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

From: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
To: "jacktby(at)gmail(dot)com" <jacktby(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Give me more details of some bits in infomask!!
Date: 2023-02-26 15:23:03
Message-ID: 3936d2fb-f229-63fd-26e1-130fbe918f51@enterprisedb.com
Views: Raw Message | Whole Thread | Download mbox | Resend email
Thread:
Lists: pgsql-hackers

On 2/26/23 15:30, 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_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

In response to

Responses

Browse pgsql-hackers by date

  From Date Subject
Next Message Tomas Vondra 2023-02-26 15:35:53 Re: Why the lp_len is 28 not 32?
Previous Message jacktby@gmail.com 2023-02-26 15:11:36 Re: Re: Why the lp_len is 28 not 32?