这篇文章主要介绍“PostgreSQL中Tuple可见性判断的基本逻辑是什么”，在日常操作中，相信很多人在PostgreSQL中Tuple可见性判断的基本逻辑是什么问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”PostgreSQL中Tuple可见性判断的基本逻辑是什么”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！
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判断逻辑
((Xmin == my-transaction && inserted by the current transaction
Cmin < my-command && before this command, and
(Xmax is null || the row has not been deleted, or
(Xmax == my-transaction && it was deleted by the current transaction
Cmax >= my-command))) but not before this command,
|| or
(Xmin is committed && the row was inserted by a committed transaction, and
(Xmax is null || the row has not been deleted, or
(Xmax == my-transaction && the row is being deleted by this transaction
Cmax >= my-command) || but it’s not deleted “yet”, or
(Xmax != my-transaction && the row was deleted by another transaction
Xmax is not committed)))) that has not been committed
其中,my-command在PG中通过cmin/cmax体现.
cmin/cmax
如下例所示,游标在删除命令前定义,因此Tuple可见.
10:51:21 (xdb@[local]:5432)testdb=# DELETE FROM mvcc_demo; DELETE 3 10:51:22 (xdb@[local]:5432)testdb=# 10:51:22 (xdb@[local]:5432)testdb=# BEGIN WORK; BEGIN 10:51:22 (xdb@[local]:5432)testdb=#* INSERT INTO mvcc_demo VALUES (1); INSERT 0 1 10:51:22 (xdb@[local]:5432)testdb=#* INSERT INTO mvcc_demo VALUES (2); INSERT 0 1 10:51:22 (xdb@[local]:5432)testdb=#* INSERT INTO mvcc_demo VALUES (3); INSERT 0 1 10:51:22 (xdb@[local]:5432)testdb=#* SELECT xmin, cmin, xmax, cmax, * FROM mvcc_demo; xmin | cmin | xmax | cmax | val ------+------+------+------+----- 2400 | 0 | 0 | 0 | 1 2400 | 1 | 0 | 1 | 2 2400 | 2 | 0 | 2 | 3 (3 rows) 10:51:23 (xdb@[local]:5432)testdb=#* DECLARE c_mvcc_demo CURSOR FOR [local] xdb@testdb-# SELECT xmin, cmin, xmax, cmax, * FROM mvcc_demo; --> 定义游标 DECLARE CURSOR 10:51:34 (xdb@[local]:5432)testdb=#* DELETE FROM mvcc_demo; --> 删除数据 DELETE 3 10:51:34 (xdb@[local]:5432)testdb=#* SELECT xmin, cmin, xmax, cmax, * FROM mvcc_demo; xmin | cmin | xmax | cmax | val ------+------+------+------+----- (0 rows) 10:51:35 (xdb@[local]:5432)testdb=#* FETCH ALL FROM c_mvcc_demo; --> 游标在删除命令前定义,因此可见 xmin | cmin | xmax | cmax | val ------+------+------+------+----- 2400 | 0 | 2400 | 0 | 1 2400 | 1 | 2400 | 1 | 2 2400 | 2 | 2400 | 2 | 3 (3 rows)
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