均匀分布。这种场景可以把sequence字段替换为uuid

postgres=# DROP TABLE public.test_serial;
DROP TABLE
Time: 19.871 ms
postgres=# CREATE TABLE public.test_serial(a text DEFAULT sys_guid(), b int) DISTRIBUTE BY HASH(a);
CREATE TABLE
Time: 19.245 ms

或者使用roundrobin分布替换，然后删除sequence相关列

postgres=# ALTER TABLE public.test_serial DISTRIBUTE BY ROUNDROBIN;
ALTER TABLE
Time: 218.796 ms
postgres=# ALTER TABLE public.test_serial DROP COLUMN a;
ALTER TABLE
Time: 9.509 ms

• 保证字段数据的唯一性。这种场景可以使用uuid()+主键的方式实现

postgres=# DROP TABLE public.test_serial;
DROP TABLE
Time: 18.786 ms
postgres=# CREATE TABLE public.test_serial(a text DEFAULT sys_guid(), b int, primary key(a)) DISTRIBUTE BY HASH(a);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "test_serial_pkey" for table "test_serial"
CREATE TABLE

注：理论上一个集群内的uuid不会重复，但还是建议表上增加主键约束

点击关注，第一时间了解华为云新鲜技术~