On Sun, 2023-02-26 at 22:29 +0800, jacktby@gmail.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
