事务查看开始后对数据库行所做的更改

Question

我发现了一些在使用 Postgres 和 Go GORM 时无法解决的问题。

在下面的代码片段中，我尝试查找、删除和创建相同的项目，但是在 2 个不同的事务中，但是我开始第二个事务并在提交第一个事务之前找到项目。

在第二笔交易中，发生了一些奇怪的事情。当我调用 t2.Delete() 时，它不会删除任何行。在数据库中，调用t1.Commit()后，item的主键变成了2，但是由于已经得到了旧的item，所以对主键1.[=18=进行了删除]

为什么事务 2 看到新行，尽管它是在提交第一个之前创建的？

db, err := gorm.Open("postgres", "host=localHost port=5432 user=postgres dbname=test password=postgres sslmode=disable")
defer db.Close()
db.DropTableIfExists(&Product{})
db.AutoMigrate(&Product{})
db.LogMode(true)

db.Create(&Product{Code: "A", Price: 1000})
// SQL: INSERT  INTO "products" ("code","price") VALUES ('A',1000) RETURNING "products"."id"

// Start transaction 1, find item, delete it
t1 := db.Begin()
product := &Product{}

err = t1.Find(product, "code = ?", "A").Error
// SQL: SELECT * FROM "products"  WHERE (code = 'A')

err = t1.Delete(product).Error
// SQL: DELETE FROM "products"  WHERE "products"."id" = 1

// Start transaction 2 and get item, before transaction 1 creates new item and commits
t2 := db.Begin()

product2 := &Product{}
err = t2.Find(product2, "code = ?", "A").Error
// SQL: SELECT * FROM "products"  WHERE (code = 'A')

err = t1.Create(&Product{Code: "A", Price: 3000}).Error
// SQL: INSERT  INTO "products" ("code","price") VALUES ('A',3000) RETURNING "products"."id"

err = t1.Commit().Error
// Database now contains

err = t2.Delete(product2).Error
// SQL: DELETE FROM "products"  WHERE "products"."id" = 1
// [0 rows affected or returned ]

err = t2.Save(&Product{Code: "A", Price: 4000}).Error
// SQL: INSERT  INTO "products" ("code","price") VALUES ('A',4000) RETURNING "products"."id"
// ERROR HERE: pq: duplicate key value violates unique constraint "products_code_key"

err = t2.Commit().Error

注意： GORM 默认使用 Read Committed isolation level。如果它正常工作，我知道这会导致完整性问题。我将改为使用 Serializable，如果指定 "FOR UPDATE;"

，这会在 Commit 时出错，或者在 Get 时阻塞

Answer 1

来自 PostgreSQL docs

Read Committed is the default isolation level in PostgreSQL. When a transaction uses this isolation level, a SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began; it never sees either uncommitted data or changes committed during query execution by concurrent transactions. In effect, a SELECT query sees a snapshot of the database as of the instant the query begins to run.

...

Also note that two successive SELECT commands can see different data, even though they are within a single transaction, if other transactions commit changes after the first SELECT starts and before the second SELECT starts.

第二段似乎概括了你的情况。在 t1 提交后启动的 t2 中的任何 "SELECT" 都可以看到 t1 应用的更新。

事务查看开始后对数据库行所做的更改

Transaction viewing changes to database rows made after it began

postgresql

transactions

go

isolation-level

go-gorm