PostgreSQL未触发预期的Serializable访问错误问题咨询

Hey there, let's break down why you might not be seeing the expected serialization error in your PostgreSQL test. First, let's recap what you've shared so far, then dive into common pitfalls and how to fix your test.

Your Test Setup #

First, here's your table creation statement (formatted for clarity):

CREATE TABLE concurrency_test ( id serial PRIMARY KEY, sum INT NOT NULL );

And your partial test steps:

It looks like your step list got cut off—those missing steps are probably the key here! Serialization failures don't happen by accident; they require specific conflicting operations between transactions.

Why You're Not Seeing the Error #

Let's go through the most likely reasons:

• No conflicting operations: If your transactions aren't reading and writing the same data (e.g., each is modifying a unique id row), there's no overlap to trigger a serializability check. PostgreSQL only cares when two transactions' actions can't be reordered into a valid serial sequence.
• Transactions aren't committing: Serialization checks happen at commit time. If you're not committing both transactions, or if one rolls back before the other commits, PostgreSQL never evaluates the conflict.
• Transactions run in a valid serial order: If one transaction finishes all its reads/writes before the other starts touching the same data, there's no anomaly to detect. The SERIALIZABLE level allows this—it only blocks when the order of operations creates an impossible serial scenario.
• Missing critical read/write steps: For example, if you don't have both transactions read the same sum value before modifying it, PostgreSQL can't detect the classic "lost update" anomaly that would trigger a serialization failure.

A Working Test That Triggers the Error #

To see the serialization failure you expect, try this complete flow:

1. First, seed some test data (run in either connection):

   INSERT INTO concurrency_test (sum) VALUES (100);
   

2. Connection 1:

   START TRANSACTION ISOLATION LEVEL SERIALIZABLE;
   -- Read the current sum value
   SELECT sum FROM concurrency_test WHERE id = 1; -- You'll get 100
   

3. Connection 2:

   START TRANSACTION ISOLATION LEVEL SERIALIZABLE;
   -- Read the same sum value
   SELECT sum FROM concurrency_test WHERE id = 1; -- Also gets 100
   -- Modify and commit
   UPDATE concurrency_test SET sum = sum + 50 WHERE id = 1;
   COMMIT;
   

4. Connection 1:

   -- Now try to modify and commit
   UPDATE concurrency_test SET sum = sum + 50 WHERE id = 1;
   COMMIT; -- This should throw: ERROR:  could not serialize access due to read/write dependencies among transactions
   

Quick Reminder #

PostgreSQL's SERIALIZABLE implementation is smart—it doesn't throw errors just because two transactions are running at the same time. It only intervenes when there's a genuine serializability anomaly that can't be resolved safely. So double-check your full test steps to make sure you're creating that conflict!

内容的提问来源于stack exchange，提问作者Ivan Alikin