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

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

On Sun, Feb 26, 2023 at 8:36 AM jacktby@gmail.com <jacktby@gmail.com> wrote:

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

I think the word "session" is wrong. It should be "transaction".

IIUC, it is what is changed when one issues CommandCounterIncrement within
a transaction. And you need somewhere to save which CCI step deletes rows,
in particular due to the savepoint feature.

David J.