Re: Give me details of some attributes!!

On Sun, 2023-02-26 at 22:29 +0800, jacktby(at)gmail(dot)com wrote:
>
> here are the source codes from src/include/access/htup_details.h.
> /*
>  * information stored in t_infomask:
>  */
> #define HEAP_HASNULL 0x0001 /* has null attribute(s) */
> #define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
> #define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
> #define HEAP_HASOID_OLD 0x0008 /* has an object-id field */
> #define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
> #define HEAP_COMBOCID 0x0020 /* t_cid is a combo CID */
> #define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
> #define HEAP_XMAX_LOCK_ONLY 0x0080 /* xmax, if valid, is only a locker */
>
> And I can't understand these attrs:
> 1. external stored attribute(s), what is this?  can you give a create statement to show me?

Those are columns stored out of line. See
https://www.postgresql.org/docs/current/storage-toast.html

> 2. xmax is a key-shared locker/exclusive locker/only a locker, so how you use this? can you give me a  scenario?

https://www.cybertec-postgresql.com/en/whats-in-an-xmax/

> let me try to explain it:
>  if there is a txn is trying to read this heaptuple, the HEAP_XMAX_KEYSHR_LOCK bit will be set to 1.

No, reading a row won't lock it.
It's SELECT ... FOR KEY SHARE that does that.

>  if there is a txn is trying to delete/update this heaptuple, the HEAP_XMAX_EXCL_LOCK bit will be set to 1.

Correct.

>  but for HEAP_XMAX_LOCK_ONLY, I can't understand.

See the comment a few source lines below:

/*
* A tuple is only locked (i.e. not updated by its Xmax) if the
* HEAP_XMAX_LOCK_ONLY bit is set; or, for pg_upgrade's sake, if the Xmax is
* not a multi and the EXCL_LOCK bit is set.

Essentially, it is set after SELECT ... FOR UPDATE.

> And another thought is that these three bit can have only one to be set 1 at most.

If a tuple is only locked FOR KEY SHARE, bit not updated or deleted, both are set.

> 3. t_cid is a combo CID? what's a CID? give me an example please.

The command ID of the command in the transaction.

Yours,
Laurenz Albe