幂等性的一个要求是多次操作的结果一致。对于update操作，多次直接的结果都是最后update的值，是满足需求的。
但对于insert，如果已经插入，第二次会报错，duplicate error， 主键重复或者unique key duplicate。所以需要做一下处理。
最简单的就是，try-catch，当报错的时候，调用update去更新，或者策略更简单点，直接返回就行，不需要更新，以第一条为准。
PostgreSQL从9.5之后就提供了原子的upsert语法： 不存在则插入，发生冲突可以update。
## Inert语法
官方文档： https://www.postgresql.org/docs/devel/sql-insert.html
[ WITH [ RECURSIVE ] with_query [, ...] ]
INSERT INTO table_name [ AS alias ] [ ( column_name [, ...] ) ]
[ OVERRIDING { SYSTEM | USER} VALUE ]
{ DEFAULT VALUES | VALUES ( { expression | DEFAULT } [, ...] ) [, ...] | query }
[ ON CONFLICT [ conflict_target ] conflict_action ]
[ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]
where conflict_target can be one of:
( { index_column_name | ( index_expression ) } [ COLLATE collation ] [ opclass ] [, ...] ) [ WHERE index_predicate ]
ON CONSTRAINT constraint_name
and conflict_action is one of:
DO NOTHING
DO UPDATE SET { column_name = { expression | DEFAULT } |
( column_name [, ...] ) = [ ROW ] ( { expression | DEFAULT } [, ...] ) |
( column_name [, ...] ) = ( sub-SELECT )
} [, ...]
[ WHERE condition ]index_column_name
The name of a table_name column. Used to infer arbiter indexes. Follows CREATE INDEX format. SELECT privilege on index_column_name is required.
index_expression
Similar to index_column_name, but used to infer expressions on table_name columns appearing within index definitions (not simple columns).
Follows CREATE INDEX format. SELECT privilege on any column appearing within index_expression is required.
## 使用示例
创建表CREATE TABLE "test"."upsert_test" ( "id" int4 NOT NULL, "name" varchar(255) COLLATE "pg_catalog"."default" ) ;
当主键id冲突时，更新其他字段INSERT INTO test.upsert_test(id, "name") VALUES(1, 'm'),(2, 'n'),(4, 'c')
ON conflict(id) DO UPDATE SET "name" = excluded.name;
- did 冲突的主键
- EXCLUDED 代指要插入的记录
当主键或者unique key发生冲突时，什么都不做
INSERT INTO test.upsert_test(id, "name")
VALUES(1, 'm'),(2, 'n'),(4, 'c')
ON conflict(id) DO NOTHING;