Postgresql source code (66) insert on conflict grammar introduction and kernel execution process analysis

1 语法介绍

insert on conflict语法实现了upsert的功能,That is, a primary key conflict occurs on the insert、or a unique constraint violation,执行on conflict后面的语句,将insert变成update或do nothing避免报错.

语法手册：https://www.postgresql.org/docs/current/sql-insert.html

测试用例：

drop table decoding_test;
CREATE TABLE decoding_test(x integer primary key, y text);

postgres=# select * from decoding_test;
 x  | y 
----+---
 12 | 9

postgres=# INSERT INTO decoding_test(x,y) values(12,9) on conflict (x) do nothing;
INSERT 0 1

postgres=# select * from decoding_test;
 x  | y 
----+---
 12 | 9

-- No primary key conflict was reported,As a result, the insertion has no effect.
postgres=# INSERT INTO decoding_test(x,y) values(12,9) on conflict (x) do nothing;
INSERT 0 0

postgres=# select * from decoding_test;
 x  | y 
----+---
 12 | 9
(1 row)

postgres=# INSERT INTO decoding_test(x,y) values(101,20) on conflict (x) do update set y=EXCLUDED.y;
INSERT 0 1
postgres=# 
postgres=# select * from decoding_test;
  x  | y  
-----+----
  12 | 9
 101 | 20
 
-- A primary key violation occurred while inserting,执行后面的update语句,将y更新为400,EXCLUDEDIndicates that this row of data is ready to be newly inserted.
postgres=# INSERT INTO decoding_test(x,y) values(101,400) on conflict (x) do update set y=EXCLUDED.y;
INSERT 0 1
postgres=# select * from decoding_test;
  x  |  y  
-----+-----
  12 | 9
 101 | 400
(2 rows)

2 Kernel execution flow

注意：后面提到的speculative insert等价与insert on conflict语法.

2.1 Observed from the execution flowspeculative insert

执行流程：

• spec insertThe execution flow and commoninsert是分开的,走两个分支.
• specThe special thing is that there is a retry mechanism,即：
  • In the first check if no unique key conflict is found,正常是可以直接insert的.
  • But due to no lock check,Possibly in realinsertA unique key violation occurred again（Checked before,other concurrencyinsertA piece of conflicting data）
  • 那么这时xlogOne has already been successfulinsert了,Need to add another onedelete（The fourth step in the figure conflict occurs）.
    

2.2 Observed from a log perspectivespeculative insert

• INSERT INTO decoding_test(x,y) values(12,9) on conflict (x) do nothing;
  • 情况一：插入成功
    • heap_insert,生成XLOG_HEAP_INSERT日志.
    • heap_finish_speculative,生成XLOG_HEAP_CONFIRM日志.
  • 情况二：插入失败
    • 不生成日志
  • 情况三：There are no conflicts yet when inserting,But other processes insert conflicting rows concurrently（Concurrency conflict locations are analyzed later）
    • heap_insert,生成XLOG_HEAP_INSERT日志.
    • heap_abort_speculative,生成XLOG_HEAP_DELETE日志.
• INSERT INTO decoding_test(x,y) values(20,9) on conflict (x) do update set y=100;
  • 插入成功
    • heap_insert,生成XLOG_HEAP_INSERT日志.
    • heap_finish_speculative,生成XLOG_HEAP_CONFIRM日志.
  • 更新成功：转换为update语句执行
    • log_heap_update,生成XLOG_HEAP_HOT_UPDATE日志.
  • There are no conflicts yet when inserting,But other processes insert conflicting rows concurrently（Concurrency conflict locations are analyzed later）
    • heap_insert,生成XLOG_HEAP_INSERT日志.
    • heap_abort_speculative,生成XLOG_HEAP_DELETE日志.

So it may be seen from the log3种情况：

情况一： 第一条XLOG_HEAP_INSERT      第二条XLOG_HEAP_CONFIRM
情况二： 第一条XLOG_HEAP_INSERT      第二条XLOG_HEAP_DELETE
情况三： 第一条XLOG_HEAP_HOT_UPDATE

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The next article will continue to introduce these types of logs after they are parsed by logical replication.