Re: Re: Give me more details of some bits in infomask!!

 

From: Tomas Vondra

Date: 2023-02-26 23:23

To: jacktby@gmail.com; pgsql-hackers

Subject: Re: Give me more details of some bits in infomask!!

On 2/26/23 15:30, jacktby@gmail.com wrote:

> here are the source codes from src/include/access/htup_details.h.

> /*

>  * information stored in t_infomask:

>  */

> #define HEAP_HASNULL0x0001/* has null attribute(s) */

> #define HEAP_HASVARWIDTH0x0002/* has variable-width attribute(s) */

> #define HEAP_HASEXTERNAL0x0004/* has external stored attribute(s) */

> #define HEAP_HASOID_OLD0x0008/* has an object-id field */

> #define HEAP_XMAX_KEYSHR_LOCK0x0010/* xmax is a key-shared locker */

> #define HEAP_COMBOCID0x0020/* t_cid is a combo CID */

> #define HEAP_XMAX_EXCL_LOCK0x0040/* xmax is exclusive locker */

> #define HEAP_XMAX_LOCK_ONLY0x0080/* xmax, if valid, is only a locker */

>

> And I can't understand these attrs:

 

I suggest you try something like 'git grep HEAP_HASEXTERNAL' which shows

you where the flag is used, which should tell you what it means. These

short descriptions generally assume you know enough about the internals.

 

> 1. external stored attribute(s), what is this?  can you give a create

> statement to show me?

 

external = value stored in a TOAST table

 

> 2. xmax is a key-shared locker/exclusive locker/only a locker, so how

> you use this? can you give me a  scenario?

> 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.

>  if there is a txn is trying to delete/update this heaptuple,

> the HEAP_XMAX_EXCL_LOCK bit will be set to 1.

>  but for HEAP_XMAX_LOCK_ONLY, I can't understand.

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

> 1 at most.

 

I believe HEAP_XMAX_LOCK_ONLY means the xmax transaction only locked the

tuple, without deleting/updating it.

 

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

 

CID means "command ID" i.e. sequential ID assigned to commands in a

single session (for visibility checks, so that a query doesn't see data

deleted by earlier commands in the same session). See

src/backend/utils/time/combocid.c for basic explanation of what "combo

CID" is.

 

 

regards

 

--

Tomas Vondra

EnterpriseDB: http://www.enterprisedb.com

The Enterprise PostgreSQL Company

> I believe HEAP_XMAX_LOCK_ONLY means the xmax transaction only locked the

> tuple, without deleting/updating it.

if so, you mean when I read this tuple, this bit will be set 1, but I think this is duplicat with HEAP_XMAX_KEYSHR_LOCK.

> CID means "command ID" i.e. sequential ID assigned to commands in a

> single session (for visibility checks, so that a query doesn't see data

> deleted by earlier commands in the same session). See

> src/backend/utils/time/combocid.c for basic explanation of what "combo

> CID" is.

I think if cid is used for  visibility checks in one session, that's meaingless, beacause we can use the t_xmin and t_xmax to 

get this goal. Is tis