Question about tuple´s lock

[Carlos Alves]

Hi! 

I need some help to understand why a transaction wiht a row delete
aquire a AccessExclusiveLock and a row update a ExclusiveLock. 

To ilustrate, a made this scenario: 

create table tblock 

( 

  id int primary key, 

  value text 

); 

insert into tblock values (1, 'somevalue'); 

First start Transaction 1: 

begin; 

update tblock set value = 'othervalue' where id = 1; 

-- keep this transaction opened 

After, start Transaction 2: 

begin; 

update tblock set value = 'onemorevalue' where id = 1; 

-- at this point, this transaction assumes a ExclusiveLock in the tuple;


-- keep this transaction opend 

Then, start Transaction 3: 

begin; 

update tblock set value = 'lastofthevalues' where id = 1; 

-- here i can see this transaction trying to aquire a
AccessExclusiveLock in this tuple 

So, my question is: why a delete row requires a AccessExclusiveLock in
the tuple insteead of ExclusiveLock? 

Bellow, a image of a query over pg_locks: 

PostgreSQL version: 13 

OS: Centos 7 

Thanks in advance.

-- 

 Carlos Alves 
Especialista

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71205-080 || Guará || Brasília, DF 0800-6020097 

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Re: Question about tuple´s lock

[Carlos Alves]

Em 24/10/2023 13:27, Carlos Alves escreveu:

> Hi! 
> 
> I need some help to understand why a transaction wiht a row delete aquire a 
> AccessExclusiveLock and a row update a ExclusiveLock. 
> 
> To ilustrate, a made this scenario: 
> 
> create table tblock 
> 
> ( 
> 
> id int primary key, 
> 
> value text 
> 
> ); 
> 
> insert into tblock values (1, 'somevalue'); 
> 
> First start Transaction 1: 
> 
> begin; 
> 
> update tblock set value = 'othervalue' where id = 1; 
> 
> -- keep this transaction opened 
> 
> After, start Transaction 2: 
> 
> begin; 
> 
> update tblock set value = 'onemorevalue' where id = 1; 
> 
> -- at this point, this transaction assumes a ExclusiveLock in the tuple; 
> 
> -- keep this transaction opend 
> 
> Then, start Transaction 3: 
> 
> begin; 
> 
> update tblock set value = 'lastofthevalues' where id = 1; 
> 
> -- here i can see this transaction trying to aquire a AccessExclusiveLock in 
> this tuple 
> 
> So, my question is: why a delete row requires a AccessExclusiveLock in the 
> tuple insteead of ExclusiveLock? 
> 
> Bellow, a image of a query over pg_locks: 
> 
> PostgreSQL version: 13 
> 
> OS: Centos 7 
> 
> Thanks in advance.

I typed the last command wrong. Should be:

Transaction 3: 

begin; 

delete from tblock where id = 1; 

Sorry!

Re: Question about tuple´s lock

[Carlos Alves]

Em 24/10/2023 17:31, Tom Lane escreveu:

> Carlos Alves  writes: 
> 
>> I need some help to understand why a transaction wiht a row delete
>> aquire a AccessExclusiveLock and a row update a ExclusiveLock.
> 
> UPDATE can use the weaker lock type if it's not modifying any
> column that is part of a unique index.  This is to allow concurrency
> with foreign-key checks that might wish to grab a read-only (shared)
> lock on such a tuple.
> 
> A DELETE, or an UPDATE that is modifying key columns, has to
> conflict with foreign-key checks.
> 
> regards, tom lane

Tom, 

thank you very much for your straight to the point answer!

regards
Carlos Alves