可序列化的隔离级别 - 它会阻止读取吗？

Question

I have read that serializable isolation level blocks only: insert, update, delete but NOT read.

I have run in one window:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;  

UPDATE [dbo].[Categories]
SET name = 'aaa'
WHERE categoryid = 4;

-- without commit

And in the second window:

SELECT TOP (1000) [CategoryId]
      ,[Name]
  FROM [dbo].[Categories]

And the above query is waiting for the end of the first query. So does serialization isolation level block also read?

Answer 1

Yes it blocks reads. Your transaction is not committed, so you can still do a rollback. That means the data you have updated should not be read by any other process before you commit.

Answer 2

"does serialization isolation level block also read": No. Serializable acquires some lock (in Ms Sql its shared lock + range lock) which prevents writes from other transactions. Its the UPDATE statement's exclusive lock (the above shared lock now converts to exclusive lock) that prevents second transaction's SELECT (Read Commited or Repeatable Read by deafult depending on db) to wait.