Questions of 'for update'

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От Zhenghua Lyu
Тема Questions of 'for update'
Дата
Msg-id CAO0i4_QCf8LUCO9xDgDpJ0zdsyM7q83APtqHamdsswQ6NVT3ZQ@mail.gmail.com
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Ответы Re: Questions of 'for update'
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Hi,

    I am reading the code that generating plan for `rowmarks` of Postgres 9.4 (https://github.com/postgres/postgres/blob/REL9_4_STABLE/src/backend/optimizer/plan/planner.c#L2070) 

  After emitting the `LockRows` plannode, the results cannot be considered in order, and there are comments there:
/*
 * The result can no longer be assumed sorted, since locking might
 * cause the sort key columns to be replaced with new values.
 */

I do not understand the reason and after some guess, I come up with a case:

```
create table t(c int);
insert into t values (1), (2), (3), (4);

-- Transaction 1
begin;
update t set c = 999 where c = 1; -- change the smallest value to a very big one
-- transaction 1 not commit yet

-- Transaction 2, another session
begin;
select * from t order by c limit 1 for update; -- Want to find the smallest value, and then update it
-- this transaction will be blocked by transaction 1

-- then, transaction 1 commit and transaction 2 will return the tuple with value 999
```

I think the reason is that EvalPlanQual does not check the order.

I try this case under mysql, it will output 2 (which is the correct value for the meaning of smallest).

So, in summary, my questions are:

1. why after emitting `lockrows` plannode,  the result can no longer be assumed sorted?
2. Is the case above a bug or a feature?

Thanks!

Best Regards,
Zhenghua Lyu

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